Chapter 4. Modification of Mendelian Ratios

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1 Chapter 4. Modification of Mendelian Ratios

2 Inheritance Patterns are Often More Complex than Predicted by Simple Mendelian Genetics The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied. Many heritable characters are not determined by only one gene with two alleles. However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance.

3 Outline of Extensions to Mendel s Analysis Single-gene inheritance In which pairs of alleles show deviations from complete dominance and recessiveness In which different forms of the gene are not limited to two alleles Where one gene may determine more than one trait Multifactorial inheritance in which the phenotype arises from the interaction of one or more genes with the environment, chance, and each other

4 Extending Mendelian Genetics for a Single Gene Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: When alleles are not completely dominant or recessive. When a gene has more than two alleles. When a gene produces multiple phenotypes.

5 The Relation Between Dominance and Phenotype A dominant allele does not subdue a recessive allele; alleles don t interact. Alleles are simply variations in a gene s nucleotide sequence. For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype.

6 Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical. In incomplete dominance, the phenotype of F 1 hybrids is somewhere between the phenotypes of the two parental varieties. In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways.

7 Dominance is Not Always Complete Crosses between true-breeding strains can produce hybrids with phenotypes different from both parents. Incomplete dominance F 1 hybrids that differ from both parents express an intermediate phenotype. Neither allele is dominant or recessive to the other. Phenotypic ratios are same as genotypic ratios. Codominance F 1 hybrids express phenotype of both parents equally. Phenotypic ratios are same as genotypic ratios.

8 Incomplete Dominance in Snapdragons Copyright 2010 Pearson Education, Inc.

9 P generation Red RR White rr Gametes R r F 1 generation Pink Rr Gametes 1 2 R 1 2 r 1 2 R Sperm 1 2 r F 2 generation 1 2 R RR rr Eggs 1 2 r Rr rr

10 Codominant Blood Group Alleles

11 The MN Blood Type Genotype Phenotype L M L M L M L N L N L N M MN N

12 Summary of Dominance Relationships

13 A Gene Can Have More Than Two Alleles Genes may have multiple alleles that segregate in populations. Alleles may be unique to every pair of alleles in an Alleles may be unique to every pair of alleles in an individual.

14 Multiple Alleles Most genes exist in populations in more than two allelic forms. For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: I A, I B, and i. The enzyme encoded by the I A allele adds the A carbohydrate, whereas the enzyme encoded by the I B allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither.

15 ABO Types are Determined by Three Alleles

16 Allele I A I B i Carbohydrate A B none (a) The three alleles for the ABO blood groups and their associated carbohydrates Genotype Red blood cell appearance Phenotype (blood group) I A I A or I A i A I B I B or I B i B I A I B AB ii O (b) Blood group genotypes and phenotypes

17 Multiple Alleles for the ABO Blood Groups

18 Bombay Phenotype Child from both O type parents can have A or B type. Bombay phenotype actually arises from homozygosity for a mutant recessive allele (hh) of a second gene that masks the effects of any ABO alleles that might be present.

19 Frequency of Dominant Alleles Dominant alleles are not necessarily more common in populations than recessive alleles. For example, one baby out of 400 in the United States is born with extra fingers or toes. The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage. In this example, the recessive allele is far more prevalent than the population s dominant allele.

20 Copyright 2010 Pearson Education, Inc. Dual Role of Allele

21

22 Genes and Traits

23 Pleiotropy The phenomenon of a single gene determining a number of distinct and seemingly unrelated characteristics Most genes have multiple phenotypic effects, a property called pleiotropy: one gene influencing many characteristics. If proteins encoded by important genes play pivotal roles in diverse biological processes, mutations in those genes may lead to pleiotropic effects.

24 For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease. The gene for sickle cell disease Affects the type of hemoglobin produced Affects the shape of red blood cells Causes anemia Causes organ damage Is related to susceptibility to malaria

25 This sickling creates a cascade of symptoms, demonstrating the pleiotropic effects of this allele. Doctors can use regular blood transfusions to prevent brain damage and new drugs to prevent or treat other problems.

