Patterns of Inheritance

Patterns of Inheritance
Chapter 9 Patterns of Inheritance

Introduction Dogs are one of man’s longest genetic experiments.
Dogs are one of man’s longest genetic experiments. Over thousands of years, humans have chosen and mated dogs with specific traits. The result has been an incredibly diverse array of dogs with distinct body types and behavioral traits. 2 © 2012 Pearson Education, Inc.

Variations on Mendel’s Laws
Figure 9.0_1 Chapter 9: Big Ideas Mendel’s Laws Variations on Mendel’s Laws Figure 9.0_1 Chapter 9: Big Ideas The Chromosomal Basis of Inheritance Sex Chromosomes and Sex-Linked Genes 3

9.1 The science of genetics has ancient roots
Pangenesis, proposed around 400 BCE by Hippocrates, was an early explanation for inheritance that suggested that particles called pangenes came from all parts of the organism to be incorporated into eggs or sperm and characteristics acquired during the parents’ lifetime could be transferred to the offspring. Aristotle rejected pangenesis and argued that instead of particles, the potential to produce the traits was inherited. 5 © 2012 Pearson Education, Inc.

9.1 The science of genetics has ancient roots
The idea that hereditary materials mix in forming offspring, called the blending hypothesis, was suggested in the 19th century by scientists studying plants but later rejected because it did not explain how traits that disappear in one generation can reappear in later generations. 6 © 2012 Pearson Education, Inc.

9.2 Experimental genetics began in an abbey garden
Heredity is the transmission of traits from one generation to the next. Genetics is the scientific study of heredity. Gregor Mendel began the field of genetics in the 1860s, deduced the principles of genetics by breeding garden peas, and relied upon a background of mathematics, physics, and chemistry. 7 © 2012 Pearson Education, Inc.

In 1866, Mendel correctly argued that parents pass on to their offspring discrete “heritable factors” and stressed that the heritable factors (today called genes), retain their individuality generation after generation. A heritable feature that varies among individuals, such as flower color, is called a character. Each variant for a character, such as purple or white flowers, is a trait. 8 © 2012 Pearson Education, Inc.

True-breeding varieties result when self-fertilization produces offspring all identical to the parent. The offspring of two different varieties are hybrids. The cross-fertilization is a hybridization, or genetic cross. True-breeding parental plants are the P generation. Hybrid offspring are the F1 generation. A cross of F1 plants produces an F2 generation. 9 © 2012 Pearson Education, Inc.

Parents (P) Offspring (F1)
Figure 9.2C_s3 White 1 Removal of stamens Stamens Carpel 2 Transfer of pollen Parents (P) Purple 3 Carpel matures into pea pod Figure 9.2C_s3 Mendel’s technique for cross-fertilization of pea plants (step 3) 4 Seeds from pod planted Offspring (F1) 10

Figure 9.2D Character Traits Dominant Recessive Flower color Purple White Flower position Axial Terminal Seed color Yellow Green Figure 9.2D The seven pea characters studied by Mendel Seed shape Round Wrinkled Pod shape Inflated Constricted Pod color Green Yellow Stem length Tall Dwarf 11

9.3 Mendel’s law of segregation describes the inheritance of a single character
A cross between two individuals differing in a single character is a monohybrid cross. Mendel performed a monohybrid cross between a plant with purple flowers and a plant with white flowers. The F1 generation produced all plants with purple flowers. A cross of F1 plants with each other produced an F2 generation with ¾ purple and ¼ white flowers. 12 © 2012 Pearson Education, Inc.

of plants have purple flowers of plants have white flowers
Figure 9.3A_s3 The Experiment P generation (true-breeding parents)  Purple flowers White flowers F1 generation All plants have purple flowers Figure 9.3A_s3 Crosses tracking one character (flower color) (step 3) Fertilization among F1 plants (F1  F1) F2 generation of plants have purple flowers 3 4 of plants have white flowers 1 4 13

The all-purple F1 generation did not produce light purple flowers, as predicted by the blending hypothesis. Mendel needed to explain why white color seemed to disappear in the F1 generation and white color reappeared in one quarter of the F2 offspring. Mendel observed the same patterns of inheritance for six other pea plant characters. 14 © 2012 Pearson Education, Inc.

