Overview
This lecture covers a complex exam question on dihybrid genetic crosses, focusing on trait inheritance, genotypes, and typical exam pitfalls.
Dihybrid Crosses: Concepts and Definitions
- A dihybrid cross examines the inheritance of two different genes simultaneously.
- Each gene has two alleles: a dominant and a recessive variant.
- For PTC tasting, tasting is dominant (T), taste-blind is recessive (t).
- For skin pigmentation, normal is dominant (N), albinism is recessive (n).
- Use the allele symbols provided in the question key; do not invent your own.
Genotype and Phenotype Determination
- Heterozygous means having one dominant and one recessive allele for a gene (e.g., Tt or Nn).
- Homozygous recessive means having two recessive alleles (e.g., tt or nn).
- The genotype determines the phenotype, which is the observable trait.
Exam Question Breakdown
- The question asks why it is a dihybrid cross: it involves the combination of two genes.
- Given: Man is heterozygous for both traits (TtNn); woman is homozygous recessive (ttnn).
- Genotype of the woman: ttnn.
- Possible gametes from the man (TtNn): TN, Tn, tN, tn.
- This step requires accuracy; all four correct gametes must be listed for marks.
Interpreting Offspring Information
- To determine a child’s phenotype from their genotype, refer to the given key.
- Example: ttNn child is taste-blind (tt), but has normal pigmentation (Nn).
Determining Parent Genotypes for Set Offspring
- If a homozygous recessive woman (ttnn) and a man only produce heterozygous children (TtNn), the man must be homozygous dominant (TTNN).
Key Terms & Definitions
- Gene — a unit of heredity controlling a trait.
- Allele — a variant form of a gene.
- Genotype — the allele combination for a trait.
- Phenotype — the physical expression of a genotype.
- Homozygous — two identical alleles for a trait.
- Heterozygous — two different alleles for a trait.
- Dihybrid Cross — genetic cross involving two different genes.
Action Items / Next Steps
- Review the use of Punnett squares for dihybrid crosses.
- Practice identifying genotypes and phenotypes from allele combinations.
- Remember to use the provided allele symbols in all exam answers.