Overview
This lecture covers the genetic phenomena of incomplete dominance and codominance, both exceptions to Mendelian inheritance, using examples from plants, animals, and humans.
Incomplete Dominance
- Incomplete dominance occurs when the dominant allele does not completely mask the effect of the recessive allele.
- The resulting offspring show an intermediate phenotype (blend) between the parents, not the usual dominant trait.
- Example: Crossing homozygous red (RR) and white (ww) 4 o'clock or snapdragon plants produces pink (Rw) flowers.
- The F1 generation is all intermediate (pink), and the F2 generation exhibits a 1:2:1 ratio for both genotype and phenotype.
- Human hair type is another example: Curly (CC) and straight (ss) crossed yield wavy hair (Cs) in F1; F2 ratio is 1 curly : 2 wavy : 1 straight.
Codominance
- Codominance occurs when both alleles are equally dominant and both are fully expressed in the offspring.
- Example: Crossing black-feathered (BB) and white-feathered (WW) chickens produces offspring with both black and white (BW) speckled or checkered feathers.
- The F2 generation shows 1 black : 2 checkered : 1 white for both genotypic and phenotypic ratios.
- In humans, the ABO blood group system illustrates codominance; IA and IB alleles expressed together result in AB blood type.
- Additional example: Cattle with roan coat color exhibit both red and white hairs due to codominance.
Key Terms & Definitions
- Incomplete Dominance — A genetic situation where neither allele is completely dominant, resulting in an intermediate phenotype.
- Codominance — A genetic pattern where both alleles in a gene pair are fully expressed and observed in the phenotype.
- Phenotype — The observable characteristics or traits of an organism.
- Genotype — The genetic constitution of an organism with respect to a trait.
- Allele — Different forms of a gene found at a specific locus.
- F1 Generation — The first generation of offspring from a genetic cross.
- F2 Generation — The second generation, offspring of the F1 generation.
Action Items / Next Steps
- Review genetic crosses for incomplete dominance (e.g. 4 o’clock plants, hair type) and codominance (e.g. chickens, blood groups).
- Prepare to discuss the next exception to Mendelian genetics in the upcoming lecture.