Genetics
What the exam tests
This domain tests Mendelian inheritance, monohybrid and dihybrid crosses, deviations such as incomplete dominance and codominance, multiple alleles, linkage and crossing over, sex determination, mutation, and chromosomal disorders.
Core rules and concepts
- Law of segregation: alleles separate during gamete formation.
- Law of independent assortment: alleles of different genes assort independently when genes are on different chromosomes.
- Monohybrid F2 ratio is 3:1 for complete dominance; 1:2:1 for incomplete dominance.
- Dihybrid F2 ratio is 9:3:3:1 when genes assort independently.
- A test cross uses a homozygous recessive parent to reveal the genotype of an individual with a dominant phenotype.
- Linkage reduces recombination; crossing over increases it.
- Mutation is a change in DNA sequence and can be point or chromosomal.
Worked example
Cross RrYy × RrYy. Each parent produces four gametes: RY, Ry, rY, ry. A 4 × 4 Punnett square gives phenotypes in the ratio 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green. The ratio assumes independent assortment. If R and Y were linked, the ratio would be distorted.
Traps that cost marks
Do not use 9:3:3:1 for linked genes. A back cross is not always a test cross; a test cross is specifically to a homozygous recessive. Codominance shows both alleles, while incomplete dominance shows an intermediate. Do not confuse genotype ratios with phenotype ratios.