Overview
This lecture explains non-Mendelian inheritance patterns, which do not follow classic Mendelian dominance, and covers incomplete dominance, codominance, polygenic traits, and epistasis.
Non-Mendelian Traits
- Non-Mendelian traits do not follow the rule that a dominant allele always determines the phenotype.
- Examples include snapdragons, human height, and speckled chickens.
Incomplete Dominance
- Incomplete dominance occurs when neither allele is completely dominant, resulting in an intermediate phenotype.
- Example: Crossing red (RR) and white (rr) snapdragons produces pink (Rr) offspring.
- Crossing two pink (Rr) snapdragons yields red, pink, or white flowers.
Codominance
- Codominance means both alleles are fully expressed in the phenotype.
- Example: Crossing black (BB) and white (WW) chickens results in speckled (BW) offspring showing both colors.
- Codominance uses different letter notation, and sometimes exponents, depending on classroom conventions.
Polygenic Traits
- Polygenic traits are influenced by multiple genes, not just one gene pair.
- Human height and skin color are polygenic; many gene pairs contribute to these traits.
- Environmental factors like nutrition and sun exposure can affect the expression of polygenic traits but do not change the underlying genetics.
Epistasis
- Epistasis occurs when one gene affects the expression of another gene.
- Example: In llamas, a gene for wool pigment (B/b) is only expressed if another gene (C/c) is not homozygous recessive (cc).
- If the llama's genotype is cc, no pigment is produced, resulting in albinism regardless of wool color genes.
Key Terms & Definitions
- Non-Mendelian trait — a genetic trait that does not follow standard Mendelian inheritance.
- Incomplete dominance — a pattern where heterozygotes display an intermediate phenotype.
- Codominance — a pattern where both alleles in a gene pair are fully expressed.
- Polygenic trait — a trait controlled by multiple genes.
- Epistasis — when one gene’s expression depends on another gene.
Action Items / Next Steps
- Watch videos on multiple alleles and sex-linked traits for further examples of non-Mendelian inheritance.