Unformatted text preview: lete Crosses between truebreeding strains can produce hybrids with phenotypes different from both parents. Incomplete dominance Codominance F1 hybrids that differ from both parents express an intermediate phenotype. Neither allele is dominant or recessive to the other. Phenotypic ratios are same as genotypic ratios F1hybrids express phenotype of both parents equally. Phenotypic ratios are same as genotypic ratios. 3-6 Single Gene Inheritance Models
COMPLETE DOMINANCE (pea plant traits) Crosses between truebreeding strains produces offspring completely like one of the parents INCOMPLETE DOMINANCE (plant flower colors) F1 hybrids that differ from both parents express an intermediate phenotype. Neither allele is dominant or recessive to the other Phenotypic ratios are same as genotypic ratios CODOMINANCE (human blood types, lentil coat patten) F1 hybrids express phenotype of both parents equally Phenotypic ratios are same as genotypic ratios Summary of dominance relationships Fig. 3.2 COMPLETE DOMINANCE (F2 cross) R R r r ________ genotype ratio ________ phenotype ratio Incomplete dominance in snapdragons Fig. 3.3 INCOMPLETE DOMINANCE (F2 cross) R R r r ________ genotype ratio ________ phenotype ratio Codominant blood group alleles Fig. 3.4b Codominant lentil coat patterns Fig. 3.4a CO-DOMINANCE (F2 cross) A B B A _____ genotype ratio (all ___ genotype) _____ phenotype ratio (all type ___ blood) Do variations on dominance relations negate Mendel's law of segregation? Dominance relations affect phenotype and have no bearing on the segregation of alleles. Alleles still segregate randomly. Gene products control expression of phenotypes differently. Mendel's law of segregation still applies. Interpretation of phenotype/genotype relation is more complex. A gene can have more than two alleles Genes may have multiple alleles that segregate in populations. Although there may be many alleles in a population, each individual carries only 2 of the alternatives. ABO blood group Dominance relations are unique to a pair of alleles. 3 alleles 6 possible ABO genotypes: IAIA, IBIB, IAIB, IAi, IBi, or ii Dominance or recessiveness is always relative to a second allele. ABO blood group
IA is completely dominant to i but codominant to IB. 6 genotypes generate 4 phenotypes. (AA) (AO) (B...
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- Fall '03