Crop Improvement
Crop Improvement
This domain tests how useful genetic variation is created and fixed into improved varieties. Expect objectives, selection methods, hybridization, mutation, polyploidy, and the difference between a pure-line variety and a hybrid.
Core rules and concepts
- Crop improvement aims at higher yield, better quality, resistance to pests and diseases, earliness, and tolerance to drought or salinity.
- Genetic variation comes from germplasm collection, hybridization, mutation, polyploidy and biotechnology. Selection does not create new genes; it changes gene frequencies.
- Pure-line selection is effective in self-pollinated crops such as rice and wheat. Hybrid varieties are used where heterosis is exploitable, and hybrid seed must usually be purchased each season.
- A variety must be tested for yield and adaptation before release; breeder seed, foundation seed and certified seed are different multiplication classes.
Worked example
Two pure lines differ at one gene for plant height, with tall dominant to dwarf. The F₁ is all tall. Selfing gives an F₂ expected ratio of 3 tall : 1 dwarf. If 400 F₂ plants are raised, expected tall = 300 and dwarf = 100. The reasoning is a monohybrid cross: F₁ heterozygotes segregate 1 TT : 2 Tt : 1 tt, and both TT and Tt are tall.
Traps that cost marks
Do not say the F₁ shows a 3:1 ratio; the 3:1 ratio is expected in F₂. Mutation is random and often harmful, so it is not automatically an improvement. A hybrid is not the same as a composite or a pure-line variety. Do not claim that selection creates variation; it uses existing variation.