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Incomplete Dominance – heterozygotes have a third, intermediate phenotype Stephen V. Evans, NSLS Co-dominance – both alleles expressed C = agouti cch = chinchilla ch = himilayan c = albino Rabbit Color – Multiple Alleles C > cch > ch > c C __ cch cch, cchch, cchc chch, chc cc C > cch > ch > c Can this mating produce a ? What phenotypic ratio of offspring do you expect from this mating? C > cch > ch > c Can this mating produce a ? What phenotypic ratio of offspring do you expect from this mating? Can each of these phenotypes be true breeders? If you cross a true breeding walnut with a true breeding single, what offspring result in the F1 generation? If you cross two F1 individuals, what does the F2 generation look like? Chicken Combs – Polygenic Trait Rose (R_, pp) Walnut (R_, P_) Pea (rr, P_) Single (rr, pp) RrPp x RrPp gametes: RP, Rp, rP, rp Epistasis – one gene interferes with the expression of another Complimentary Gene Action Example Enzyme pathway producing a pigment Intermediate Molecule Precursor Molecule Pigment Product Gene A Gene B Complementary Gene Action: need a dominant at both loci to express trait AaBb x AaBb gametes: AB, Ab, aB, ab Only two phenotypes – 9:7 Duplicate Gene Action Example Enzyme pathway producing a pigment Precursor Molecule Pigment Product Gene A Gene B Pigment Product Duplicate Gene Action: need a dominant at one locus to express trait AaBb x AaBb gametes: AB, Ab, aB, ab Only two phenotypes – 15:1 Suppressive Gene Action Examples Recessive: Albinism Dominant: Eye Color Gene A controls pigment deposition Gene M controls pigment production If aa, then no matter how much pigment is made, none can be deposited Gene B controls brown pigment production Gene G controls green pigment production If BB or Bb, then eyes are brown regardless of genotype at the G locus Suppressive Gene Action: recessive at one locus overrides any genotype at other AaBb x AaBb gametes: AB, Ab, aB, ab Three phenotypes – 9:3:4 Suppressive Gene Action: dominant at one locus overrides any genotype at other AaBb x AaBb gametes: AB, Ab, aB, ab Three phenotypes – 12:3:1 Pleiotropy – one gene has multiple phenotypic effects Terms to get straight Incomplete Dominance vs. Codominance: When the heterozygote shows an intermediate phenotype it is ID but when the heterozygote expresses both phenotypes, it is CD. Epistasis: Interactions between different genes. Complementary (Dup. Rec.) – two genes have to be dominant Duplicate – one gene of a pair has to be dominant Suppressive – (Rec.) recessive for one gene inhibits the other could also be dominant – eye color Pleiotropy: When a single gene affects more than one phenotypic trait. Penetrance vs. Expressivity: Penetrance is the % of individuals with a genotype that express the phenotype. Expressivity is the degree to which the genotype is expressed in an individual Summarizing the Ratios If you’re dealing with a single trait (e.g., flower color, chicken comb pattern) use the ratios to help you determine the inheritance relationships. How many different phenotypes are there? 2 – could be simple dominance or one of the epistatic interactions 3 – could be incomplete dominance, codominance, or an epistatic interaction 4 – could be polygenic or multiple alleles 4+ - likely multiple alleles or possibly polygenic 2. What do the phenotypic ratios of the offspring tell you? Do they match expected ratios for simple Mendelian inheritance? If not, consider the epistatic interactions and lethal alleles. Lethal alleles – is there a phenotype class missing or is one phenotype category lower than expected? Epistatic interactions – consider these if ratios are skewed away from expected ones; consider examples of each interaction and see if the ratios match your results.