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Unformatted text preview: sis, One Gene's Alleles Mask the Effects of Another Gene's Alleles
0 This is where we see further modifications of the 9:3:3:1 ratio Type of gene interaction in which the effects of an allele at one gene hide the effects of alleles at another gene The allele doing the masking is "epistatic" to the gene that is being masked (the "hypostatic" gene) Let's look at 3 types of epistasis... 1. Recessive Epistasis Coat color in Labrador retrievers 3 coat colors Black Brown Yellow Modified ratio is... 9:3:4 from 0 9:3:4 The "ee" allele combination maskes the effects of the B or b alleles 2. Dominant Epistasis I 0 Summer squash color 3 colors White Green Yellow Modified ratio is... 12:3:1 from 0 12:3:1 Presence of "B" allele hides any action of the "A" or "a" allele 3. Dominant Epistasis II 0 Leg color in chickens 2 colors White Colored Modified ratio is... 13:3 from 0 13:3 At least one copy of "A" and the absence of "B" is needed to produce color GENE INTERACTION SUMMARY
0 For Some Traits, Homozygosity for a Mutant Allele at Any One of Two or More Genes Produces the Phenotype It takes many genes to generate a functional pathway Good example: Human hearing ~50 genes must have at least one dominant allele present in order for hearing to be normal Heterogeneous trait: mutation at any one of a number of genes can give rise to the same phenotype (i.e. impaired hearing) Complementation test: method of discovering whether a particular phenotype arises from mutations in the Impaired Impaired OK So, the recessive alleles are in different genes, while the other gene is homozygous dominant Impaired Impaired Impaired So, the recessive alleles are in the same gene Breeding Studies Help Decide How a Trait is Inherited
0 Specific breeding tests can help decide between two hypothesis Based on the results of additional crosses, you may be able to determine which of two hypotheses is correct Let's consider mouse coat colors... 0 HYPOTHESIS 1: 2 genes with recessive epistasis 0 HYPOTHESIS 2: 1 gene with incomplete dominance 0 The outcome of crossing a white F2 mouse with a PURE BREEDING brown mouse will help you decide which Hypothesis is correct (why could you not use a black mouse???) The Same Genotype Does Not Always Produce the Same Phenotype
0 Phenotype depends on penetrance and expressivity Modifier genes produce 2 effects on phenotype Penetrance: how many members of a population with a particular genotype show the expected phenotype High penetrance = high incidence of the phenotype Low penetrance = low incidence of the phenotype Expressivity: degree to which a phenotype is expressed Modifier gene: genes having a subtle, secondary effect Siamese car coloring & egg incubation temp. in alligators Environment can affect the phenotypic expression Continuous Variation: Still Mendelian Applications Discontinuous trait: clearcut traits, easy to score Many of the traits Mendel used in pea plants Some of ones we did in class (i.e. ear lobes) Continuous traits: trait that has a wide range of values Height Skin tone Weight Often referred to as "qualtitative traits" Usually polygenic: controlled by multiple genes Mouse Coat Color & Tail Length 0 Gene 1: agouti or other color patterns Dominance series epistasis epistasis Gene 2: black or brown Gene 3: albino or pigmented Gene 4: Short, long or somewhere in between tail polygenic...
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- Fall '03