Genetics and Heredity
What this domain tests
Questions cover Mendelian ratios, genotype versus phenotype, test crosses, deviations such as incomplete dominance and codominance, human sex determination, sex-linked inheritance, mutation, and the DNA-to-protein pathway.
Core rules
- Law of dominance, law of segregation (alleles separate during gamete formation) and law of independent assortment (two pairs of factors behave independently).
- Monohybrid Tt × Tt gives phenotypic ratio 3 : 1 and genotypic ratio 1 TT : 2 Tt : 1 tt.
- Test cross: crossing an unknown dominant phenotype with the recessive parent; a 1 : 1 result proves heterozygosity.
- Incomplete dominance (Mirabilis jalapa): heterozygote shows an intermediate phenotype, so F2 is 1 : 2 : 1 for both phenotype and genotype.
- Codominance: ABO blood groups, alleles I^A, I^B and i; I^A and I^B are codominant, both dominant over i.
- Human sex determination: female XX, male XY; the father's sperm decides the child's sex.
- Sex-linked genes sit on the X chromosome (haemophilia, red–green colour blindness), so affected males are far more common.
- DNA replication is semi-conservative; transcription makes mRNA in the nucleus; translation at the ribosome builds the polypeptide.
Worked example
A man of blood group AB marries a woman of group O. Father's genotype I^A I^B, mother's ii. Gametes: I^A or I^B from the father, only i from the mother. Children are I^A i (group A) or I^B i (group B) in a 1 : 1 ratio. No child can be group O or AB, because neither parent supplies two compatible alleles.
Traps that cost marks
Writing 3 : 1 as a genotypic ratio; assuming the dominant allele is the commoner one in a population; saying the mother determines sex; confusing a test cross with a back cross; forgetting that haemophilia is X-linked recessive.