26 Degree of dominance Description Example Complete dominance of one allele Heterozygous phenotype same as that of homozygous dominant PP Pp Incomplete dominance of either allele Heterozygous phenotype intermediate between the two homozygous phenotypes C R C R C R C W C W C W Codominance Heterozygotes: Both phenotypes expressed I A I B Multiple alleles Pleiotropy In the whole population, some genes have more than two alleles One gene is able to affect multiple phenotypic characters ABO blood group alleles I A, I B, i Sickle-cell disease

27 Different Dominance Relations

28 Do Variations on Dominance Relations Negate Mendel s Law of Segregation? Dominance relations affect phenotype and have no bearing on the segregation of alleles. Alleles still segregate randomly. Gene products control expression of phenotypes differently. Mendel s law of segregation still applies. Interpretation of phenotype/genotype relation is more complex.

29 Modified Ratio of the Dihybrid Inheritance Copyright 2010 Pearson Education, Inc.

30 Two Genes can Interact to Determine One Trait A gene interaction in which the effects of an alleles at another gene is known as epistasis. In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus. The allele that is doing the masking is epistatic to the gene that is being masked. Recessive vs. Dominant epistasis

31 Recessive Epistasis For example, in mice and many other mammals, coat color depends on two genes. One gene determines the pigment color (with alleles B for black and b for brown). The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair.

32 BbCc BbCc Eggs Sperm 1 / 4 1 / 4 1 / 4 1 / 4 BC bc Bc bc 1 / 4 1 / 4 BC bc BBCC BbCC BBCc BbCc BbCC bbcc BbCc bbcc 1 / 4 Bc BBCc BbCc BBcc Bbcc 1 / 4 bc BbCc bbcc Bbcc bbcc 9 : 3 : 4

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34 Dominant Epistasis Summer squash exhibiting the fruit-shape phenotypes disc (white), long (orange gooseneck), and sphere (bottom left).

35 An Example: Fruit Color in Summer Squash P: AABB (white) X aabb (green) F 1 : AaBb (white) X AaBb (white) F 2 : 9/16 A-B- : white 3/16 A-bb : white 3/16 aab- : yellow 1/16 aabb : green Finally, the phenotypic ratio is 12:3:1.

36 Epistasis in which the dominant allele of one gene hides the effects of another gene. When two white F 1 dihybrids (AaBb) are crossed the F 2 phenotypic ratio is 12 white: 3 yellow: 1 green. 12 white includes two genotypic classes (9 A-B- and 3 aab-). The dominant allele A results in white fruit color regardless of the genotype at a second locus B.

37 Complementary Gene Action

38 9:7 ratio of purple to white F 2 plants indicates that at least one dominant allele for each gene is necessary for the development of purple color. Enzymes specified by the dominant alleles of the two genes may both be necessary for completion of a biochemical pathway for pigment production. Recessive alleles of both genes specify inactive enzymes.

39 The basis of modified dihybrid F 2 phenotypic ratios, resulting from crosses between doubly heterozygous F 1 individuals.

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41 Do Variations on Allelic Interation Negate Mendel s Law of Independent Assortment? The added complexity of inheritance does not detract from the validity of Mendel s conclusions. The F 2 phenotypic ratio in each example has been expressed in sixteenth.

42 p85: Insights and Solutions Question Copyright 2010 Pearson Education, Inc.

43 Consider the problem of comb-shape inheritance in chickens, where walnut, pea, rose, and single are the observed distinct phenotypes. How is comb shape inherited, what are the genotypes of the P 1 generation of each cross? Use the following data: Cross 1 : single X single à all single Cross 2 : walnut X walnut à all walnut Cross 3 : rose X pea à all walnut Cross 4 : F 1 X F 1 of cross 3 walnut X walnut à 93 walnut 28 rose 32 pea 10 single

44 Question p87: Problem #7 In some plants, a red pigment, cyanidin, is synthesized from a colorless precursor. The addition of a hydroxyl group (-OH) to the cyanidin molecule causes it to become purple. In a cross between two randomly selected purple plants, the following results are obtained: 94 purple: 31 red: 43 colorless 1) How many genes are involved in determining these flower colors? 2) Which genotypic combinations produce which phenotypes?

