Overview
This lecture reviews four types of non-Mendelian inheritance: incomplete dominance, codominance, multiple alleles, and polygenic inheritance, emphasizing their differences from classic Mendelian genetics.
Incomplete Dominance
- In incomplete dominance, neither allele is completely dominant, resulting in a heterozygous phenotype that is a blend of both parents.
- Example: Crossing true breeding black and white chickens produces all grayish-blue heterozygotes in the F1 generation.
- Crossing two F1 heterozygotes yields a 1:2:1 phenotypic ratio (black : grayish-blue : white).
Codominance
- In codominance, both alleles are fully expressed in the heterozygote, showing both traits distinctly.
- Example: A chicken with both black and white feathers due to codominant alleles.
- The F1 generation expresses both traits and F2 follows a 1:2:1 ratio.
Multiple Alleles and Blood Types
- Some traits, like human blood type, are controlled by more than two alleles (multiple alleles).
- Blood type alleles: A and B are each dominant over O; O is recessive.
- Type AB blood expresses both A and B proteins, demonstrating codominance.
- Possible genotypes for parents (heterozygous for A and B) can produce offspring with type A, B, AB, or O blood.
Polygenic Inheritance
- Polygenic inheritance occurs when a single trait is influenced by multiple genes.
- Example: Human skin color is determined by proteins from gene A, B, and C at different locations.
- Multiple genetic combinations from heterozygous parents lead to a wide variety of offspring phenotypes.
Key Terms & Definitions
- Non-Mendelian Genetics — Patterns of inheritance that do not fit classic Mendelian ratios.
- Incomplete Dominance — Phenotype of heterozygote is intermediate between two homozygotes.
- Codominance — Both alleles in a heterozygote are fully expressed.
- Multiple Alleles — More than two possible alleles exist for a genetic locus.
- Polygenic Inheritance — A trait is controlled by two or more genes.
Action Items / Next Steps
- Identify examples of incomplete dominance and codominance in practice problems.
- Practice Punnett squares with multiple alleles (e.g., blood type crosses).
- Review textbook sections on polygenic traits and prepare examples.