Concepts and Connections Benjamin A. Pierce Benjamin A. Pierce GENETICS Essentials Concepts and Connections SECOND EDITION CHAPTER 4 Extensions and Modifications of Basic Principles PART II © 2013 W. H. Freeman and Company

4.3 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 population. ABO blood group Fig. 4.16

Figure 4.16 ABO blood types and possible blood transfusions.

4.4 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. Fig. 4.17 Gene interaction with epistasis Epistasis: one gene masks the effect of another gene.

Gene Interaction that produces novel Phenotype C. annuum comes in four colors Red, peach, yellow, cream

Gene Interaction that produces novel Phenotype F2 generation

Gene Interaction that produces novel Phenotype Calculate probabilities to see gene interactions: Testcross reveals different ratios Do the math of such testcross: Y+y C+ cross with yy cc What are the ratios?

4.4 Gene Interaction Takes Place When Genes at Multiple Loci Determine a Single Phenotype Gene interaction with epistasis Epistasis: one gene masks the effect of another gene. Recessive epistasis Dominant epistasis: Fig. 4.18 Table 5.2

Recessive epistasis Coat color determination in labrador retriever dogs i. Gene B/_ makes black pigment, while b/b makes brown. ii. Another gene, E/_, allows expression of the B gene, while e/e does not. The E locus encodes the melanocortin 1 receptor, a regulator of hair and skin color. Iii. Genotypes and their corresponding phenotypes: (1) B/_ E/_ is black. (2) b/b E/_ is brown (chocolate). (3) _/_ e/e produces yellow with nose and lips either dark (B/_ e/e) or pale (b/b e/e). Ratio 9black:3brown:4yellow

Dominant epistasis In dominant epistasis A/_ B/_ and A/_ b/b have the same phenotype, producing an F2 ratio of 12:3:1. Examples: a. Summer squash fruit have three common colors: white, yellow, and green. i. Yellow is recessive to white but dominant to green. ii. Gene pairs are W/w and Y/y. (1)W/_ are white no matter the genotype of the other locus. (2) w/w are yellow in Y/_ and green in y/y.

Duplicate recessive epistasis The case of the sweet pea flowers:

4.4 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. Single locus

4.5 Sex Influences the Inheritance and Expression of Genes in a Variety of Ways Sex-influenced and sex-limited characteristics Sex-influenced characteristics Sex-limited characteristics Cytoplasmic inheritance Figs. 4.19 & 4.20

SEX-INFLUENCED AND SEX-LIMITED Sex-influenced: somatic gene with Mendelian inheritance that is differently expressed in males and females Sex-limited: somatic gene that is exclusively expressed in only one sex; zero penetrance in the other.

Cytoplasmic inheritance Genes that are not on nuclear chromosomes; Mitochondria and chloroplasts DNA inherited from one parent, usually female egg Reciprocal cross gives different outcomes Extensive phenotypic variations Leber’s hereditary optic neuropathy (LHON, OMIM 585000). Optic nerve degeneration results in complete or partial blindness in midlife adults. LHON is caused by mutations in mtDNA genes for electron transport chain proteins. Kearns-Sayre syndrome (OMIM 530000) produces three types of neuromuscular defects: i. Progressive paralysis of certain eye muscles. ii. Abnormal pigment accumulation on the retina, and degeneration of the retina. iii. Heart disease.

4.5 Sex Influences the Inheritance and Expression of Genes in a Variety of Ways Sex-influenced and sex-limited characteristics Genetic maternal effect Fig. 4.21 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 affect the way in which the DNA sequences are expressed.

Genetic maternal effect Phenotype of the offspring is determined by the genotype of the mother. (nuclear genes) Mostly from the genes expresses in the eggs (oocytes) Example snail Limnea peregra shell coiling

Genomic imprinting Only the allele of one parent is expressed and determines the phenotype.

epigenetics Modification of nucleotides; methylation, acetylation Changes gene expression and can be different in gametes from each parent Epigenetic modifications change through life and is influenced by diet (bee queen and workers are females yet different) Cell differentiation and signaling Chemicals

4.6 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.

Inheritance of continuous characteristics Characteristics with easily distinguishable phenotypes- discontinuous characteristics Human height- continuous characteristic (quantitative characteristics- can be described-measured) Polygenic characteristics- many loci influence the characteristic (3n where n is number of loci) Pleiotropic characteristics- one gene incluences many characteristics