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Extensions to Mendel: Complexities in Relating Genotype to Phenotype. Outline of extensions to Mendel’s analysis. Single-gene inheritance In which pairs of alleles show deviations from complete dominance and recessiveness In which different forms of the gene are not limited to two alleles
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Extensions to Mendel: Complexities in Relating Genotype to Phenotype
Outline of extensions to Mendel’s analysis • Single-gene inheritance • In which pairs of alleles show deviations from complete dominance and recessiveness • In which different forms of the gene are not limited to two alleles • Where one gene may determine more than one trait • Multifactorial inheritance in which the phenotype arises from the interaction of one or more genes with the environment, chance, and each other
Dominance is not always complete • Crosses between true-breeding strains can produce hybrids with phenotypes different from both parents • Incomplete dominance • 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 • F1hybrids express phenotype of both parents equally • Phenotypic ratios are same as genotypic ratios
There Are Often More Than Two Alleles of a Gene The ABO blood group system is determined by one gene with three alleles.
There Are Often More Than Two Alleles of a Gene Note that the ABO blood system shows both complete dominance and codominance.
Multiple Alleles Can be Grouped in a Dominance Series Dominance series for lentil bean coat color.
Do variations on dominance relations negate Mendel’s law of segregation? • Dominance relations affect only the relationship between genotype and phenotype • Alleles still segregate randomly and unite randomly • Gene products control expression of phenotypes • Interpretation of phenotype/genotype relationship can be complex