Plant Breeding, Genetics and Seed Technology
Plant Breeding, Genetics and Seed Technology
This domain tests Mendelian inheritance, breeding methods for self- and cross-pollinated crops, and the production and certification of quality seed.
Core rules and concepts
- A gene is a hereditary unit; alleles are alternative forms. Genotype is genetic constitution; phenotype is observable expression.
- The law of segregation explains 3:1 phenotypic and 1:2:1 genotypic ratios in a monohybrid cross. Independent assortment applies to genes on different chromosomes.
- A test cross uses a homozygous recessive parent to reveal the genotype of a dominant phenotype.
- Breeding methods include pureline selection, mass selection, hybridisation, backcrossing, mutation breeding and heterosis breeding.
- Seed classes in certification are nucleus, breeder, foundation and certified seed. Genetic purity, germination, moisture and freedom from weeds are key quality parameters.
Worked example
Cross two heterozygous tall pea plants, Tt × Tt. Gametes are T and t on each side. Offspring genotypes are TT:Tt:tt = 1:2:1; phenotypes are tall:short = 3:1. If the tall parent were test-crossed with tt, offspring would be Tt:tt = 1:1. Reason: the test cross exposes whether the tall parent is TT or Tt, because tt contributes only recessive alleles.
Traps that cost marks
Mixing the 3:1 phenotypic ratio with the 1:2:1 genotypic ratio; calling certified seed the highest class when nucleus and breeder seed are earlier in the chain; assuming one isolation distance fits every crop; ignoring seed moisture in storage questions.