Chapter 4 Lecture. Concepts of Genetics. Tenth Edition. Extensions of Mendelian Genetics. Copyright 2009 Pearson Education, Inc.

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1 Chapter 4 Lecture Concepts of Genetics Tenth Edition Extensions of Mendelian Genetics Copyright 2009 Pearson Education, Inc.

2 Factors that cause deviation from normal monohybrid and dihybrid ratios: X-linkage lethal alleles codominance and incomplete dominance multiallelism epistasis genetic linkage environmental conditions epigenetics segregation distortion

3 4.1 Alleles Alter Phenotypes in Different Ways New alleles are produced by mutation. The solution to understanding the laws of heredity were found by finding and studying mutations. wild-type vs. mutant loss-of-function mutation (e.g. albinism; tyrosinase mutation) gain-of-function mutation (e.g. lactose tolerance; lactase persistence) neutral mutation (non-coding, silent mutation) Copyright 2009 Pearson Education, Inc.

4 4.2 Geneticists use a variety of symbols for Alleles The initial letter of the name of a recessive trait, lowercased and italicized, denotes the recessive allele. The same letter in uppercase refers to the dominant allele. d, dwarf; D, tall in the garden pea example. Wild type= + mutant= initials of the name of the mutant trait examples: +/+; e+/e; +/e; e/e. e stands for ebony body color. Copyright 2009 Pearson Education, Inc.

5 4.3 Incomplete (partial) dominance is a condition when neither allele is dominant over the other Copyright 2009 Pearson Education, Inc.

6 occurs when one allele does not mask the phenotype of the other allele, and the phenotype of the heterozygote is intermediate Example: flower color in snapdragons, Antirrhinum Copyright 2009 Pearson Education, Inc. Figure 4.1

7 4.4 In codominance, the influence of both alleles in a heterozygote Is clearly evident (There s no blending of phenotypes) Copyright 2009 Pearson Education, Inc.

8 Codominance Occurs when phenotypes of both alleles are expressed Simple ratios in monohybrid F 2 because phenotype=genotype 1:2:1 Classic example is the ABO blood group system gene encodes surface antigens on red blood cells antigen = molecule recognized by immune system antibody three alleles exist, I A and I B are codominant and I O is recessive to both

9 Codominance and multiple alleles ABO blood groups Genotype Antigen Phenotype Antibodies in serum I A I A, I A I O A A Ab against B I B I B, I B I O B B Ab against A I A I B A, B AB No Ab I O I O Neither O Ab against A&B Recipient A B AB O Donor blood type (red cells) A B AB O

10 4.5 Multiple alleles of a gene may exist in a population Copyright 2009 Pearson Education, Inc.

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13 Copyright 2009 Pearson Education, Inc. Bombay Phenotype

14 Copyright 2009 Pearson Education, Inc. More than 100 alleles

15 4.6 Lethal Alleles Represent Essential Genes Copyright 2009 Pearson Education, Inc.

16 Copyright 2009 Pearson Education, Inc. Recessive Lethal Mutations

17 Dominant Lethal Mutations Huntington s disease typical age of onset is about 40 causes gradual nervous and motor degeneration Copyright 2009 Pearson Education, Inc.

18 4.7 Combinations of Two Gene Pairs With Two Modes of Inheritance Modify the 9:3:3:1 Ratio Copyright 2009 Pearson Education, Inc.

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21 4.8 Phenotypes Are Often Affected by More Than One Gene One gene masks or modifies the effect of another gene in epistasis. Epistasis causes deviation from Mendelian phenotypic ratios. Let s mate a AB blood type male Bombay carrier with an AB blood type female Bombay carrier. Expectation of blood types if did not consider Bombay locus (H)=1:2:1 Copyright 2009 Pearson Education, Inc.

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24 Copyright 2009 Pearson Education, Inc. Epistasis causes deviation from Mendelian dihybrid phenotypic ratios

25 Examples of modified dihybrid ratios Copyright 2009 Pearson Education, Inc.

26 Two locus interactions may show: dominant epistasis: a dominant allele at one locus masks the expression of the alleles at a second locus Pea flower color: 9:7 recessive epistasis: a recessive genotype at one locus masks the expression of the alleles at a second locus Mouse coat color: 9:3:4 novel phenotypes: can emerge when two loci act simultaneously to produce a trait mouse coat color 9:3:4 Squash shape: 9:6:1 Note: these ratios are just examples, and can vary depending on the traits in question

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28 4.9 Complementation Analysis Can Determine if Two Mutations Causing a Similar Phenotype are Alleles Copyright 2009 Pearson Education, Inc.

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30 4.10 Expression of a Single Gene May Have Multiple Effects Pleiotropy Copyright 2009 Pearson Education, Inc.

31 4.11 X-Linkage Describes Genes on the X Chromosome Copyright 2009 Pearson Education, Inc.

32 X-Linkage in Drosophila Copyright 2009 Pearson Education, Inc.

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34 Copyright 2009 Pearson Education, Hemophilia Inc. A

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36 4.12 In Sex-Limited and Sex-Influenced Inheritance, an Individual s Sex Influences the Phenotype Sex-limited traits are found in one sex only. Genotype HH Hh hh Phenotype Female Male Hen-feathered Hen-feathered Hen-feathered Hen-feathered Hen-feathered Cock-feathered Sex-influenced traits show up in both sexes, but expression of such traits may differ between the two sexes. Genotype Phenotype Female Male BB Bald Bald Bb Not bald Bald bb Not bald Not bald Copyright 2009 Pearson Education, Inc.

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39 The same genotype does not always produce the same phenotype Penetrance is the percentage of individuals that show at least some degree of expression of a mutant genotype. Retinoblastoma, the most malignant form of eye cancer, arises from a dominant mutation of one gene, but only 75% of people who carry the mutant allele develop the disease. Expressivity reflects the range of expression of a mutant genotype. For instance, one or both eyes may be affected in retinoblastoma.

40 Copyright 2009 Pearson Education, Inc. Variable expressivity as illustrated among eyeless homozygous mutants in Drosophila.

41 Genetic suppression: mutant alleles of Gene 2 suppresses the expression of the mutant phenotype of Gene 1. Positional effect: the physical location of a gene in relation to other genetic material may influence its expression. Copyright 2009 Pearson Education, Inc.

42 4.13 Genetic Background and the Environment May Alter Phenotypic Expression Copyright 2009 Pearson Education, Inc.

43 Temperature Effects: An Introduction to Conditional Mutations Temperature-sensitive mutation in Siamese cats 3 wks old 11 wks old 5 months old Copyright 2009 Pearson Education, Inc.

44 a Himalayan rabbit and a Siamese cat show dark fur color on the muzzle, ears, and paws Copyright 2009 Pearson Education, Inc.

45 Genomic (or parental) imprinting: a condition where the expression of a trait depends on whether the trait has been inherited from a male or a female parent. Imprinting is a heritable phenotype but is not controlled by the genotype of the DNA (epigenetic) Copyright 2009 Pearson Education, Inc.

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