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

Three Different Forms of Dominance

Incomplete Dominance for Flower Color in Snapdragon

Codominance of Spotted and Dotted Coat Pattern Alleles

Codominance of IA and IB Blood Group Alleles

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