Mendel developed four hypotheses, described below using modern terminology. 1. Alleles are alternative versions of genes that account for variations in inherited characters. 2. For each characteristic, an organism inherits two alleles, one from each parent. The alleles can be the same or different. A homozygous genotype has identical alleles. A heterozygous genotype has two different alleles. 15 © 2012 Pearson Education, Inc.

If the alleles of an inherited pair differ, then one determines the organism’s appearance and is called the dominant allele. The other has no noticeable effect on the organism’s appearance and is called the recessive allele. The phenotype is the appearance or expression of a trait. The genotype is the genetic makeup of a trait. The same phenotype may be determined by more than one genotype. 16 © 2012 Pearson Education, Inc.

A sperm or egg carries only one allele for each inherited character because allele pairs separate (segregate) from each other during the production of gametes. This statement is called the law of segregation. Mendel’s hypotheses also explain the 3:1 ratio in the F2 generation. The F1 hybrids all have a Pp genotype. A Punnett square shows the four possible combinations of alleles that could occur when these gametes combine. 17 © 2012 Pearson Education, Inc.

Genetic makeup (alleles)
Figure 9.3B_s3 The Explanation P generation Genetic makeup (alleles) Purple flowers White flowers PP pp Gametes All P All p F1 generation (hybrids) All Pp Figure 9.3B_s3 An explanation of the crosses in Figure 9.3A (step 3) Alleles segregate Gametes 2 1 P 2 1 p Fertilization F2 generation Sperm from F1 plant P p Phenotypic ratio 3 purple : 1 white P PP Pp Eggs from F1 plant Genotypic ratio 1 PP : 2 Pp : 1 pp p Pp pp 18

9.4 Homologous chromosomes bear the alleles for each character
A locus (plural, loci) is the specific location of a gene along a chromosome. For a pair of homologous chromosomes, alleles of a gene reside at the same locus. Homozygous individuals have the same allele on both homologues. Heterozygous individuals have a different allele on each homologue. P a B b PP aa Bb Dominant allele Recessive allele Gene loci Homologous chromosomes Genotype: Heterozygous, with one dominant and one recessive allele Homozygous for the recessive allele Homozygous for the dominant allele 19 © 2012 Pearson Education, Inc.

9.5 The law of independent assortment is revealed by tracking two characters at once
A dihybrid cross is a mating of parental varieties that differ in two characters. Mendel performed the following dihybrid cross with the following results: P generation: round yellow seeds  wrinkled green seeds F1 generation: all plants with round yellow seeds F2 generation: 9/16 had round yellow seeds 3/16 had wrinkled yellow seeds 3/16 had round green seeds 1/16 had wrinkled green seeds 20 © 2012 Pearson Education, Inc.

Figure 9.5A Two hypotheses for segregation in a dihybrid cross
P generation RRYY rryy Gametes RY  ry Sperm F1 generation RrYy 4 1 RY 4 1 rY 4 1 Ry 4 1 ry Sperm 2 1 RY 2 1 ry 4 1 RY Figure 9.5A Two hypotheses for segregation in a dihybrid cross RRYY RrYY RRYy RrYy 2 1 RY 4 1 rY 16 9 Yellow round F2 generation Eggs RrYY rrYY RrYy rrYy Eggs 2 1 ry 4 1 16 3 Green round Ry RRYy RrYy RRyy Rryy 16 3 Yellow wrinkled 4 1 ry 16 1 Green wrinkled RrYy rrYy Rryy rryy The hypothesis of dependent assortment Data did not support; hypothesis refuted The hypothesis of independent assortment Actual results; hypothesis supported 21

Mendel needed to explain why the F2 offspring had new nonparental combinations of traits and a 9:3:3:1 phenotypic ratio. Mendel suggested that the inheritance of one character has no effect on the inheritance of another, suggested that the dihybrid cross is the equivalent to two monohybrid crosses, and called this the law of independent assortment. 22 © 2012 Pearson Education, Inc.

