Extensions and Modifications of Benjamin A. Pierce GENETICS A Conceptual Approach FIFTH EDITION CHAPTER 5 Extensions and Modifications of Basic Principles © 2014 W. H. Freeman and Company

The direction of shell coiling in snails is determined by genetic maternal effect.

5.1 Additional Factors at a Single Locus Can Affect the Results of Genetic Crosses Genes at the same locus—two versions of the same gene; each version of the same gene is defined as allele. Types of dominance Complete dominance Incomplete dominance Codominance

Figure 5.1 The dominance exhibited by a trait depends on how the phenotype of the heterozygote relates to the phenotypes of the homozygotes.

Figure 5.2 Incomplete dominance in eggplant.

5.1 Additional Factors at a Single Locus Can Affect the Results of Genetic Crosses Penetrance: the percentage of individuals having a particular genotype that express the expected phenotype. Expressivity: the degree to which a character is expressed.

Figure 5.3 Human polydactyly is an example of incomplete dominance and variable expressivity.

Concept Check 1 Assume that long fingers are inherited as a recessive trait with 80% penetrance. Two people heterozygous for long fingers mate. What is the probability that their first child will have long fingers?

Concept Check 1 Assume that long fingers are inherited as a recessive trait with 80% penetrance. Two people heterozygous for long fingers mate. What is the probability that their first child will have long fingers? ¼ x 80% = 20%

5.1 Additional Factors at a Single Locus Can Affect the Results of Genetic Crosses A lethal allele: causes death at an early stage of development, and so some genotypes may not appear among the progeny Affects the Mendelian genotypic and phenotypic ratios in progeny

Concept Check 2 A cross between two green corn plants yields 2/3 progeny that are green and 1/3 progeny that are white. What is the genotype of the green progeny and the white progeny?

Concept Check 2 A cross between two green corn plants yields 2/3 progeny that are green and 1/3 progeny that are white. What is the genotype of the green progeny and the white progeny? White genotype: GG Green genotype: Gg gg: lethal allele causing death in homozygous

5.1 Additional Factors at a Single Locus Can Affect the Results of Genetic Crosses Multiple alleles: for a given locus, more than two alleles are present within a group of individuals. ABO blood group

Figure 5.5 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).

Figure 5.6 ABO blood types and possible blood transfusions.

Concept Check 3 What blood types are possible among the children of a cross between a man who is blood-type A and a woman of blood-type B?

Concept Check 3 What blood types are possible among the children of a cross between a man who is blood-type A and a woman of blood-type B? Could be: A type with a genotype of IAIA and IAi; Could be B type with a genotype of IBIB or IBi Could be AB type with a genotype of IAIB Could also be a O type with a genotype of ii

5.2 Gene Interaction Takes Place 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 Gene interaction with epistasis: one gene masks the effect of another gene

Figure 5.7 Gene interaction in which two loci determine a single characteristic, fruit color, in the pepper Capsicum annuum.

5.2 Gene Interaction Takes Place When Genes At Multiple Loci Determine a Single Phenotype Epistasis: one gene masks the effect of another gene. Recessive epistasis: Fig. 5.8 Dominant epistasis: Fig. 5.9 Duplicate recessive epistasis: Fig. 5.10

Figure 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.

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

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

5.2 Gene Interaction Takes Place When Genes At Multiple Loci Determine a Single Phenotype Complementation: determine 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

Figure 5.11 Coat color in dogs is determined by interactions between genes at a number of loci.

Sex-influenced and sex-limited characteristics 5.3 Sex Influences the Inheritance and Expression of Genes in a Variety of Ways Sex-influenced and sex-limited characteristics Sex-influenced characteristics (Fig. 5.12) Sex-limited characteristics (Fig. 5.13) Cytoplasmic inheritance (Fig. 5.15 & 5.16)

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

Figure 5.13 A sex-limited characteristic is encoded by autosomal genes that are expressed in only one sex.

Sex-limited characteristics are inherited according to Mendel’s principles.

Figure 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.

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

5.3 Sex Influences the Inheritance and Expression of Genes in a Variety of Ways Sex-influenced and sex-limited characteristics Genetic maternal effect (Fig. 5.17) Genomic imprinting: differential expression of genetic material depending on whether it is inherited from the male or female parent (Fig. 5.18) Epigenetics: phenomena due to alterations to DNA that do not include changes in the base sequence; often affect the way in which the DNA sequences are expressed

Figure 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.

Figure 5.18 Genomic imprinting of the Igf2 gene in mice and humans affects fetal growth.

5.4 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. Occurs due to expansion of an unstable region of DNA from generation to generation

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

The Inheritance of Continuous Characteristics Discontinuous characteristics: relatively few phenotypes Continuous characteristics: continuous distribution of phenotypes; occurs when genes at many loci interact Polygenic characteristics: characteristics encoded by genes at many loci Pleiotropy: one gene affects multiple characteristics