1. Lecture Outlines by Gregory Ahearn, University of North Florida Copyright © 2011 Pearson Education Inc. Chapter 10 Patterns of Inheritance

2. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Chapter 10 At a Glance  10.1 What Is the Physical Basis of Inheritance?  10.2 How Were the Principles of Inheritance Discovered?  10.3 How Are Single Traits Inherited?  10.4 How Are Multiple Traits Inherited?  10.5 How Are Genes Located on the Same Chromosome Inherited?  10.6 How Is Sex Determined?

3. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Chapter 10 At a Glance (continued)  10.7 How Are Sex-Linked Genes Inherited?  10.8 Do the Mendelian Rules of Inheritance Apply to All Traits?  10.9 How Are Human Genetic Disorders Investigated?  10.10 How Are Human Disorders Caused by Single Genes Inherited?  10.11 How Do Errors in Chromosome Number Affect Humans?

4. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.1 What Is the Physical Basis of Inheritance?  Genes are sequences of nucleotides at specific locations on chromosomes –Inheritance is the process by which the characteristics of individuals are passed to their offspring –A gene is a unit of heredity that encodes information needed to produce proteins, cells, and entire organisms –Genes comprise segments of DNA ranging from a few hundred to many thousands of nucleotides in length –The location of a gene on a chromosome is called its locus (plural, loci)

5. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.1 What Is the Physical Basis of Inheritance?  Genes are sequences of nucleotides at specific locations on chromosomes (continued) –Homologous chromosomes carry the same kinds of genes for the same characteristics –Genes for the same characteristic are found at the same loci on both homologous chromosomes

6. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.1 What Is the Physical Basis of Inheritance?  Genes are sequences of nucleotides at specific locations on chromosomes (continued) –Genes for a characteristic found on homologous chromosomes may not be identical –Alternative versions of genes found at the same gene locus are called alleles

7. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.1 What Is the Physical Basis of Inheritance?  Mutations are the source of alleles –Alleles arise as mutations in the nucleotide sequence in genes –If a mutation occurs in the cells that become sperm or eggs, it can be passed on from parent to offspring

8. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.1 What Is the Physical Basis of Inheritance?  An organism’s two alleles may be the same or different –Each cell carries two alleles per characteristic, one on each of the two homologous chromosomes –If both homologous chromosomes carry the same allele (gene form) at a given gene locus, the organism is homozygous at that locus

9. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.1 What Is the Physical Basis of Inheritance?  An organism’s two alleles may be the same or different (continued) –If two homologous chromosomes carry different alleles at a given locus, the organism is heterozygous at that locus (a hybrid)

10. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Both chromosomes carry the same allele of the gene at this locus; the organism is homozygous at this locus This locus contains another gene for which the organism is homozygous Each chromosome carries a different allele of this gene, so the organism is heterozygous at this locus a pair of homologous chromosomes gene loci the chromosome from the male parent the chromosome from the female parent The Relationship Among Genes, Alleles, and Chromosomes Fig. 10-1

11. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.2 How Were the Principles of Inheritance Discovered?  Doing it right: The secrets of Mendel’s success –Who was Gregor Mendel? –Mendel was an Austrian monk in a monastery in the late 1800s –He discovered the common patterns of inheritance and many essential facts about genes, alleles, and the distribution of alleles in gametes and zygotes during sexual reproduction –He chose the edible pea plant for his experiments, which took place in the monastery garden –Mendel’s background allowed him to see patterns in the way plant characteristics were inherited

12. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Gregor Mendel Fig. 10-2

13. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.2 How Were the Principles of Inheritance Discovered?  Doing it right: The secrets of Mendel’s success (continued) –Mendel was the first to perform experiments by correctly applying three key scientific steps to his research: –Choosing the right organism –Designing and performing the experiment correctly –Analyzing the data properly

14. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.2 How Were the Principles of Inheritance Discovered?  Doing it right: The secrets of Mendel’s success (continued) –Pea plants have qualities that make it a good organism for studying inheritance –Pea flowers have stamens, the male structures that produce pollen, that in turn contain the sperm (male gametes); sperm are gametes and pollen is the vehicle –Pea flowers have carpels, female structures housing the ovaries, which produce the eggs (female gametes) –Pea flower petals enclose both male and female flower parts and prevent entry of pollen from another pea plant

15. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e intact pea flowerflower dissected to show its reproductive structures Carpel (female, produces eggs) Stamen (male, produces pollen that contain sperm) Flowers of the Edible Pea Fig. 10-3

16. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.2 How Were the Principles of Inheritance Discovered?  Doing it right: The secrets of Mendel’s success (continued) –Because of their structure, pea flowers naturally self-fertilize –Pollen from the stamen of a plant transfers to the carpel of the same plant, where the sperm then fertilizes the plant’s eggs

17. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.2 How Were the Principles of Inheritance Discovered?  Doing it right: The secrets of Mendel’s success (continued) –Mendel was able to mate two different plants by hand (cross-fertilization) –Female parts (carpels) were dusted with pollen from other selected plants

18. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.2 How Were the Principles of Inheritance Discovered?  Doing it right: The secrets of Mendel’s success (continued) –Unlike previous researchers, Mendel chose a simple experimental design –He chose to study individual characteristics (called traits) that had unmistakably different forms, such as white versus purple flowers –He started out by studying only one trait at a time

19. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.2 How Were the Principles of Inheritance Discovered?  Doing it right: The secrets of Mendel’s success (continued) –Mendel employed numerical analysis in studying the traits –He followed the inheritance of these traits for several generations, counting the numbers of offspring with each type of trait –By analyzing these numbers, he saw the basic patterns of inheritance emerge

20. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.3 How Are Single Traits Inherited?  Research into inheritance begins with parental organisms that have easily identified traits that are inherited consistently from generation to generation  Pea plants that are homozygous for a particular characteristic always produce the same physical forms –If a plant is homozygous for purple flowers, it will always produce offspring with purple flowers –Plants homozygous for a characteristic are true- breeding

21. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.3 How Are Single Traits Inherited?  The language of a genetic cross –A genetic cross is the mating of pollen and eggs (from same or different parents) –The parents used in a cross are part of the parental generation (known as P) –The offspring of the P generation are members of the first filial generation (F 1 ) –Offspring of the F 1 generation are members of the F 2 generation

22. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.3 How Are Single Traits Inherited?  Mendel’s flower color experiments –Mendel crossed a true-breeding purple flower plant with a true-breeding white-flower plant (the P generation) –The F 1 generation consisted of all purple- flowered plants –What had happened to the white-flowered trait?

23. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e true-breeding, purple-flowered plant true-breeding, white-flowered plant cross-fertilize pollen all purple-flowered plants Parental generation (P) First-generation offspring (F 1 ) Cross of Pea Plants True-Breeding for White or Purple Flowers Fig. 10-4

24. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.3 How Are Single Traits Inherited?  Mendel’s flower color experiments (continued) –Mendel allowed the F 1 generation to self-fertilize –The F 2 was composed of 3/4 purple-flowered plants and 1/4 white-flowered plants, a ratio of 3:1 –The results showed that the white trait had not disappeared in the F 1 but merely was hidden

25. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Author Animation: Self- and Cross-Pollination of Pea Plants

26. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e First- generation offspring (F 1 ) 3/4 purple 1/4 white Second- generation offspring (F 2 ) self-fertilize Fig. 10-5 Self-Fertilization of F 1 Pea Plants with Purple Flowers

27. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.3 How Are Single Traits Inherited?  Mendel’s flower color experiments (continued) –Mendel then self-fertilized the F 2 generation –In the F 3 generation, all the white-flowered F 2 plants produced white-flowered offspring –These proved to be true-breeding

28. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.3 How Are Single Traits Inherited?  Mendel’s flower color experiments (continued) –In the F 3 generation, self-fertilized purple-flowered F 2 plants produced two types of offspring –About 1/3 were true-breeding for purple –The other 2/3 were hybrids that produced both purple- and white-flowered offspring, again, in the ratio of 3 purple to 1 white –Therefore, the F 2 generation included 1/4 true- breeding purple-flowered plants, 1/2 hybrid purple, and 1/4 true-breeding white-flowered plants

29. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.3 How Are Single Traits Inherited?  The inheritance of dominant and recessive alleles on homologous chromosomes can explain the results of Mendel’s crosses –A five-part hypothesis explains the inheritance of single traits 1.Each trait is determined by pairs of genes; each organism has two alleles for each gene, one on each homologous chromosome –True-breeding white-flowered plants have different alleles than true-breeding purple-flowered plants 2.When two different alleles are present in an organism, the dominant allele may mask the recessive allele, even though the recessive allele is still present –In edible peas the purple-flower trait is dominant to the white-flower trait

30. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.3 How Are Single Traits Inherited?  A five-part hypothesis explains the inheritance of single traits (continued) 3.The pairs of alleles on homologous chromosomes separate, or segregate, from each other during meiosis, which is known as Mendel’s law of segregation 4.Chance determines which allele is included in a given gamete—because homologous chromosomes separate at random during meiosis; the distribution of alleles to the gametes is also random

31. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.3 How Are Single Traits Inherited?  A five-part hypothesis explains the inheritance of single traits (continued) 5.True-breeding organisms have two copies of the same allele for a given gene and are homozygous for that gene; hybrid organisms have two different alleles for a given gene and are heterozygous for that gene

32. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e homozygous parent (a) Gametes produced by a homozygous parent (b) Gametes produced by a heterozygous parent AA AA gametes heterozygous parent A a a A gametes The Distribution of Alleles in Gametes Fig. 10-6

33. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.3 How Are Single Traits Inherited?  The hypothesis explains Mendel’s results with peas –The particular combination of the two alleles carried by an individual is called the genotype –The physical expression of the genotype is known as the phenotype (for example, purple or white flowers)

34. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.3 How Are Single Traits Inherited?  The hypothesis explains Mendel’s results with peas (continued) –There are two alleles for a given gene characteristic (such as flower color) –Let P stand for the dominant purple-flowered allele: A homozygous purple-colored plant has two alleles for purple flower color (PP) and produces only P gametes –Let p stand for the recessive white-flowered allele: A homozygous white-colored plant has two alleles for white flower color (pp) and produces only p gametes

35. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.3 How Are Single Traits Inherited?  The hypothesis explains Mendel’s results with peas (continued) –A cross between a purple-flowered plant (PP) and a white-flowered plant (pp) produces all purple-flowered F 1 offspring, with a Pp genotype –Dominant P gametes from purple-flowered plants combined with recessive p gametes from white-flowered plants to produce hybrid purple-flowered plants (Pp)

36. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.3 How Are Single Traits Inherited?  The hypothesis explains Mendel’s results with peas (continued) –The F 1 offspring were all heterozygous (Pp) for flower color –When the F 1 offspring were allowed to self- fertilize, four types of gametes were produced from the Pp parents –Sperm: Pp –Eggs: Pp

37. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.3 How Are Single Traits Inherited?  The hypothesis explains Mendel’s results with peas (continued) –A heterozygous plant produces equal numbers of P and p sperm and equal numbers of P and p eggs –When a Pp plant self-fertilizes, each type of sperm has an equal chance of fertilizing each type of egg –Combining these four gametes into genotypes in every possible way produces offspring PP, Pp, Pp, and pp –The probabilities of each combination (and therefore the genotypic fraction each genotype is of the total offspring) are 1/4 PP, 1/2 Pp, and 1/4 pp

38. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e (a) Gametes produced by homozygous parents PP P P purple parent all P sperm and eggs pp p white parent all p sperm and eggs p + + Fig. 10-7a Segregation of Alleles and Fusion of Gametes

39. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e (b) Fusion of gametes produces F 1 offspring p p + + Pp spermeggs P P or F 1 offspring Fig. 10-7b Segregation of Alleles and Fusion of Gametes

40. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Fig. 10-7c Segregation of Alleles and Fusion of Gametes pp p + Pp p + p + + gametes from F 1 Pp plants eggs F 2 offspring p P P P P PP sperm Pp (c) Fusion of gametes from the F 1 generation produces F 2 offspring

41. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.3 How Are Single Traits Inherited?  Simple “genetic bookkeeping” can predict genotypes and phenotypes of offspring –The Punnett square method predicts offspring genotypes and phenotypes from combinations of parental gametes 1.First, assign letters to the different alleles of the characteristic under consideration (uppercase for dominant, lowercase for recessive) 2.Determine the gametes and their fractional proportions (out of all the gametes) from both parents

42. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.3 How Are Single Traits Inherited?  The Punnett square method predicts offspring genotypes from combinations of parental gametes (continued) 3.Write the gametes from each parent, together with their fractional proportions, along each side of a 2 x 2 grid (Punnett square) 4.Fill in the genotypes of each pair of combined gametes in the grid, including the product of the fractions of each gamete (e.g., 1/4 PP, 1/4 Pp and 1/4 pP, and 1/4 pp)

43. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.3 How Are Single Traits Inherited?  The Punnett square method predicts offspring genotypes from combinations of parental gametes (continued) 5.Add together the fractions of any genotypes of the same kind (1/4 Pp + 1/4 pP = 1/2 Pp total) 6.From the sums of all the different kinds of offspring genotypes, create a genotypic fraction –1/4 PP, 1/2 Pp, 1/4 pp is in the ratio 1 PP : 2 Pp : 1 pp

44. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.3 How Are Single Traits Inherited?  The Punnett square method predicts offspring genotypes from combinations of parental gametes (continued) 7.Based on dominant and recessive rules, determine the phenotypic fraction –A genotypic ratio of 1 PP : 2 Pp : 1 pp yields 3 purple-flowered plants : 1 white- flowered plant

45. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Author Animation: The Inheritance of Single Traits

46. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e P p eggs Pp self-fertilize 1 2 1 2 P p 1 2 1 2 P 1 2 P 1 2 PP 1 4   1 4 P 1 2 p 1 2 Pp 1 4   purple 3 4 p 1 2 P 1 2 pP 1 4   Pp 1 2 p 1 2 p 1 2 pp 1 4   1 4 white 1 4 sperm eggs offspring genotypes genotypic ratio (1:2:1) phenotypic ratio (3:1) PP pp Pp 1 4 1 4 1 4 1 4 pP (a) Punnett square of a single-trait cross(b)Using probabilities to determine the offspring of a single-trait cross sperm Determining the Outcome of a Single-Trait Cross Fig. 10-8

47. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.3 How Are Single Traits Inherited?  Mendel’s hypothesis can be used to predict the outcome of new types of single-trait crosses –A test cross is used to deduce whether an organism with a dominant phenotype is homozygous for the dominant allele or heterozygous 1.Cross the unknown dominant-phenotype organism (P_) with a homozygous recessive organism (pp) 2.If the dominant-phenotype organism is homozygous dominant (PP), only dominant-phenotype offspring will be produced (Pp) 3.If the dominant-phenotype organism is heterozygous (Pp), approximately half the offspring will be of recessive phenotype (pp)

48. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Punnett Square of a Test Cross Fig. 10-9 pollen all sperm pp P pp PpPp all eggs PP or Pp sperm unknown if PP if Pp eggs p 1 2 1 2 1 2 P p 1 2 all Pp sperm

49. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.4 How Are Multiple Traits Inherited?  Mendel next tested his hypothesis that multiple traits are inherited independently –Mendel performed genetic crosses in which he followed the inheritance of two traits at the same time

50. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Seed shape Seed color Pod color Pod shape Flower color Flower location at leaf junctions at tips of branches tall (about 6 feet) dwarf (about 8 to 16 inches) Plant size smooth Dominant formTraitRecessive form wrinkled yellow inflated green white constricted purple Fig. 10-10 Traits of Pea Plants Studied by Gregor Mendel

51. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.4 How Are Multiple Traits Inherited?  Mendel next tested his hypothesis that traits are inherited independently (continued) –From the many pea plant phenotypes, he chose seed color (yellow vs. green peas) and seed shape (smooth vs. wrinkled peas) –Yellow color is dominant to green color –Smooth shape is dominant to wrinkled –The allele symbols were assigned, as follows: –Y = yellow (dominant), y = green (recessive) –S = smooth (dominant), s = wrinkled (recessive)

52. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.4 How Are Multiple Traits Inherited?  Mendel next tested his hypothesis that traits are inherited independently (continued) –The two-trait cross was between two true-breeding varieties for each characteristic, one dominant for both traits, the other recessive for both traits –P: SSYY (smooth, yellow)  ssyy (wrinkled, green –The SSYY plant produced only SY gametes, and the ssyy plant produced only sy gametes –Therefore, the F 1 consisted solely of SsYy individuals, with smooth skins and yellow coloring

53. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.4 How Are Multiple Traits Inherited?  Mendel next tested his hypothesis that traits are inherited independently (continued) –Mendel next allowed the F 1 individuals to self- fertilize: SsYy  SsYy –Crossing the F 1 plants yielded 315 plants with smooth, yellow seeds; 101 with wrinkled, yellow seeds; 108 with smooth, green seeds; and 32 with wrinkled, green seeds –This is a ratio of approximately 9:3:3:1 –Two-trait crosses of other traits produced similar proportions of phenotype combinations

54. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 1 4 1 4 3 4 1 4 1 4 1 4 1 4 1 4 1 4 1 16 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 SY SSYYSsYY ssYY ssyY SsyY SSYySsYy self-fertilize ssYy ssyy SsyySSyy sSyYsSyy sSYYsSYy SSyY sY sy Sy eggs seed shapeseed colorphenotypic ratio (9:3:3:1) smooth 3 4 yellow 9 16 smooth yellow 3 16 smooth green 3 16 wrinkled yellow 1 16 wrinkled green 3 4 yellow 1 4 green 1 4 3 4 smooth 1 4 wrinkled 1 4         (a) Punnett square of a two-trait cross(b) Using probabilities to determine the offspring of a two-trait cross sperm Predicting Genotypes and Phenotypes for a Cross between Parents That Are Heterozygous for Two Traits Fig. 10-11

55. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.4 How Are Multiple Traits Inherited?  Mendel’s results supported his hypothesis that traits are inherited independently –Mendel predicted that if the two traits were inherited independently, then for each trait, three-quarters of the offspring should show the dominant phenotype and one-quarter should show the recessive phenotype — a 3:1 ratio, as he had found for the single trait flower color

56. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.4 How Are Multiple Traits Inherited?  Mendel’s results supported his hypothesis that traits are inherited independently (continued) –He found 423 plants with smooth seeds (of either color) and 133 with wrinkled seeds (a ratio of about 3:1) –He found 416 plants produced yellow seeds (of either shape) and 140 produced green seeds (also about 3:1)

57. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.4 How Are Multiple Traits Inherited?  Mendel’s results supported his hypothesis that traits are inherited independently (continued) –The independent inheritance of two or more traits is called the law of independent assortment –Multiple traits are inherited independently because the alleles of one gene are distributed to gametes independently of the alleles for other genes –Independent assortment will occur when the traits being studied are controlled by genes on different pairs of homologous chromosomes

58. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Author Animation: The Inheritance of Multiple Traits

59. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.4 How Are Multiple Traits Inherited?  Mendel’s results supported his hypothesis that traits are inherited independently (continued) –The physical basis of independent assortment has to do with the way homologous pairs line up during meiosis –Which of the two homologues is “on top” occurs randomly for all pairs, so the homologues assort randomly and independently of one another at anaphase I

60. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Independent Assortment of Alleles Fig. 10-12 independent assortment produces four equally likely allele combinations during meiosis pairs of alleles on homologous chromosomes in diploid cells chromosomes replicate replicated homologous pair during metaphase of meiosis I, orienting like this or like this meiosis II meiosis I S S S S S S S S S SY sY s s s s s s s s sy Sy s s s Y Y Y Y Y S Y Y Y Y YY y y y y y y y y y y S y

61. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.4 How Are Multiple Traits Inherited?  In an unprepared world, genius may go unrecognized –Mendel’s work was published in 1865 but went unnoticed –Three biologists—Carl Correns, Hugo de Vries, and Erich Tschermak—independently (of Mendel and each other) rediscovered Mendel’s principles of inheritance in 1900 –Mendel was credited in new papers as laying the groundwork of genetics 30 years previously

62. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.5 How Are Genes Located on the Same Chromosome Inherited?  Genes on the same chromosome tend to be inherited together –Mendel’s law of independent assortment works only for genes whose loci are on different pairs of homologous chromosomes –Alleles that are on the same chromosome do not line up independently of one another on the metaphase plate and are not separated at anaphase I

63. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.5 How Are Genes Located on the Same Chromosome Inherited?  Genes on the same chromosome tend to be inherited together (continued) –Different gene loci located on the same chromosome tend to be inherited together –Characteristics whose genes tend to assort together are said to be linked

64. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.5 How Are Genes Located on the Same Chromosome Inherited?  Genes on the same chromosome tend to be inherited together (continued) –An example of genetic linkage is flower color and pollen in sweet peas –The genes for flower color and pollen shape are linked; that is, their loci are on the same chromosome –Purple flower color is dominant to red; long pollen shape is dominant to round –Let P = purple flowers and p = red flowers –Let L = long pollen shape and l = round shape

65. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.5 How Are Genes Located on the Same Chromosome Inherited?  Genes on the same chromosome tend to be inherited together (continued) –The pattern of inheritance for linked genes is different from genes that assort independently –What are the expected gametes from parent PpLl, where P is linked with L and p is linked with l ? –Independent assortment would yield gametes in a genetic proportion of 1/4 PL, 1/4 Pl, 1/4 pL, 1/4 pl –Instead, the gametes are mostly PL and pl

66. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Fig. 10-13 Linked Genes on Homologous Chromosomes flower-color genepollen-shape gene purple allele, P long allele, L red allele, p round allele, l

67. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.5 How Are Genes Located on the Same Chromosome Inherited?  Crossing over creates new combinations of linked alleles –Genes on the same chromosome do not always sort together –Crossing over, or genetic recombination, in prophase I of meiosis creates new gene combinations –Crossing over involves the exchange of DNA between chromatids of paired homologous chromosomes in synapsis (the tight association of homologous chromosomes on the metaphase plate)

68. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.5 How Are Genes Located on the Same Chromosome Inherited?  Crossing over creates new combinations of linked alleles (continued) –The farther apart two linked gene loci are on a chromosome, the more likely crossing over is to occur between them –Crossing over occurs so often between loci far apart on a chromosome that they appear to assort independently –They appear to assort randomly because roughly as many gametes are produced with the genes exchanged by crossing over as are produced in which the original, linked combination of alleles occurs

69. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Fig. 10-14a, b Crossing Over Recombines Alleles on Homologous Chromosomes flower-color gene purple allele, Plong allele, L red allele, pround allele, l pollen-shape gene sister chromatids homologous chromosomes (duplicated) at meiosis I sister chromatids (a) Replicated chromosomes in prophase of meiosis I(b) Crossing over during prophase I P P p p L L l l

70. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Fig. 10-14c, d Crossing Over Recombines Alleles on Homologous Chromosomes recombined chromatids PL p L P l p l unchanged chromatids (c) Homologous chromosomes separate at anaphase I PL P L p l p l recombined chromosomes unchanged chromosomes (d) Unchanged and recombined chromosomes after meiosis II

71. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.6 How Is Sex Determined?  Mammals have a set of sex chromosomes that dictate gender –Females have two X chromosomes –Males have an X chromosome and a Y chromosome –The Y chromosome is much smaller than the X chromosome –A small section of the X and Y chromosomes is homologous, allowing them to pair in prophase I and segregate during meiosis –The rest of the (non-sex) chromosomes occur in identical pairs and are called autosomes

72. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e X chromosome Y chromosome Fig. 10-15 Photomicrograph of Human Sex Chromosomes

73. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.6 How Is Sex Determined?  Mammals have a set of sex chromosomes that dictate gender (continued) –For organisms in which males are XY and females are XX, the sex chromosome carried by the sperm determines the sex of the offspring –During sperm formation, each sperm receives either the X or the Y chromosome, along with a copy of each of the autosomes –Because the female has only X sex chromosomes, the unfertilized egg must have an X chromosome –If the egg is fertilized by a sperm with a Y chromosome, a male results; if fertilized by an X- bearing sperm, a female is produced

74. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e eggs female parent female offspring male offspring male parent X1X1 X1X1 X2X2 X2X2 XmXm XmXm Y Y XmXm XmXm X1X1 X2X2 Y Y X1X1 X2X2 sperm Sex Determination in Mammals Fig. 10-16

75. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.7 How Are Sex-Linked Genes Inherited?  Sex-linked genes are found only on the X or only on the Y chromosome –Genes carried on one sex chromosome, but not on the other, are sex-linked –In humans, the X chromosome is much larger than the Y and carries over 1,000 genes –In contrast, the human Y chromosome is smaller and carries only 78 genes

76. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.7 How Are Sex-Linked Genes Inherited?  Sex-linked genes are found only on the X or only on the Y chromosomes (continued) –During embryonic life, the action of the Y-linked gene SRY sets in motion the entire male developmental pathway –Under normal conditions, SRY causes the male gender to be linked 100 percent to the Y chromosome

77. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.7 How Are Sex-Linked Genes Inherited?  Sex-linked genes are found only on the X or only on the Y chromosomes (continued) –Few of the genes on the X chromosome have a specific role in female reproduction –Most of the genes on the X chromosome have no counterpart on the Y chromosome –Some genes found only on the X chromosome are important to both sexes, such as genes for color vision, blood clotting, and certain structural proteins in muscles

78. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.7 How Are Sex-Linked Genes Inherited?  Sex-linked genes are found only on the X or only on the Y chromosomes (continued) –The X and the Y have very few genes in common –Females (XX) can be homozygous or heterozygous for a characteristic –Males (XY) have only one copy of the genes on the X or the Y

79. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.7 How Are Sex-Linked Genes Inherited?  Sex-linked genes are found only on the X or only on the Y chromosomes (continued) –Because females have two X chromosomes, recessive sex-linked genes on an X chromosome may or may not be expressed –Because males, with only one X chromosome, have no second copy to mask recessive genes, they fully express all the X-linked alleles they have, whether those alleles are dominant or recessive

80. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.7 How Are Sex-Linked Genes Inherited?  Sex-linked genes are found only on the X or only on the Y chromosomes (continued) –Red-green color blindness in humans is a sex- linked trait –Color blindness is caused by recessive alleles of either of two genes located on the X chromosome –The normal, dominant alleles of these genes (called C) encode proteins that allow one set of eye cones to be most sensitive to red light and another to be most sensitive to green light

81. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.7 How Are Sex-Linked Genes Inherited?  Sex-linked genes are found only on the X or only on the Y chromosomes (continued) –There are several defective recessive alleles of these genes, called c –The afflicted person cannot distinguish between red and green –A man can have the genotype CY or cY, which means that he has a color-vision allele C or c on his X chromosome and no corresponding gene on his Y chromosome

82. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.7 How Are Sex-Linked Genes Inherited?  Sex-linked genes are found only on the X or only on the Y chromosomes (continued) –He will have normal color vision if his X chromosome bears the C allele, or be color-blind if his X chromosome bears the c allele –A woman may be CC, Cc, or cc because she has two X chromosomes that each can carry an allele for the trait and will only be color-blind if her genotype is cc

83. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.7 How Are Sex-Linked Genes Inherited?  Sex-linked genes are found only on the X or only on the Y chromosomes (continued) –A color-blind man (cY) will pass his defective allele only to his daughters because only his daughters inherit his X chromosome –A heterozygous woman (Cc), although she has normal color vision, will pass her defective allele to half her sons, who will be color-blind

84. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e eggs female parent female offspring male offspring male parent XCXC XCXC XcXc XcXc XCXC XCXC Y Y XCXC XCXC XCXC XcXc Y Y XCXC XcXc (a) Normal color vision(b) Red-green color blindness (c) Expected children of a man with normal color vision ( C Y), and a heterozygous woman ( Cc ) The individual cannot distinguish red from green sperm Fig. 10-17 Sex-Linked Inheritance of Color Blindness

85. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.8 Do the Mendelian Rules of Inheritance Apply to All Traits?  Many traits do not follow simple Mendelian rules of inheritance –Not all traits are completely controlled by a single gene –A trait may not be completely dominant to another

86. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.8 Do the Mendelian Rules of Inheritance Apply to All Traits?  Incomplete dominance –In the genes studied by Mendel, one allele was dominant over the other, which was recessive –Some alleles, however, are incompletely dominant over others –When the heterozygous phenotype is intermediate between the two homozygous phenotypes, the pattern of inheritance is called incomplete dominance

87. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.8 Do the Mendelian Rules of Inheritance Apply to All Traits?  Incomplete dominance (continued) –Human hair texture is influenced by a gene with two incompletely dominant alleles, C 1 and C 2 –A person with two copies of the C 1 allele has curly hair –Someone with two copies of the C 2 allele has straight hair –Heterozygotes (with the C 1 C 2 genotype) have wavy hair

88. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.8 Do the Mendelian Rules of Inheritance Apply to All Traits?  Incomplete dominance (continued) –If two wavy-haired people marry, their children could have any of the three hair types: curly (C 1 C 1 ), wavy (C 1 C 2 ), or straight (C 2 C 2 )

89. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Incomplete Dominance C1C1 C2C2 eggs C1C1 father C1C2C1C2 C1C2C1C2 C2C2C2C2 C1C1C1C1 C1C2C1C2 C1C2C1C2 mother C2C2 sperm Fig. 10-18

90. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.8 Do the Mendelian Rules of Inheritance Apply to All Traits?  A single gene may have multiple alleles –An individual may have at most two different gene alleles –A species may have multiple alleles for a given characteristic –However, each individual still carries two alleles for this characteristic

91. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.8 Do the Mendelian Rules of Inheritance Apply to All Traits?  A single gene may have multiple alleles (continued) –The human blood types are an example of multiple alleles of a single gene – Human blood group genes produce blood types A, B, AB, and O –There are three alleles in this system: A, B, and o

92. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.8 Do the Mendelian Rules of Inheritance Apply to All Traits?  A single gene may have multiple alleles (continued) –Alleles A and B code for enzymes that add different sugar molecules to the ends of glycoproteins that protrude from red blood cells –Allele o codes for a nonfunctional enzyme that doesn’t add any sugar molecules –Blood types A, B, AB, and O arise as a result of the actions of these alleles

93. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.8 Do the Mendelian Rules of Inheritance Apply to All Traits?  A single gene may have multiple alleles (continued) –Alleles A and B are dominant to allele o –People with AA or Ao genotypes have blood type A; people with BB or Bo genotypes have blood type B; people with oo genotypes have blood type O –AB individuals have both the A and the B allele, so they produce both types of enzymes –Consequently, the plasma membranes of their red blood cells have both A and B glycoproteins –When heterozygotes express the phenotypes of both of the homozygotes (in this case, both A and B glycoproteins), the pattern of inheritance is called codominance

94. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.8 Do the Mendelian Rules of Inheritance Apply to All Traits?  A single gene may have multiple alleles (continued) –People make antibodies to the type of glycoproteins they lack –People with type A blood make B antibodies; people with type B blood make A antibodies –People with type O blood make both type A and type B antibodies; type AB blood groups make no antibodies

95. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.8 Do the Mendelian Rules of Inheritance Apply to All Traits?  A single gene may have multiple alleles (continued) –The antibodies cause red blood cells that bear foreign glycoproteins to clump together and rupture –The presence of such antibodies dictates that blood type must be determined and matched carefully before a blood transfusion is made

96. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.8 Do the Mendelian Rules of Inheritance Apply to All Traits?  A single gene may have multiple alleles (continued) –Type O blood, lacking any sugars, is not attacked by antibodies in A, B, or AB blood, so it can be transfused safely to all –Type O blood is called the universal donor –The A and B antibodies in type O blood become too dilute to cause problems in the recipient of transfused type O blood –Because people with type O blood produce both A and B antibodies, they can receive blood only from other type O donors –Their antibodies would attack any donated blood cells bearing A or B glycoproteins

97. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Table 10-1

98. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.8 Do the Mendelian Rules of Inheritance Apply to All Traits?  Polygenic inheritance –Some characteristics show a range of continuous phenotypes instead of discrete, defined phenotypes –Examples of this include human height, skin color, and body build, and in wheat, grain color

99. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.8 Do the Mendelian Rules of Inheritance Apply to All Traits?  Polygenic inheritance (continued) –Phenotypes produced by polygenic inheritance are governed by the interaction of more than two genes at multiple loci –Human skin color is controlled by at least three genes, each with pairs of incompletely dominant alleles

100. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Polygenic Inheritance of Skin Color in Humans Fig. 10-19

101. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.8 Do the Mendelian Rules of Inheritance Apply to All Traits?  Single genes typically have multiple effects on phenotype –Some alleles of a characteristic may have multiple phenotypic effects (pleiotropy) influencing a number of gene loci –The SRY gene on the Y chromosome in male humans encodes a protein that activates other genes –The SRY gene stimulates development of gonads into testes, which in turn stimulate development of the prostate, seminal vesicles, penis, and scrotum

102. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.8 Do the Mendelian Rules of Inheritance Apply to All Traits?  The environment influences the expression of genes –The environment in which an organism lives profoundly affects its phenotype –Newborn Siamese cats demonstrate the effect of environment on phenotype –A Siamese cat has the genotype for dark fur all over its body –However, the enzyme that produces the dark pigment is inactive at temperatures above 93°F (34°C)

103. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.8 Do the Mendelian Rules of Inheritance Apply to All Traits?  Newborn Siamese cats demonstrate the effect of environment on phenotype (continued) –When kittens are in the all-encompassing warmth of their mother’s uterus, the enzyme is inactive and they are born with pale fur everywhere on their bodies –After birth, the ears, nose, paws, and tail become cooler than the rest of the body, and dark pigment is produced there in the pattern characteristic of the breed

104. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Environmental Influence on Phenotype Fig. 10-20

105. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.9 How Are Human Genetic Disorders Investigated?  Many human diseases are influenced by genetics –Human geneticists trying to understand the relationship between genetics and disease search medical, historical, and family records to study past crosses –Geneticists studying humans are proscribed from using breeding techniques employed with plants and other animals –Records of gene expression over several generations of a family can be diagrammed –Records extending across several generations can be arranged in the form of family pedigrees, diagrams that show the genetic relationships between a set of related individuals

106. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.9 How Are Human Genetic Disorders Investigated?  Many human diseases are influenced by genetics (continued) –Pedigree analysis is often combined with molecular genetics technology to elucidate gene action and expression –As a result, scientists now know the genes responsible for sickle-cell anemia, hemophilia, muscular dystrophy, Marfan syndrome, and cystic fibrosis

107. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Family Pedigrees Fig. 10-21 (a) A pedigree for a dominant trait (b) A pedigree for a recessive trait How to read pedigrees  generations  male  female  parents  offspring or  shows trait or  does not show trait or  known carrier (heterozygote) for recessive trait or  cannot determine the genotype from this pedigree ???? ??? ??

108. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.10 How Are Human Disorders Caused by Single Genes Inherited?  Some human genetic disorders are caused by recessive alleles –New alleles produced by mutation usually code for nonfunctional proteins –Alleles coding for nonfunctional proteins are recessive to those coding for functional ones –The presence of one normal allele may generate enough functional protein to enable heterozygotes to be phenotypically indistinguishable from homozygotes with two normal alleles

109. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.10 How Are Human Disorders Caused by Single Genes Inherited?  Some human genetic disorders are caused by recessive alleles (continued) –Heterozygous individuals are carriers of a recessive genetic trait (but otherwise have a normal phenotype) –Recessive genes are more likely to occur in a homozygous combination (expressing the defective phenotype) when related individuals have children –Close relatives are more likely than the general population to each be heterozygous for a particular recessive allele and, so, are more likely to produce the homozygous recessive phenotype

110. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.10 How Are Human Disorders Caused by Single Genes Inherited?  Albinism results from a defect in melanin production –Melanin is the dark pigment that colors skin cells –Melanin is produced by the enzyme tyrosinase –An allele known as TYR (for tyrosinase) encodes a defective tyrosinase protein in skin cells, producing no melanin and a condition called albinism

111. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.10 How Are Human Disorders Caused by Single Genes Inherited?  Albinism results from a defect in melanin production (continued) –Humans and other mammals who are homozygous for TYR have no color in their skin, fur, or eyes (the skin and hair appear white, and the eyes are pink)

112. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Albinism Fig. 10-22

113. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.10 How Are Human Disorders Caused by Single Genes Inherited?  Sickle-cell anemia is caused by a defective allele for hemoglobin synthesis –Hemoglobin is an oxygen-transporting protein found in red blood cells –A mutant hemoglobin gene causes hemoglobin molecules in blood cells to clump together –Red blood cells take on a sickle (crescent) shape and easily break –Blood clots can form, leading to oxygen starvation of downstream tissues and paralysis –The condition is known as sickle-cell anemia

114. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.10 How Are Human Disorders Caused by Single Genes Inherited?  Sickle-cell anemia is caused by a defective allele for hemoglobin synthesis (continued) –People homozygous for the sickle-cell allele synthesize only defective hemoglobin and therefore produce mostly sickled cells –Although heterozygotes have about half normal and half abnormal hemoglobin, they usually have few sickled cells and are not seriously affected –Because only people who are homozygous for the sickle- cell allele usually show symptoms, sickle-cell anemia is considered a recessive disorder

115. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.10 How Are Human Disorders Caused by Single Genes Inherited?  Sickle-cell anemia is caused by a defective allele for hemoglobin synthesis (continued) –About 20 to 40 percent of sub-Saharan Africans are heterozygous for sickle-cell anemia, but the allele is very rare in Caucasians –The large proportion of heterozygotes in Africa exists because heterozygotes have some resistance to the parasite that causes malaria –The rarity of heterozygotes in Caucasians corresponds with the rarity of malaria in northern climes, where immunity (and therefore, heterozygosity) has no selective advantage

116. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Sickle-Cell Anemia Fig. 10-23

117. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.10 How Are Human Disorders Caused by Single Genes Inherited?  Some human genetic disorders are caused by dominant alleles –A dominant disease can be transmitted to offspring if at least one parent suffers from the disease and lives long enough to reproduce –Dominant disease alleles also arise as new mutations in the DNA of eggs or sperm of unaffected parents

118. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.10 How Are Human Disorders Caused by Single Genes Inherited?  Some human genetic disorders are caused by dominant alleles (continued) –Various mechanisms create an allele’s dominance over the normal allele –Some dominant alleles encode an abnormal protein that interferes with the function of the normal protein –Some dominant alleles encode proteins that carry out toxic reactions –An allele may be dominant if it encodes a protein that is overactive or is active at inappropriate times and places

119. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.10 How Are Human Disorders Caused by Single Genes Inherited?  Some human genetic disorders are caused by dominant alleles (continued) –Huntington disease is a dominant disorder that causes a slow, progressive deterioration of parts of the brain –The disease results in a loss of coordination, flailing movements, personality disturbances, and eventual death –The disease becomes manifest in adulthood, ensuring its maintenance in the population –The gene encodes for a protein, called huntingtin, of unknown function –Mutant huntington seems both to interfere with the action of normal huntington and to form large aggregates in nerve cells that ultimately kill the cells

120. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.10 How Are Human Disorders Caused by Single Genes Inherited?  Some human genetic disorders are sex-linked –The X chromosome contains many genes that have no counterpart on the Y chromosome –Because males have only one X chromosome, they have no other allele to exert dominance over a sex-linked (X-linked) allele causing disease –Consequently, sex-linked diseases tend to occur in males

121. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.10 How Are Human Disorders Caused by Single Genes Inherited?  Some human genetic disorders are sex-linked (continued) –Sex-linked disorders caused by a recessive allele have a unique pattern of inheritance –A son receives his X chromosome from his mother and passes it on only to his daughters, since the gene doesn’t exist on his Y chromosome –Sex-linked genes typically skip generations because the affected male passes the trait to a phenotypically normal carrier daughter, who in turn bears affected sons –Several defective alleles for characteristics encoded on the X chromosome are known, including red-green color blindness and hemophilia

122. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.10 How Are Human Disorders Caused by Single Genes Inherited?  Some human genetic disorders are sex-linked (continued) –Hemophilia is caused by a recessive allele on the X chromosome that results in a deficiency in one of the proteins needed for blood clotting –Hemophiliacs often have anemia owing to blood loss and bruise easily –The hemophilia gene in Queen Victoria of England was passed among the royal families of Europe

123. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Hemophilia Among the Royal Families of Europe Fig. 10-24 unaffected male unaffected female Edward Duke of Kent Victoria Princess of Saxe-Coburg Albert Prince of Saxe- Coburg-Gotha Victoria Queen of England Louis IV Grand Duke of Hesse-Darmstadt Alice Princess of Hesse Victoria Mary Elizabeth Alexandra Tsarina FrederickErnest Mary Victoria Irene OlgaTatianaMariaAnastasia Alexis Tsarevitch Edward VII King of England Alexandra of Denmark Leopold Duke of Albany Helen Princess of Waldeck-Pyrmont Henry Prince of Battenburg Beatrice present British royal family (unaffected) Alexander Albert Alfonso XII Victoria Queen of Spain LeopoldMaurice Alfonso Crown Prince JuanBeatrice died in infancy MarieJaimeGonzalo carrier daughter and hemophiliac grandson several unaffected chidren carrier female hemophiliac male Nicholas II of Russia ??????

124. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.11 How Do Errors in Chromosome Number Affect Humans?  The incorrect separation of chromosomes or chromatids in meiosis is known as nondisjunction –Nondisjunction causes gametes to have too many and too few chromosomes –Most embryos that arise from fusion of gametes with abnormal chromosome numbers spontaneously abort, but some survive to birth and beyond

125. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Normal meiosis Nondisjunction during meiosis I Nondisjunction during meiosis II Parent cell nnnn Meiosis I Meiosis II n  1 n  1 nn n  1n  1 Effects of Nondisjunction during Meiosis Fig. 10-25

126. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.11 How Do Errors in Chromosome Number Affect Humans?  Some genetic disorders are caused by abnormal numbers of sex chromosomes –Nondisjunction of sex chromosomes in males or females produces abnormal numbers of X and Y chromosomes –Nondisjunction of sex chromosomes in males produces sperm with either no sex chromosomes (called “O” sperm), or two sex chromosomes (sperm may be XX, YY, or XY) –Nondisjunction of sex chromosomes in females can produce eggs that are O or XX eggs instead of eggs with one X chromosome

127. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.11 How Do Errors in Chromosome Number Affect Humans?  Some genetic disorders are caused by abnormal numbers of sex chromosomes (continued) –When normal gametes fuse with these defective sperm or eggs, the zygotes have normal numbers of autosomes but abnormal numbers of sex chromosomes –The most common abnormalities are XO, XXX, XXY, and XYY –Some sex chromosome abnormalities allow affected individuals to survive –The genes on the X chromosome are so essential to survival, any embryo without at least one X chromosome spontaneously aborts very early in development

128. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.11 How Do Errors in Chromosome Number Affect Humans?  Some genetic disorders are caused by abnormal numbers of sex chromosomes (continued) –Turner syndrome (XO) occurs in females with only one X chromosome –At puberty, hormone deficiencies prevent XO females from menstruating or developing secondary sexual characteristics –Hormone treatment promotes physical development, but because affected women lack mature eggs, they remain infertile –Additional symptoms include short stature, folds of skin around the neck, and increased risk of cardiovascular disease, kidney defects, and hearing loss –Because they have only one X chromosome, women with Turner’s syndrome are more susceptible to recessive disorders such as red-green color blindness and hemophilia

129. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.11 How Do Errors in Chromosome Number Affect Humans?  Some genetic disorders are caused by abnormal numbers of sex chromosomes (continued) –Trisomy X (XXX) results in a fertile “normal” woman with an extra X chromosome –Most affected women show no abnormal symptoms –There is an increased chance of learning disabilities and a tendency toward tallness associated with trisomy X –By some unknown mechanism that prevents an extra X chromosome from being included in their eggs, women with trisomy X bear normal XX and XY children

130. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.11 How Do Errors in Chromosome Number Affect Humans?  Some genetic disorders are caused by abnormal numbers of sex chromosomes (continued) –Men with Klinefelter syndrome (XXY) have an extra X chromosome –Most afflicted males show no symptoms, although some may show mixed secondary sexual characteristics, including partial breast development, broadening of the hips, and small testes –XXY men are often infertile because of low sperm count but are not impotent

131. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.11 How Do Errors in Chromosome Number Affect Humans?  Males with Jacob syndrome (XYY) have an extra Y chromosome (XYY) –Men with this malady have high levels of testosterone, tend to develop severe acne, and may be exceptionally tall

132. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Table 10-2

133. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.11 How Do Errors in Chromosome Number Affect Humans?  Some genetic disorders are caused by abnormal numbers of autosomes –Nondisjunction of autosomes can occur during meiosis in the father or mother, resulting in eggs or sperm that are missing an autosome or that have two copies of an autosome –Fusion of these gametes with a normal sperm or egg results in a zygote with one or three copies of the affected autosome –Single-copy autosome embryos usually abort very early in development –Embryos with three copies of an autosome (trisomy) also usually spontaneously abort; however, a small fraction of embryos with three copies of chromosomes 13, 18, or 21 survive to birth –The frequency of nondisjunction increases with the age of the parents

134. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 10.11 How Do Errors in Chromosome Number Affect Humans?  Some genetic disorders are caused by abnormal numbers of autosomes (continued) –In trisomy 21 (Down syndrome), afflicted individuals have three copies of chromosome 21 –Down syndrome includes several distinctive physical characteristics, including weak muscle tone, a small mouth held partially open because it cannot accommodate the tongue, and distinctively shaped eyelids –Down syndrome is also characterized by low resistance to infectious diseases, heart malformations, and varying degrees of mental retardation, often severe

135. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Fig. 10-26 Trisomy 21, or Down Syndrome