The following figure demonstrates the law of independent assortment as it applies to two characters in Labrador retrievers: black versus chocolate color, normal vision versus progressive retinal atrophy. 23 © 2012 Pearson Education, Inc.

Figure 9.5B Blind Blind Phenotypes Black coat, normal vision B_N_ Black coat, blind (PRA) B_nn Chocolate coat, normal vision bbN_ Chocolate coat, blind (PRA) bbnn Genotypes Figure 9.5B Independent assortment of two genes in the Labrador retriever Mating of double heterozygotes (black coat, normal vision) BbNn  BbNn Blind Blind Phenotypic ratio of the offspring 9 Black coat, normal vision 3 Black coat, blind (PRA) 3 Chocolate coat, normal vision 1 Chocolate coat, blind (PRA) 24

9.6 Geneticists can use the testcross to determine unknown genotypes
A testcross is the mating between an individual of unknown genotype and a homozygous recessive individual. A testcross can show whether the unknown genotype includes a recessive allele. Mendel used testcrosses to verify that he had true-breeding genotypes. The following figure demonstrates how a testcross can be performed to determine the genotype of a Lab with normal eyes. 25 © 2012 Pearson Education, Inc.

What is the genotype of the black dog?
Figure 9.6 What is the genotype of the black dog? Testcross  Genotypes B_? bb Figure 9.6 Using a testcross to determine genotype Two possibilities for the black dog: BB or Bb Gametes B B b b Bb b Bb bb Offspring All black 1 black : 1 chocolate 26

9.7 Mendel’s laws reflect the rules of probability
Using his strong background in mathematics, Mendel knew that the rules of mathematical probability affected the segregation of allele pairs during gamete formation and the re-forming of pairs at fertilization. The probability scale ranges from 0 to 1. An event that is certain has a probability of 1 and certain not to occur has a probability of 0. 27 © 2012 Pearson Education, Inc.

9.7 Mendel’s laws reflect the rules of probability
The probability of a specific event is the number of ways that event can occur out of the total possible outcomes. Determining the probability of two independent events uses the rule of multiplication, in which the probability is the product of the probabilities for each event. The probability that an event can occur in two or more alternative ways is the sum of the separate probabilities, called the rule of addition. 28 © 2012 Pearson Education, Inc.

F1 genotypes Bb female Bb male Formation of eggs Formation of sperm B
Figure 9.7 F1 genotypes Bb female Bb male Formation of eggs Formation of sperm 2 1 2 1 B b Sperm Figure 9.7 Segregation and fertilization as chance events 2 1 2 1 ( )  2 1 B B B b B 4 1 4 1 F2 genotypes Eggs b B b b 2 1 b 4 1 4 1 29

9.8 CONNECTION: Genetic traits in humans can be tracked through family pedigrees
In a simple dominant-recessive inheritance of dominant allele A and recessive allele a, a recessive phenotype always results from a homozygous recessive genotype (aa) but a dominant phenotype can result from either the homozygous dominant genotype (AA) or a heterozygous genotype (Aa). Wild-type traits, those prevailing in nature, are not necessarily specified by dominant alleles. 30 © 2012 Pearson Education, Inc.

Dominant Traits Recessive Traits Freckles No freckles Widow’s peak
Figure 9.8A Dominant Traits Recessive Traits Freckles No freckles Figure 9.8A Examples of single-gene inherited traits in humans Widow’s peak Straight hairline Free earlobe Attached earlobe 31

9.8 CONNECTION: Genetic traits in humans can be tracked through family pedigrees
The inheritance of human traits follows Mendel’s laws. A pedigree shows the inheritance of a trait in a family through multiple generations, demonstrates dominant or recessive inheritance, and can also be used to deduce genotypes of family members. 32 © 2012 Pearson Education, Inc.

