Crop Improvement
Crop Improvement tests genetics and plant breeding: inheritance, breeding methods, variety development and seed classes. The exam expects you to match a breeding method to the crop's pollination system and the trait being improved.
Core rules and concepts
- Qualitative traits are controlled by one or few genes and scored in classes; quantitative traits are polygenic and measured. Heritability is the proportion of phenotypic variance that is genetic; high heritability does not guarantee high genetic advance unless variability and selection intensity are adequate.
- Pollination system dictates method. Self-pollinated crops use pure-line selection, pedigree or bulk hybridization; cross-pollinated crops use mass selection, recurrent selection, synthetic or composite varieties; asexual crops use clonal selection.
- Backcross breeding transfers one or few genes into an adapted recurrent parent. Pedigree breeding handles several traits; bulk breeding delays selection; mutation creates variability; heterosis is exploited in hybrids.
- Seed classes are breeder, foundation, certified and truthfully labelled. Breeder seed is produced under breeder control and is the source of foundation seed.
Worked example
A resistant donor is crossed with a high-yielding susceptible recurrent parent. The F1 is backcrossed to the recurrent parent, and resistant progeny are selected each generation. After six backcrosses, expected recurrent-parent genome recovery is 1 − (1/2)^6 = 98.44%, assuming selection only for the dominant resistance gene. With a recessive gene, testcrossing or progeny testing is needed before backcrossing.
Traps that cost marks
Do not use pedigree breeding when a single gene must be moved into an elite variety; that is backcross work. Do not confuse heritability with genetic advance. Do not select before creating segregating variation. Do not reverse seed classes. Check whether the trait is dominant or recessive before choosing selection.