Biology
225: Genetics Lecture 12
Outline: I. Modification of Mendelian Ratios
involving Two Genes
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A. Gene
Interactions
1.
Example #1: Comb Shape in Chickens
2.
Example #2: Coat color
in Duroc-Jersey Pigs
B. Epistasis: a form of gene
interaction in which one gene interferes with the phenotypic expression of
another nonallelic gene
1. Recessive Epistasis (9:3:4 ratio): Presence of a pair of recessive alleles
at one locus masks (hides) the expression of a dominant or recessive allele(s)
at a second locus.
2. Duplicate Recessive Epistasis (9:7 ratio): Interaction in which the products of 2
genes interact to give rise to a specific phenotype, so that in order for the
phenotype to be expressed, a dominant allele must be present at each loci (A _
B _).
3. Dominant Epistasis (12:3:1 ratio): Presence of a dominant allele at one
locus blocks (masks) the expression of gene(s) or gene pairs (recessive or
dominant) at a second genetic locus.