First generation (grandparents) Ff Ff ff Ff
Figure 9.8B First generation (grandparents) Ff Ff ff Ff Second generation (parents, aunts, and uncles) FF or Ff ff ff Ff Ff ff Figure 9.8B A pedigree showing the inheritance of attached versus free earlobes in a hypothetical family Third generation (two sisters) ff FF or Ff Female Male Attached Free 33

9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene
Inherited human disorders show either recessive inheritance in which two recessive alleles are needed to show disease, heterozygous parents are carriers of the disease-causing allele, and the probability of inheritance increases with inbreeding, mating between close relatives. dominant inheritance in which one dominant allele is needed to show disease and dominant lethal alleles are usually eliminated from the population. 34 © 2012 Pearson Education, Inc.

Normal Dd Normal Dd Parents  D Sperm d Dd Normal (carrier) DD Normal
Figure 9.9A Normal Dd Normal Dd Parents  D Sperm d Dd Normal (carrier) Figure 9.9A Offspring produced by parents who are both carriers for a recessive disorder, a type of deafness DD Normal D Eggs Offspring Dd Normal (carrier) dd Deaf d 35

9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene
The most common fatal genetic disease in the United States is cystic fibrosis (CF), resulting in excessive thick mucus secretions. The CF allele is recessive and carried by about 1 in 31 Americans. Dominant human disorders include achondroplasia, resulting in dwarfism, and Huntington’s disease, a degenerative disorder of the nervous system. 36 © 2012 Pearson Education, Inc.

Table 9.9 Table 9.9 Some Autosomal Disorders in Humans 37

9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy
New technologies offer ways to obtain genetic information before conception, during pregnancy, and after birth. Genetic testing can identify potential parents who are heterozygous carriers for certain diseases. 38 © 2012 Pearson Education, Inc.

Several technologies can be used for detecting genetic conditions in a fetus. Amniocentesis extracts samples of amniotic fluid containing fetal cells and permits karyotyping and biochemical tests on cultured fetal cells to detect other conditions, such as Tay-Sachs disease. Chorionic villus sampling removes a sample of chorionic villus tissue from the placenta and permits similar karyotyping and biochemical tests. 39 © 2012 Pearson Education, Inc.

Chorionic Villus Sampling (CVS)
Figure 9.10A Amniocentesis Chorionic Villus Sampling (CVS) Amniotic fluid extracted Tissue extracted from the chorionic villi Ultrasound transducer Ultrasound transducer Fetus Fetus Placenta Placenta Uterus Chorionic villi Cervix Cervix Uterus Centrifugation Amniotic fluid Figure 9.10A Testing a fetus for genetic disorders Biochemical and genetics tests Fetal cells Fetal cells Several hours Several hours Cultured cells Several weeks Several weeks Several hours Karyotyping 40

Blood tests on the mother at 14–20 weeks of pregnancy can help identify fetuses at risk for certain birth defects. Fetal imaging enables a physician to examine a fetus directly for anatomical deformities. The most common procedure is ultrasound imaging, using sound waves to produce a picture of the fetus. Newborn screening can detect diseases that can be prevented by special care and precautions. 41 © 2012 Pearson Education, Inc.

New technologies raise ethical considerations that include the confidentiality and potential use of results of genetic testing, time and financial costs, and determining what, if anything, should be done as a result of the testing. 42 © 2012 Pearson Education, Inc.

VARIATIONS ON MENDEL’S LAWS
43 © 2012 Pearson Education, Inc.

9.11 Incomplete dominance results in intermediate phenotypes
Mendel’s pea crosses always looked like one of the parental varieties, called complete dominance. For some characters, the appearance of F1 hybrids falls between the phenotypes of the two parental varieties. This is called incomplete dominance, in which neither allele is dominant over the other and expression of both alleles occurs. 44 © 2012 Pearson Education, Inc.

Red RR White rr Pink hybrid Rr
Figure 9.11A P generation  Red RR White rr Gametes R r F1 generation Pink hybrid Rr Figure 9.11A Incomplete dominance in snapdragon flower color 2 1 2 1 Gametes R r F2 generation Sperm 2 1 2 1 R r 2 1 R RR rR Eggs 2 1 Rr rr r 45

9.11 Incomplete dominance results in intermediate phenotypes
Incomplete dominance does not support the blending hypothesis because the original parental phenotypes reappear in the F2 generation. One example of incomplete dominance in humans is hypercholesterolemia, in which dangerously high levels of cholesterol occur in the blood and heterozygotes have intermediately high cholesterol levels. 46 © 2012 Pearson Education, Inc.

