Deviations from Mendelism
Incomplete dominance, co-dominance, multiple alleles and inheritance of blood groups, pleiotropy and polygenic inheritance.
The essentials
Later studies showed that many traits do not follow Mendel's simple dominant–recessive pattern.
Incomplete dominance: the F₁ phenotype is intermediate between the two parents. In snapdragon (Antirrhinum), a cross of red (RR) and white (rr) flowers gives pink (Rr) F₁. The F₂ shows red : pink : white in 1:2:1, so the phenotypic and genotypic ratios are the same.
Co-dominance: both alleles express fully in the heterozygote. In human ABO blood groups, the gene I has three alleles: Iᴬ, Iᴮ and i. Iᴬ and Iᴮ are co-dominant, and both are dominant over i. A person with IᴬIᴮ has blood group AB, showing both sugars on red blood cells.
Multiple alleles: more than two alleles of a gene exist in a population, although one individual carries only two. ABO has three alleles, giving six genotypes and four phenotypes (A, B, AB, O).
Pleiotropy: a single gene affects several phenotypic traits. In phenylketonuria, a mutation in the gene for phenylalanine hydroxylase causes mental retardation and reduced hair and skin pigmentation. Starch grain size in pea is controlled by one gene that also affects seed shape.
Polygenic inheritance: traits like human skin colour and height are controlled by three or more genes, each adding a small effect, so the trait shows continuous variation.
Key points
- Blood groups: A = IᴬIᴬ or Iᴬi; B = IᴮIᴮ or Iᴮi; AB = IᴬIᴮ; O = ii.
- In incomplete dominance, the heterozygote has an intermediate phenotype; in co-dominance, both parental phenotypes appear together.
Exam tips
Blood-group problems are common: write out all possible genotypes of parents before predicting children.