Agricultural Biology and Genetics
Agricultural Biology and Genetics tests cell division, inheritance and the breeding methods used to create and multiply improved varieties - with the emphasis on predicting progeny ratios rather than memorising names.
Core rules
- Mitosis preserves chromosome number in somatic growth; meiosis halves it and creates variation through crossing over and independent assortment.
- Mendel's laws: segregation of alleles and independent assortment of genes on different chromosomes.
- A monohybrid cross of heterozygotes gives 3:1 phenotype and 1:2:1 genotype; a dihybrid gives 9:3:3:1. Modified ratios signal interaction: 9:7 complementary genes, 15:1 duplicate genes, 12:3:1 dominant epistasis.
- A testcross (cross with the homozygous recessive) reveals the genotype behind a dominant phenotype; a backcross is to a parent.
- Heritability is a property of a population in a given environment; narrow-sense heritability determines response to selection (R = h² × S).
Worked example
Cross RrYy × RrYy (round/yellow, independent assortment). Round yellow = 9/16 of the progeny. With 320 plants, expected = 320 × 9/16 = 180. Round green = 3/16 → 60; wrinkled yellow 3/16 → 60; wrinkled green 1/16 → 20. If only 150 round yellow appear, test the fit with chi-square before blaming linkage - small samples fluctuate.
Traps that cost marks
Swapping genotype and phenotype ratios; assuming independent assortment for genes on the same chromosome; treating heritability as fixed across environments; confusing testcross with backcross; expecting hybrid vigour to pass to the next generation.