Genotypes Hh Heterozygous
Figure 9.11B Genotypes Hh Heterozygous HH Homozygous for ability to make LDL receptors hh Homozygous for inability to make LDL receptors Phenotypes Figure 9.11B Incomplete dominance in human hypercholesterolemia LDL LDL receptor Cell Normal Mild disease Severe disease 47

9.12 Many genes have more than two alleles in the population
Although an individual can at most carry two different alleles for a particular gene, more than two alleles often exist in the wider population. Human ABO blood group phenotypes involve three alleles for a single gene. The four human blood groups, A, B, AB, and O, result from combinations of these three alleles. The A and B alleles are both expressed in heterozygous individuals, a condition known as codominance. In codominance, neither allele is dominant over the other and expression of both alleles is observed as a distinct phenotype in the heterozygous individual. AB blood type is an example of codominance. 48 © 2012 Pearson Education, Inc.

Figure 9.12 Multiple alleles for the ABO blood groups
Blood Group (Phenotype) Antibodies Present in Blood Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left Carbohydrates Present on Red Blood Cells Genotypes O A B AB IAIA or IAi Carbohydrate A A Anti-B IBIB or IBi Carbohydrate B B Anti-A Figure 9.12 Multiple alleles for the ABO blood groups Carbohydrate A and Carbohydrate B AB IAIB None Anti-A O ii Neither Anti-B No reaction Clumping reaction 49

9.13 A single gene may affect many phenotypic characters
Pleiotropy occurs when one gene influences many characteristics. Sickle-cell disease is a human example of pleiotropy. This disease affects the type of hemoglobin produced and the shape of red blood cells and causes anemia and organ damage. Sickle-cell and nonsickle alleles are codominant. Carriers of sickle-cell disease are resistant to malaria. 50 © 2012 Pearson Education, Inc.

An individual homozygous for the sickle-cell allele
Figure 9.13B An individual homozygous for the sickle-cell allele Produces sickle-cell (abnormal) hemoglobin The abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped Sickled cell Figure 9.13B Sickle-cell disease, an example of pleiotropy The multiple effects of sickled cells Damage to organs Other effects Kidney failure Heart failure Spleen damage Brain damage (impaired mental function, paralysis) Pain and fever Joint problems Physical weakness Anemia Pneumonia and other infections 51

9.14 A single character may be influenced by many genes
Many characteristics result from polygenic inheritance, in which a single phenotypic character results from the additive effects of two or more genes. Human skin color is an example of polygenic inheritance. 52 © 2012 Pearson Education, Inc.

Figure 9.14 A model for polygenic inheritance of skin color
P generation  aabbcc (very light) AABBCC (very dark) F1 generation  AaBbCc AaBbCc Sperm Figure 9.14 A model for polygenic inheritance of skin color 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 F2 generation 8 1 8 1 8 1 8 1 Eggs 8 1 8 1 Fraction of population 8 1 8 1 64 1 64 6 64 15 64 20 64 15 64 6 64 1 53 Skin color

9.15 The environment affects many characters
Many characters result from a combination of heredity and the environment. For example, skin color is affected by exposure to sunlight, susceptibility to diseases, such as cancer, has hereditary and environmental components, and identical twins show some differences. Only genetic influences are inherited. 54 © 2012 Pearson Education, Inc.

THE CHROMOSOMAL BASIS OF INHERITANCE

9.16 Chromosome behavior accounts for Mendel’s laws
The chromosome theory of inheritance states that genes occupy specific loci (positions) on chromosomes and chromosomes undergo segregation and independent assortment during meiosis. 56 © 2012 Pearson Education, Inc.

