Animal Breeding, Genetics, and Reproduction
Animal Breeding, Genetics, and Reproduction tests inheritance, selection, breeding systems and the reproductive cycle. Questions often ask you to choose a breeding method for a defined goal or calculate selection response.
Core rules
- A gene is a unit of inheritance; alleles are alternative forms. Qualitative traits are controlled by few genes and are strongly influenced by environment, while quantitative traits such as milk yield are controlled by many genes.
- Heritability estimates the fraction of phenotypic variation due to additive genetic effects. It ranges from 0 to 1; high values mean selection on own performance is more effective.
- Selection methods include mass selection, progeny testing and family selection. Crossbreeding exploits heterosis; inbreeding increases homozygosity and can cause inbreeding depression.
- Cattle oestrus cycle is about 18–24 days, with standing heat lasting roughly 12–18 hours. Gestation is about 280 days in cattle, 310 in buffalo, 150 in goats, 148 in sheep and 114 in pigs.
- Artificial insemination uses preserved semen; timing near the end of standing heat improves conception.
Worked example
A bull is selected with a selection differential of 400 kg milk for a trait with heritability 0.25. Expected response per generation = h² × S = 0.25 × 400 = 100 kg. The reasoning shows that only one-quarter of the superiority is expected to be passed on under additive inheritance.
Traps
Do not equate heritability with economic importance. Do not use inbreeding as a general improvement tool. Do not assume crossbreeding permanently changes breed purity; it produces crossbred progeny.