LECTURE CONNECTIONS 5 | Extensions and Modifications of Basic © 2009 W. H. Freeman and Company Principles

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.

5.1 Dominance Is Interaction between Genes at the Same Locus Incomplete dominance: the heterozygote is intermediate in phenotype between the two homozygous. Codominance: the heterozygote simultaneously expresses the phenotypes of both homozygotes.

For some characters, the genotype does not always produce the expected phenotype= incomplete penetrance. Penetrance: percentage of individuals having a particular genotype that express the expected phenotype Expressivity: the degree to which a character is expressed 5.2 Penetrance and Expressivity Describe How Genes Are Expressed as Phenotype

Concept Checks 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? 2) Polydactyly is usually caused by a dominant allele. If we examined 75 people having an allele for polydactyly and found that only 50 of them were polydactylous, the penetrance would be______

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 mouse x Yellow mouse= yellow and gray litter (2:1) Never a homozygous yellow mouse.

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?

5.4 Multiple Alleles at a Locus Create a Greater Variety of Genotypes and Phenotypes than Do Two Alleles Multiple alleles: For a given locus, more than two alleles are present within a group of individuals. The inheritance of characteristics encoded by multiple alleles is no different from the inheritance of characteristics encoded by two alleles, except that a greater variety of genotypes and phenotypes are possible.

GenotypePhenotype MRMRMRMR Restricted MRMMRM MRmdMRmd MMMallard Mm d Mallard mdmdmdmd Dusky M R > M > m d

5.4 Multiple Alleles at a Locus Create a Greater Variety of Genotypes and Phenotypes than Do Two Alleles ABO blood group I A : encodes the A antigen I B : encodes the B antigen i: does not encode antigens Dominants over i and codominant with each other.

Concept Check 3 What blood types are possible among the children of a cross between a man of blood-type A and a woman of blood-type B? could be A type with a genotype of I A I A and I A i could be B type with a genotype of I B I B or I b i could be AB type with a genotype of I A I B could also be O type with a genotype of ii

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

Colors in peppers results from the relative amount of red and yellow carotenoids. The Y locus encodes one enzyme and the C locus encodes a different enzymes

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 at a different locus. Epistatic gene and hypostatic gene Recessive epistasis

The different coat colors are determined by interactions between genes at two loci. Dominant allele B encodes black; recessive allele b encodes brown. Allele E at a different locus allow dark pigment to be deposited, whereas a recessive allele e prevents the deposition of dark pigment (yellow hair). The presence of genotype ee at the second locus masks the expression of the black and brown alleles at the first locus.

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

White: W_ Green: wwyy Yellow: wwY_ Allele W is epistatic to Y and y: it suppresses the expression of these pigment-producing genes. Allele W is dominant because a single copy of the allele is sufficient to inhibit pigment production.

5.5 Gene Interaction Occurs When Genes at Multiple Loci Determine a Single Phenotype Duplicate recessive epistasis Albinism in freshwater snail Physa heterostroha can result from the presence of either of two recessive alleles at two different loci. P1 aaBB X AAbb (both albinos) F1 AaBb (albino or pigmented?) F2 9/16 A_B_ 3/16 aaB_ 3/16 A_bb 1/16 aabb Pimented? Albinos?

Concept Check 4 A number of all-white cats are crossed and they produced the following types of progeny: 12/16 all-white, 3/16 black, and 1/16 gray. Give the genotypes of the progeny, and which gene is epistatic? A_B_ and AAB_: = 12 give white color. BBA_: 3 give black color. aabb: 1 gives gray color. “A” dominant allele is epistatic to “B” allele.