9.16 Chromosome behavior accounts for Mendel’s laws
Mendel’s laws correlate with chromosome separation in meiosis. The law of segregation depends on separation of homologous chromosomes in anaphase I. The law of independent assortment depends on alternative orientations of chromosomes in metaphase I. 57 © 2012 Pearson Education, Inc.

All yellow round seeds (RrYy) Meta- phase I of meiosis
Figure 9.16_s3 F1 generation All yellow round seeds (RrYy) R y r Y R r r R Meta- phase I of meiosis Y y Y y R r r R Anaphase I Y y Y y Figure 9.16_s3 The chromosomal basis of Mendel’s laws (step 3) Metaphase II R r r R Y y Y y Gametes y Y Y y Y Y y y R R r r r r R R 4 1 4 1 4 1 4 1 RY ry rY Ry Fertilization F2 generation 9 :3 :3 :1 58

Yellow round Green round Yellow wrinkled Green wrinkled
Figure 9.16_4 Sperm 4 1 4 1 4 1 4 1 RY rY Ry ry 4 1 RY RRYY RrYY RRYy RrYy 4 1 rY RrYY rrYY RrYy rrYy Eggs 4 1 Ry Figure 9.16_4 The chromosomal basis of Mendel’s laws (Punnett square) RRYy RrYy RRyy Rryy 4 1 ry RrYy rrYy Rryy rryy 16 9 Yellow round 16 3 Green round 16 3 Yellow wrinkled 16 1 Green wrinkled 59

9.17 SCIENTIFIC DISCOVERY: Genes on the same chromosome tend to be inherited together
Bateson and Punnett studied plants that did not show a 9:3:3:1 ratio in the F2 generation. What they found was an example of linked genes, which are located close together on the same chromosome and tend to be inherited together. 60 © 2012 Pearson Education, Inc.

Observed offspring Prediction (9:3:3:1)
Figure 9.17_1 The Experiment Purple flower PpLl  PpLl Long pollen Figure 9.17_1 The experiment revealing linked genes in the sweet pea (part 1) Phenotypes Observed offspring Prediction (9:3:3:1) Purple long 284 215 Purple round 21 71 Red long 21 71 Red round 55 24 61

The Explanation: Linked Genes
Figure 9.17_2 The Explanation: Linked Genes Parental diploid cell PpLl P L p l Meiosis Most gametes P L p l Figure 9.17_2 The experiment revealing linked genes in the sweet pea (part 2) Fertilization Sperm PL pl P L P L PL Most offspring P L p l Eggs p l p l pl P L p l 3 purple long : 1 red round Not accounted for: purple round and red long 62

9.18 SCIENTIFIC DISCOVERY: Crossing over produces new combinations of alleles
Crossing over between homologous chromosomes produces new combinations of alleles in gametes. Linked alleles can be separated by crossing over, forming recombinant gametes. The percentage of recombinants is the recombination frequency. 63 © 2012 Pearson Education, Inc.

Tetrad (pair of homologous chromosomes)
Figure 9.18A p L p l P L Parental gametes Figure 9.18A Review: the production of recombinant gametes p l p L P l Tetrad (pair of homologous chromosomes) Crossing over Recombinant gametes 64

Gray body, long wings (wild type) Black body, vestigial wings
Figure 9.18C_1 The Experiment Gray body, long wings (wild type) Black body, vestigial wings GgLl ggll Female Male Offspring: Gray long Black vestigial Gray vestigial Black long Figure 9.18C_1 A fruit fly experiment demonstrating the role of crossing over in inheritance (part 1) 965 944 206 185 Parental phenotypes Recombinant phenotypes 391 recombinants 2,300 total offspring Recombination frequency   0.17 or 17% 65

The Explanation G L g l GgLl Female ggll Male g l g l G L g l G l g L
Figure 9.18C_2 The Explanation G L g l GgLl Female ggll Male g l g l Crossing over G L g l G l g L g l Figure 9.18C_2 A fruit fly experiment demonstrating the role of crossing over in inheritance (part 2) Eggs Sperm Offspring G L g l G l g L g l g l g l g l Parental Recombinant 66

9.19 Geneticists use crossover data to map genes
When examining recombinant frequency, Morgan and his students found that the greater the distance between two genes on a chromosome, the more points there are between them where crossing over can occur. Recombination frequencies can thus be used to map the relative position of genes on chromosomes. 67 © 2012 Pearson Education, Inc.

