Chapter 5 Outline 5.1 Dominance Is Interaction between Genes at the Same Locus, 100 5.2 Penetrance and Expressivity Describe How Genes Are Expressed as Phenotype, 102 5.3 Lethal Alleles May Alter Phenotypic Ratios, 103 5.4 Multiple Alleles at a Locus Create a Greater Variety of Genotypes and Phenotypes than Do Two Alleles, 103

Chapter 5 Outline 5.5 Gene Interaction Occurs When Genes at Multiple Loci Determine a Single Phenotype, 105 5.6 Sex Influences the Inheritance and Expression of Genes in a Variety of Ways, 114 5.7 Anticipation Is the Stronger or Earlier Expression of Traits in Succeeding Generations, 122 5.8 The Expression of a Genotype May Be Influenced by Environmental Effects, 122

5.1 Dominance Is Interaction between Genes at the Same Locus Genes at the same locus – two versions of the same gene; each version of the same gene is defined as allele.

5.2 The type of dominance exhibited by a trait depends on how the phenotype of the heterozygote relates to the phenotypes of the homozygotes.

5.1 Dominance Is Interaction between Genes at the Same Locus Incomplete dominance Codominance

5.2 Penetrance and Expressivity Describe How Genes Are Expressed as Phenotype Penetrance: percentage of individuals having a particular genotype that express the expected phenotype Expressivity: the degree to which a character is expressed

5.3 Lethal Alleles May Alter Phenotypic Ratios A lethal allele: causes death at an early stage of development, and so some genotypes may not appear among the progeny

Yellow coat color in mice is caused by a recessive lethal gene, producing distorted phenotypic ratios in the progeny of two yellow mice.William Castle and Clarence Little discovered the lethal nature of the yellow gene in 1910. [Reprinted with permission of Dr. Loan Phan and In Vivo, a publication of Columbia University Medical Center.]

More than 2 alleles at a locus Relationship between allele pairs determines outcome Could be exhibited as dominant/recessive relationship as in duck feathers Could be exhibited in codominance as in ABO blood type

5.4 Mendel’s principle of segregation applies to crosses with multiple alleles. In this example, three alleles determine the type of plumage in mallard ducks: MR (restricted) M (mallard) md (dusky).

5.4 Multiple Alleles at a Locus Create a Greater Variety of Genotypes and Phenotypes than Do Two Alleles ABO blood group

5.5 ABO blood types and possible blood transfusions.

5.5 Gene Interaction Occurs When Genes at Multiple Loci Determine a Single Phenotype Gene interaction: Effects of genes at one locus depend on the presence of genes at other loci. Gene interaction that produces novel phenotypes

5.6 Gene interaction in which two loci determine a single characteristic, fruit color, in the pepper Capsicum annuum.

5.5 Gene Interaction Occurs When Genes at Multiple Loci Determine a Single Phenotype Gene interaction with epistasis Epistasis: One gene masks the effect of another gene. Recessive epistasis

5. 8 Expression of the ABO antigens depend on alleles at the H locus 5.8 Expression of the ABO antigens depend on alleles at the H locus. The H locus encodes a precursor to the antigens called compound H. Alleles at the ABO locus determine which types of terminal sugars are added to compound H.

5.5 Gene Interaction Occurs When Genes at Multiple Loci Determine a Single Phenotype Dominant epistasis

5.9 Yellow pigment in summer squash is produced in a two-step pathway.

5.5 Gene Interaction Occurs When Genes at Multiple Loci Determine a Single Phenotype Duplicate recessive epistasis

5.10 Pigment is produced in a two-step pathway in snails.

5.5 Gene Interaction Occurs When Genes at Multiple Loci Determine a Single Phenotype Complementation: determines whether mutations are at the same locus or at different loci The complex genetics of coat color in dogs: Agouti (A) locus Black (B) locus Extension (E) locus Spotting (S) locus

5.6 Sex Influences the Inheritance and Expression of Genes in a Variety of Ways Sex-influenced and sex-limited characteristics

Sex-influenced and sex-limited characteristics Sex-influenced characteristics

5.12 Genes that encode sex-influenced traits are inherited according to Mendel’s principles but are expressed differently in males and females.

5.12a Genes that encode sex-influenced traits are inherited according to Mendel’s principles but are expressed differently in males and females.

5.12b Genes that encode sex-influenced traits are inherited according to Mendel’s principles but are expressed differently in males and females.

Sex-influenced and sex-limited characteristics

5.13a A sex-limited characteristic is encoded by autosomal genes that are expressed in only one sex. An example is cock feathering in chickens, an autosomal recessive trait that is limited to males. (a) Cock-feathered male. [Larry Lefever/Grant Heilman Photography.]

5.13b A sex-limited characteristic is encoded by autosomal genes that are expressed in only one sex. An example is cock feathering in chickens, an autosomal recessive trait that is limited to males. (b) Hen-feathered female. [Larry Lefever/Grant Heilman Photography.]

5.13c A sex-limited characteristic is encoded by autosomal genes that are expressed in only one sex. An example is cock feathering in chickens, an autosomal recessive trait that is limited to males. (c) Hen-feathered male. [Larry Lefever/Grant Heilman Photography.]

Sex-influenced and sex-limited characteristics Cytoplasmic inheritance

5.15 Cytoplasmically inherited characteristics frequently exhibit extensive phenotypic variation because cells and individual offspring contain various proportions of cytoplasmic genes. Mitochondria that have wild-type mtDNA are shown in red; those having mutant mtDNA are shown in blue.

5.16 Crosses for leaf type in four-oﾕclocks illustrate cytoplasmic inheritance.

Sex-influenced and sex-limited characteristics Genetic maternal effect

5.17 In genetic maternal effect, the genotype of the maternal parent determines the phenotype of the offspring. The shell coiling of a snail is a trait that exhibits genetic maternal effect.

5.17 (part 1) In genetic maternal effect, the genotype of the maternal parent determines the phenotype of the offspring. The shell coiling of a snail is a trait that exhibits genetic maternal effect.

5.17 (part 1) In genetic maternal effect, the genotype of the maternal parent determines the phenotype of the offspring. The shell coiling of a snail is a trait that exhibits genetic maternal effect.

5.17 (part 2) In genetic maternal effect, the genotype of the maternal parent determines the phenotype of the offspring. The shell coiling of a snail is a trait that exhibits genetic maternal effect.

Sex-influenced and sex-limited characteristics Genomic imprinting: differential expression of genetic material depending on whether it is inherited from the male or female parent Epigenetics: phenomena due to alterations to DNA that do not include changes in the base sequence; often affects the way in which the DNA sequences are expressed

5.7 Anticipation Is the Stronger or Earlier Expression of Traits in Succeeding Generations Anticipation: A genetic trait becomes more strongly expressed or is expressed at an earlier stage as it is passed from generation to generation.

5.8 The Expression of a Genotype May Be Influenced by Environmental Effects Temperature-sensitive allele: an allele whose product is functional only at a certain temperature