45 p86: Insights and Solutions Question In radishes, flower color may be red, purple, or white. The edible portion of the radish may be long or oval. When only flower color is studied, no dominance is evident, and red X white crosses yield all purple. If these F 1 purples are interbred, the F 2 generation consists of ¼ red: ½ purple: ¼ white. Regarding radish shape, long is dominant to oval in a normal Mendelian fashion.

46 1) Determine the F 1 and F 2 phenotypes from a cross between a true-breeding red, long radish and one that is white and oval. Be sure to define all gene symbols initially. 2) A red oval plant was crossed with a plant of unknown genotype and phenotype, yielding the following offspring: Determine the genotype and phenotype of the unknown plant. 103 red long: 101 red oval: 98 purple long: 100 purple oval

47 Continuous Traits

48 Polygenic Inheritance Quantitative characters are those that vary in the population along a continuum. Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype (many genes influence one trait). Skin color in humans is an example of polygenic inheritance.

49 AaBbCc AaBbCc A cross between two AaBbCc individuals (intermediate skin shade) would produce offspring covering a wide range of shades. Sperm 1 / 8 1 / 8 1 / 8 1 / 8 1 / 8 1 / 8 1 / 8 1 / 8 1 / 8 1 / 8 1 / 8 1 / 8 1 / 8 1 / 8 1 / 8 1 / 8 Phenotypes: 1 / 64 6 / / / / 64 6 / 64 1 / 64 Number of dark-skin alleles:

50 Individuals with intermediate skin shades would be the most likely offspring, but very light and very dark individuals are possible as well. The range of phenotypes forms a normal distribution.

51 Mendelian Explanation

52 Relationship among genes Description Example Epistasis One gene affects the expression of another BbCc BC BC bc Bc BbCc bc bc Bc bc Polygenic inheritance A single phenotypic character is affected by two or more genes AaBbCc 9 : 3 : 4 AaBbCc

53 Complementation Test To discover whether a particular phenotype arises from mutations in the same or separate genes The occurrence of complementation reveals genetic heterogeneity. Complementation test cannot be used if either of the mutations is dominant to the wildtype.

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55 Examples

56 Some Genes on the Same Chromosome Assort Together More Often Than Not In dihybrid crosses, departures from a 1:1:1:1 ratio of F1 gametes indicate that the two genes are on the same chromosome. In a sex linked cross

57 Inheritance of Sex-Linked Genes The sex chromosomes have genes for many characters unrelated to sex. A gene located on either sex chromosome is called a sex-linked gene. In humans, sex-linked usually refers to a gene on the larger X chromosome.

58 Linkage at a Sex-Linked Gene

59 Copyright 2010 Pearson Education, Inc. - The F 1 and F 2 results of T. H. Morgan's reciprocal crosses involving the X- linked white mutation in Drosophila melanogaster.

60 EXPERIMENT P Generation F 1 Generation All offspring had red eyes RESULTS F 2 Generation

61 CONCLUSION + w w P X X Generation X Y w+ F 1 Generation Eggs w+ w w+ w+ w Sperm F 2 Generation Eggs w+ w w+ w+ w+ w w w+ Sperm

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63 Inheritance of Sex-Linked Genes X N X N X n Y X N X n X N Y X N X n X n Y Sperm X n Y Sperm X N Y Sperm X n Y Eggs X N X N X n X N Y Eggs X N X N X N X N Y Eggs X N X N X n X N Y X N X N X n X N Y X n X n X N X n Y X n X n X n X n Y (a) (b) (c)

64 Sex-linked genes follow specific patterns of inheritance. For a recessive sex-linked trait to be expressed. A female needs two copies of the allele. A male needs only one copy of the allele. Sex-linked recessive disorders are much more common in males than in females. Some disorders caused by recessive alleles on the X chromosome in humans: Color blindness Duchenne muscular dystrophy Hemophilia

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67 p86: Insights and Solutions Question Consider the two very limited unrelated pedigrees shown here. Of the four combinations of X-linked recessive, X-linked dominant, autosomal recessive, and autosomal dominant, which modes of inheritance can be absolutely ruled out in each case? Copyright 2010 Pearson Education, Inc.

