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Animal Breeding Basics - Dominance & Recession

Written by Dr Clive Dalton

A dominant allele covers over the effects of a recessive allele. However, recessive alleles do not always stay covered as they can appear again in later generations.

Examples:

Animal Dominant allele Recessive allele
Cattle  Polled (no horns) Horns (with horns) 
  Black coat Red coat
  White head (Hereford) Other coloured head
  Normal leg length Dwarfed legs
  Normal palate Cleft palate
  Normal head Bulldog head
 Sheep White fleece Black fleece
  Self colour Black spotted
  Hairy (medullated) fleece Non medullated
  Normal leg length Short legged  Ancon
  Polled Horned
  Normal wool length Naked
Horses Grey coat Bay coat
  Bay coat Black coat
  Black coat Chestnut coat
Pigs White skin Black skin
  Lop ears Prick ears
  Normal nipples Inverted nipples
Dog Black coat in Labradors Chocolate coat colour
Dominance not complete
  • The action of dominance is usually complete and it completely masks the recessive allele.
  • But there is also incomplete dominance, and a good example is coat colour in Shorthorn cattle.
  • Shorthorns are either dominant red (RR) or recessive white (rr), but there is also a heterozygous roan (Rr) where the red and white hair merges into a mixed colour.
  • Mating whites to whites produces all whites, mating reds to reds produces all reds, but mating roans gives a 1:2:1 ratio of 1 red, 2 roan and 1 white.
  • Many breeds have been produced using this phenomenon, eg the Blue Albion cattle, the Blue Grey (black Galloway x White shorthorn) and the blue Andalusian fowl.