1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 14 Mendel and the gene idea

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Drawing from the Deck of Genes What genetic principles account for the transmission of traits from parents to offspring?

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings One possible explanation  “blending” hypothesis

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Alternative to the blending model is “particulate” hypothesis of inheritance: the gene idea – Parents pass on discrete heritable units, genes

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gregor Mendel – Particulate mechanism of inheritance, experimented with garden peas Figure 14.1

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Used a quantitative approach (from Christian Doppler)

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendel chose to work with peas because: – Available in many varieties – He could strictly control mating

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Crossing pea plants Figure 14.2 1 5 4 3 2 Removed stamens from purple flower Transferred sperm- bearing pollen from stamens of white flower to egg- bearing carpel of purple flower Parental generation (P) Pollinated carpel matured into pod Carpel (female) Stamens (male) Planted seeds from pod Examined offspring: all purple flowers First generation offspring (F 1 ) APPLICATION By crossing (mating) two true-breeding varieties of an organism, scientists can study patterns of inheritance. In this example, Mendel crossed pea plants that varied in flower color. TECHNIQUE When pollen from a white flower fertilizes eggs of a purple flower, the first-generation hybrids all have purple flowers. The result is the same for the reciprocal cross, the transfer of pollen from purple flowers to white flowers. TECHNIQUERESULTS

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetic vocabulary – Character: a heritable feature, e.g. flower color – Trait: a variant of a character, e.g. purple or white flowers

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendel chose to track – Characters that varied in an “either-or” manner – Varieties that were “true-breeding”

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendel mated two contrasting, true-breeding varieties (hybridization) The true-breeding parents called P generation

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Hybrid offspring – F 1 generation When F 1 individuals self-pollinate – F 2 generation produced

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Law of Segregation Crossed contrasting, true-breeding white and purple flowered pea plants – All offspring were purple When F 1 plants crossed – Many offspring plants were purple, but some were white

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendel discovered – A ratio of about three to one, purple to white flowers, in the F 2 generation Figure 14.3 P Generation (true-breeding parents) Purple flowers White flowers  F 1 Generation (hybrids) All plants had purple flowers F 2 Generation EXPERIMENT True-breeding purple-flowered pea plants and white-flowered pea plants were crossed (symbolized by  ). The resulting F 1 hybrids were allowed to self-pollinate or were cross- pollinated with other F 1 hybrids. Flower color was then observed in the F 2 generation. RESULTS Both purple-flowered plants and white- flowered plants appeared in the F 2 generation. In Mendel’s experiment, 705 plants had purple flowers, and 224 had white flowers, a ratio of about 3 purple : 1 white.

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendel reasoned that – In the F 1, only the purple flower factor was affecting flower color in these hybrids – Purple flower color was dominant, and white flower color was recessive

17. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendel observed same pattern in other pea plant characters Table 14.1

18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendel’s hypothesis: – Explains the 3:1 inheritance pattern in F 2 4 concepts make up model 

19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings First, alternative versions of genes – Variations in inherited characters, called alleles Figure 14.4 Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers

20. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Second, for each character – Organism inherits two alleles, one f/ each parent – A genetic locus is actually represented twice

21. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Third, if the two alleles at a locus differ – One, the dominant allele*, determines the organism’s appearance – The other, the recessive allele*, has no noticeable effect on appearance * The expression of the allele is dominant or recessive not the allele itself

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fourth, the law of segregation – The two alleles for a heritable character separate (segregate) during gamete formation  different gametes

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendel’s law of segregation, probability and the Punnett square Figure 14.5 P Generation F 1 Generation F 2 Generation P p P p P p P p PpPp PP pp Pp Appearance: Genetic makeup : Purple flowers PP White flowers pp Purple flowers Pp Appearance: Genetic makeup: Gametes: F 1 sperm F 1 eggs 1/21/2 1/21/2  Each true-breeding plant of the parental generation has identical alleles, PP or pp. Gametes (circles) each contain only one allele for the flower-color gene. In this case, every gamete produced by one parent has the same allele. Union of the parental gametes produces F 1 hybrids having a Pp combination. Because the purple- flower allele is dominant, all these hybrids have purple flowers. When the hybrid plants produce gametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele. 3 : 1 Random combination of the gametes results in the 3:1 ratio that Mendel observed in the F 2 generation. This box, a Punnett square, shows all possible combinations of alleles in offspring that result from an F 1  F 1 (Pp  Pp) cross. Each square represents an equally probable product of fertilization. For example, the bottom left box shows the genetic combination resulting from a p egg fertilized by a P sperm.

24. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings If homozygous f/ a particular gene – Pair of identical alleles – True-breeding If heterozygous f/ a particular gene – Pair of alleles that are different

25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phenotype – Physical appearance Genotype – Genetic makeup

26. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phenotype v. genotype Figure 14.6 3 1 1 2 1 Phenotype Purple White Genotype PP (homozygous) Pp (heterozygous) Pp (heterozygous) pp (homozygous) Ratio 3:1 Ratio 1:2:1

27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Testcross In pea plants with purple flowers, genotype is not obvious

28. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Testcross Figure 14.7  Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp If PP, then all offspring purple: If Pp, then 1 ⁄ 2 offspring purple and 1 ⁄ 2 offspring white: p p P P Pp pp Pp P p pp APPLICATION An organism that exhibits a dominant trait, such as purple flowers in pea plants, can be either homozygous for the dominant allele or heterozygous. To determine the organism’s genotype, geneticists can perform a testcross. TECHNIQUE In a testcross, the individual with the unknown genotype is crossed with a homozygous individual expressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent. RESULTS

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Law of segregation Determined by following a single trait F 1 were monohybrids, heterozygous for one character

30. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Law of Independent Assortment Follow 2 characters at the same time Crossing two, true-breeding parents differing in two characters  dihybrids in the F 1 generation, heterozygous for both characters

31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings How are two characters transmitted f/ parents to offspring? – As a package? – Independently?

32. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings YYRR P Generation GametesYRyr  yyrr YyRr Hypothesis of dependent assortment Hypothesis of independent assortment F 2 Generation (predicted offspring) 1⁄21⁄2 YR yr 1 ⁄ 2 1⁄21⁄2 yr YYRRYyRr yyrr YyRr 3 ⁄ 4 1 ⁄ 4 Sperm Eggs Phenotypic ratio 3:1 YR 1 ⁄ 4 Yr 1 ⁄ 4 yR 1 ⁄ 4 yr 1 ⁄ 4 9 ⁄ 16 3 ⁄ 16 1 ⁄ 16 YYRR YYRr YyRR YyRr YyrrYyRr YYrr YyRR YyRr yyRRyyRr yyrr yyRrYyrrYyRr Phenotypic ratio 9:3:3:1 315108 101 32 Phenotypic ratio approximately 9:3:3:1 F 1 Generation Eggs YR Yr yRyr 1 ⁄ 4 Sperm Dihybrid cross 4 phenotypes in the F 2 generation

33. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Law of independent assortment – Each pair of alleles segregates independently during gamete formation

34. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The multiplication rule – Probability that two or more independent events will occur together is the product of their individual probabilities

35. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Probability in a monohybrid cross  Rr Segregation of alleles into eggs Rr Segregation of alleles into sperm R r r R R R R 1⁄21⁄2 1⁄21⁄2 1⁄21⁄2 1⁄41⁄4 1⁄41⁄4 1⁄41⁄4 1⁄41⁄4 1⁄21⁄2 r r R r r Sperm  Eggs Figure 14.9

36. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Rule of addition – Probability that any one of two or more exclusive events will occur is sum of individual probabilities – e.g. probability of a heterozygote in a Tt xTt cross ¼ prob. of Tt and ¼ prob. Of tT  ¼ + ¼ = ½ probability of heterozygote

37. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Dihybrid – Equivalent to two independent monohybrid crosses occurring simultaneously When calculating: – Multiply individual probabilities

38. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Inheritance patterns are often more complex than predicted by simple Mendelian genetics Relationship between genotype and phenotype is rarely simple

39. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Spectrum of Dominance Complete dominance – Phenotypes of the heterozygote and dominant homozygote are identical

40. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Codominance – 2 dominant alleles affect the phenotype in separate, distinguishable ways – e.g. human blood group MN

41. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Incomplete dominance – Phenotype of F 1 hybrids between phenotypes of the two parental varieties Figure 14.10 P Generation F 1 Generation F 2 Generation Red C R Gametes CRCR CWCW  White C W Pink C R C W Sperm CRCR CRCR CRCR CwCw CRCR CRCR Gametes 1⁄21⁄2 1⁄21⁄2 1⁄21⁄2 1⁄21⁄2 1⁄21⁄2 Eggs 1⁄21⁄2 C R C R C W C W C R C W

42. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Relation Between Dominance and Phenotype Dominant and recessive alleles – Do not really “interact” – Lead to synthesis of different proteins that produce a phenotype

43. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Dominant alleles – Not necessarily more common in populations than recessive alleles

44. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Multiple alleles e.g. ABO blood group in humans More than 2 alleles Table 14.2

45. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pleiotropy – Gene has multiple phenotypic effects – e.g. sickle-cell disease

46. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings e.g. Epistasis – Gene at one locus alters the phenotypic expression of a gene at second locus

47. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Example of epistasis Figure 14.11 BC bCBc bc 1⁄41⁄4 1⁄41⁄4 1⁄41⁄4 1⁄41⁄4 BC bC Bc bc 1⁄41⁄4 1⁄41⁄4 1⁄41⁄4 1⁄41⁄4 BBCcBbCc BBcc Bbcc bbcc bbCc BbCc BbCC bbCC BbCc bbCc BBCCBbCC BBCc BbCc 9 ⁄ 16 3 ⁄ 16 4 ⁄ 16 BbCc  Sperm Eggs

48. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  AaBbCc aabbcc Aabbcc AaBbccAaBbCcAABbCc AABBCcAABBCC 20 ⁄ 64 15 ⁄ 64 6 ⁄ 64 1 ⁄ 64 Fraction of progeny Polygenic inheritance Quantitative variation  polygenic inheritance – Additive effect of two or more genes, continuum Figure 14.12

49. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Nature and Nurture: Environmental Impact on Phenotype – Phenotype depends on environment & genotype

50. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Norm of reaction – Phenotypic range of a particular genotype that is influenced by the environment Figure 14.13

51. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Multifactorial characters – Influenced by both genetics and environment

52. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phenotype – Physical appearance, internal anatomy, physiology, and behavior – Genotype + unique environmental history

53. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pedigree Inheritance patterns of particular traits Figure 14.14 A, B Ww ww Ww wwWw ww Ww WW or Ww ww First generation (grandparents) Second generation (parents plus aunts and uncles) Third generation (two sisters) Ff ffFf ff Ff ff Ff FF or Ff ff FF or Ff Widow’s peak No Widow’s peak Attached earlobe Free earlobe (a) Dominant trait (widow’s peak) (b) Recessive trait (attached earlobe)

54. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recessively inherited disorders – Show up only in individuals homozygous for the allele Carriers – Heterozygous individuals who carry the recessive allele but are phenotypically normal

55. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sickle-cell disease – Caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells

56. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Matings between relatives – Increase the probability of the appearance of a genetic disease – Called consanguineous matings

57. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Dominantly inherited disorders e.g. achondroplasia Figure 14.15

58. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Multifactorial Disorders Many human diseases – Genetic and environment components – e.g. Heart disease

59. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetic Testing and Counseling

60. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fetal testing Figure 14.17 A, B (a) Amniocentesis Amniotic fluid withdrawn Fetus PlacentaUterus Cervix Centrifugation A sample of amniotic fluid can be taken starting at the 14th to 16th week of pregnancy. (b) Chorionic villus sampling (CVS) Fluid Fetal cells Biochemical tests can be Performed immediately on the amniotic fluid or later on the cultured cells. Fetal cells must be cultured for several weeks to obtain sufficient numbers for karyotyping. Several weeks Biochemical tests Several hours Fetal cells Placenta Chorionic viIIi A sample of chorionic villus tissue can be taken as early as the 8th to 10th week of pregnancy. Suction tube Inserted through cervix Fetus Karyotyping and biochemical tests can be performed on the fetal cells immediately, providing results within a day or so. Karyotyping

61. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ultrasound