Recombination frequencies
Figure 9.19A Section of chromosome carrying linked genes g c l Figure 9.19A Mapping genes from crossover data 17% 9% 9.5% Recombination frequencies 68

Long aristae (appendages on head) Gray body (G) Red eyes (C)
Figure 9.19B Mutant phenotypes Short aristae Black body (g) Cinnabar eyes (c) Vestigial wings (l) Brown eyes Figure 9.19B A partial linkage map of a fruit fly chromosome Long aristae (appendages on head) Gray body (G) Red eyes (C) Normal wings (L) Red eyes Wild-type phenotypes 69

SEX CHROMOSOMES AND SEX-LINKED GENES

9.20 Chromosomes determine sex in many species
Many animals have a pair of sex chromosomes, designated X and Y, that determine an individual’s sex. In mammals, males have XY sex chromosomes, females have XX sex chromosomes, the Y chromosome has genes for the development of testes, and an absence of the Y allows ovaries to develop. 71 © 2012 Pearson Education, Inc.

Figure 9.20A X Figure 9.20A The human sex chromosomes Y 72

Male Female 44  XY 44  XX Parents (diploid) Gametes (haploid) 22  X
Figure 9.20B Male Female 44  XY 44  XX Parents (diploid) Gametes (haploid) 22  X 22  Y 22  X Figure 9.20B The X-Y system Sperm Egg Offspring (diploid) 44  XX 44  XY Female Male 73

Figure 9.21A Figure 9.21A Fruit fly eye color determined by sex-linked gene 74

Female Male XRXR XrY Sperm Xr Y Eggs XR XRXr XRY R  red-eye allele
Figure 9.21B Female Male XRXR XrY Figure 9.21B A homozygous, red-eyed female crossed with a white-eyed male Sperm Xr Y Eggs XR XRXr XRY R  red-eye allele r  white-eye allele 75

Female Male XRXr XRY Sperm xR Y XR XRXR XRY Eggs Xr XrXR XrY
Figure 9.21C Female Male XRXr XRY Sperm Figure 9.21C A heterozygous female crossed with a red-eyed male xR Y XR XRXR XRY Eggs Xr XrXR XrY R  red-eye allele r  white-eye allele 76

Female Male XRXr XrY Sperm Xr Y XR XRXr XRY Eggs Xr XrXr XrY
Figure 9.21D Female Male XRXr XrY Sperm Figure 9.21D A heterozygous female crossed with a white-eyed male Xr Y XR XRXr XRY Eggs Xr XrXr XrY R  red-eye allele r  white-eye allele 77

9.22 CONNECTION: Human sex-linked disorders affect mostly males
Most sex-linked human disorders are due to recessive alleles and seen mostly in males. A male receiving a single X-linked recessive allele from his mother will have the disorder. A female must receive the allele from both parents to be affected. 78 © 2012 Pearson Education, Inc.

9.22 CONNECTION: Human sex-linked disorders affect mostly males
Recessive and sex-linked human disorders include hemophilia, characterized by excessive bleeding because hemophiliacs lack one or more of the proteins required for blood clotting, red-green color blindness, a malfunction of light-sensitive cells in the eyes, and Duchenne muscular dystrophy, a condition characterized by a progressive weakening of the muscles and loss of coordination. 79 © 2012 Pearson Education, Inc.

Czar Nicholas II of Russia
Figure 9.22 Queen Victoria Albert Alice Louis Figure 9.22 Hemophilia in the royal family of Russia Female Male Alexandra Czar Nicholas II of Russia Hemophilia Carrier Normal Alexis 80

9.23 EVOLUTION CONNECTION: The Y chromosome provides clues about human male evolution
The Y chromosome provides clues about human male evolution because Y chromosomes are passed intact from father to son and mutations in Y chromosomes can reveal data about recent shared ancestry. 81 © 2012 Pearson Education, Inc.

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