Animal Breeding and Genetics
Genes, traits and inheritance
Genotype is the genetic make-up; phenotype is what you see, shaped by genotype plus environment. Know the terms allele, dominant, recessive, homozygous, heterozygous, hybrid vigour and heritability. Qualitative traits (coat colour, horned status) are controlled by few genes; quantitative traits (growth rate, milk yield, litter size) are controlled by many genes and strongly influenced by management.
Selection methods
- Individual selection: choose animals on their own performance.
- Progeny testing: judge a sire by his offspring; slow but accurate for lowly heritable traits.
- Family selection: use relatives' records when the trait cannot be measured on the animal itself.
- Pedigree selection: use ancestry for traits expressed late in life.
Selection differential is the gap between selected animals and the herd average; genetic gain depends on selection differential, heritability and generation interval.
Breeding systems
Inbreeding raises homozygosity and exposes defects; linebreeding is a mild form; outcrossing and crossbreeding raise heterozygosity and give hybrid vigour. Rotational crossbreeding maintains vigour; grading up uses a purebred sire repeatedly to improve a local population.
Traps
- Believing a high-performing animal is always genetically superior - environment may explain the performance.
- Confusing heritability with the proportion of a trait that is inherited absolutely; it is a population measure.
- Forgetting that inbreeding depression is most severe in fitness traits such as fertility and survival.
Worked rule of thumb
If a trait has heritability of 0.3 and you select parents 10 units above the herd mean, expected response in the next generation is about 3 units (0.3 × 10). Low heritability means management improvement gives faster results than selection.