Genetics, Evolution and Animal Breeding
Genetics, evolution and animal breeding covers inheritance ratios, sources of variation, sex determination, mutation, selection and practical breeding systems used in farm animals.
Core rules
- Mendel: dominance, segregation (alleles separate in gametes), independent assortment (unlinked genes). A test cross uses a homozygous recessive to reveal a dominant phenotype's genotype; a back cross is to a parent.
- Deviations: incomplete dominance gives an intermediate heterozygote; codominance shows both alleles; multiple alleles occur in the ABO blood group; sex-linked genes sit on sex chromosomes.
- Sex determination: XX-XY in humans and most mammals; ZZ-ZW in birds; haplodiploidy in bees.
- Mutation: point substitutions, frameshifts, chromosomal changes; most are neutral or harmful, rarely useful. Variation comes from recombination, mutation, migration and genetic drift.
- Evolution: Darwin's natural selection; Lamarck's inheritance of acquired characters is rejected; homologous structures share ancestry, analogous structures share function. Adaptive radiation and speciation follow ecological opportunity.
- Breeding: mass selection, progeny selection, inbreeding (fixes traits but causes inbreeding depression), outcrossing, crossbreeding (heterosis), grading/upgrading.
Worked example
Cross Tt × Tt for tall (T) dominant over dwarf (t). Gametes are T and t from each parent; Punnett gives TT:Tt:tt = 1:2:1, so phenotype 3 tall:1 dwarf. Test-cross Tt × tt gives 1 tall:1 dwarf, confirming the heterozygote.
Traps
Independent assortment applies only to unlinked genes. Dominance is not stronger inheritance. Inbreeding does not create new genes; it increases homozygosity.