BIOL 1411 1st Edition Lecture 17 Outline of Last Lecture I The Cell Cycle II Reproduction III Meiosis Errors IV Mendel Outline of Current Lecture I Dominance II Epistasis Lecture Dominance Mendel s Second Law independent assortment alleles of different genes assort independently during gamete formation o Doesn t often apply to different genes on the same chromosome but different chromosomes do segregate independently Complete dominance heterozygotes appear similar to one of the homozygotes used to define a dominant allele and a recessive allele Incomplete dominance heterozygotes have an intermediate phenotype o Petal color alleles in snapdragons o Dihybrid crosses produce a 1 2 1 ration of phenotypes in F2 o Phenotypes genotypes o Also true for crosses involving co dominant alleles o Phenotype lies between Co dominance sometimes alleles at one locus produce distinct phenotypes that are both expressed in the heterozygote o Blood types ABO blood system o Three common alleles Ia Ib i o Ia and Ib alleles are both expressed in heterozygotes Both are completely dominant to I allele Gene interactions Many traits are influenced by the genotype of more than a single gene o Hair color in domesticated mamals and humans These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute o Eye color in humans Physical characteristics reflect underlying cellular functions such as enzymatic activity within biochemical pathways Epistasis phenotypic expression of one gene is influenced by genotype of another gene o Ex coat color in Labrador retrievers involves at least 2 different genes o Results in a modification of the usual 9 3 3 1 ratio of dihybrid cross 9 3 4 Mendalian traits o Single gene affecting discreet phenotypic differences Phenotypic variation is often complex o Phenotypes vary continuously over a range quantitative variation Height weight skin pigmentation in humans Hypertension type 2 diabetes asthma o Phenotypic variation is usually due to the action of multiple different gene and also influenced by the environment Individual genes can have additive effects Environment can narrow or broaden the range Variation in human skin color
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