Spectrum of Dominance (Complete, Incomplete, & Codominance)
SPECTRUM OF DOMINANCE
Complete Dominance
- Phenotype of the heterozygote and dominant homozygote are indistinguishable
- F2 generation 3:1 dominant to recessive ratio
Codominance
- Phenotype of both alleles are present in the heterozygote
- F1 generation from a red and a white flower is pink
- F2 generation is 1:2:1 red to pink to white ratio
Incomplete dominance
- Phenotype of the heterozygote is intermediate between the phenotypes of the two homozygotes
Multiple alleles
More than two alleles for a single gene
- Group A blood – IAIA or IAi – Carbohydrate A on surface of blood cells
- Group B blood – IBIB or IBi – Carbohydrate B on surface of blood cells
- Group AB blood – IAIB – Both carbohydrate A and carbohydrate B on surface of blood cells
- Group O blood – ii – No blood group carbohydrate on the surface of blood cells
Tay-Sachs disease
- Unable to metabolize certain lipids in the brain due to lack of certain enzyme
- Organismal (individual level) – Functional allele is dominant
- Biochemical level – Disease and functional allele exhibit incomplete dominance
- Molecular level – Disease and functional allele exhibit codominance
Pleiotropy
Single gene can have multiple phenotypic effects