68 p86: Insights and Solutions Question In humans, red-green color blindness is inherited as an X-linked recessive trait. A woman with normal vision whose father is color blind marries a male who has normal vision. Predict the color vision of their male and female offspring. Copyright 2010 Pearson Education, Inc.

69 Question p88: Problem #12 In goats, development of the beard is due to a recessive gene. The following cross involving true-breeding goats was made and carried to the F 2 generation: P: beard female x beardless male F 1 : all bearded males and beardless females F 1 x F 1 : 1/8 beardless males 3/8 bearded males 3/8 beardless females 1/8 bearded females 1) Offer an explanation for the inheritance and expression of this traits. 2) Propose one or more crosses to test your hypothesis.

70 Sex-Limited vs. Sex-Influenced Inheritance Only in male with the hh genotype results in cock feathering. When females inherit the BB genotype, the phenotype is less pronounced.

71 Penetrance and Expressivity Sometimes, a genotype is not expressed at all. The trait caused by a genotype is expressed to varying degrees. Penetrance: how mant members of a population with a particular genotype show the expected phenotype (complete vs. incomplete) Expressivity: the degree or intensity with which a particular genotype is expressed in a phenotype

72 Variable Expressivity in eyeless Mutation Copyright 2010 Pearson Education, Inc.

73 Environmental Factors in Phenotypes Phenotypic variations are influenced by the environment. Skin color is affected by exposure to sunlight. Susceptibility to diseases, such as cancer, has hereditary and environmental components.

74 Environmental Effects Permissive vs. Restrictive

75 X Inactivation in Female Mammals In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development. The inactive X condenses into a Barr body. If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character.

76 Early embryo: X chromosomes Allele for orange fur Allele for black fur Two cell populations in adult cat: Active X Cell division and X chromosome inactivation Inactive X Active X Black fur Orange fur

77 Genomic Imprinting For a few mammalian traits, the phenotype depends on which parent passed along the alleles for those traits. Such variation in phenotype is called genomic imprinting. Genomic imprinting involves the silencing of certain genes that are stamped with an imprint during gamete production.

78 Paternal chromosome Normal Igf2 allele is expressed Maternal chromosome Normal Igf2 allele is not expressed Wild-type mouse (normal size) (a) Homozygote

79 Mutant Igf2 allele inherited from mother Mutant Igf2 allele inherited from father Normal size mouse (wild type) Dwarf mouse (mutant) Normal Igf2 allele is expressed Mutant Igf2 allele is expressed Mutant Igf2 allele is not expressed Normal Igf2 allele is not expressed (b) Heterozygotes

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81 It appears that imprinting is the result of the methylation (addition of CH 3 ) of DNA. Genomic imprinting is thought to affect only a small fraction of mammalian genes. Most imprinted genes are critical for embryonic development.

82 Phenocopy A change in phenotype arising from environmental factors that mimic the effects of a mutation in a gene Phenocopies are not heritable because they do not arise from a change in a gene.

83 Extracellular Inheritance Organelle heredity: the phenotype of an individual organism is affected by the expression of genes contained in the DNA of mitochondria or chloroplast. Maternal effect: the phenotype of an individual Maternal effect: the phenotype of an individual organism is determined by genetic information expressed in the gamete of the mother.

84 Copyright 2010 Pearson Education, Inc.

85 Copyright 2010 Pearson Education, Inc.

86 Copyright 2010 Pearson Education, Inc.