ASB 325 Animal Breeding_Week2_ Types of Gene actions.pdf

ASB 325 Animal Breeding Continuation of week 2 Dr K. Thutwa  Types of gene action A. For genes located on autosomes Additive action – also called co-dominance Non-additive gene action – complete dominance, partial or incomplete and over dominance Addit...

ASB 325 Animal Breeding Continuation of week 2 Dr K. Thutwa  Types of gene action A. For genes located on autosomes Additive action – also called co-dominance Non-additive gene action – complete dominance, partial or incomplete and over dominance Additive gene action – e.g. AA, AB and BB, where the performance of heterozygous is intermediate of those of the two homozygous. For example coat colour Shorthorn cattle, red and white when mated produce roan  Complete dominance – e.g. there is complete masking of one allele by the other. E.g. Polledness and horns in cattle, Angus are black and Herefords are red. Black colour is dominant while red colour is recessive. The heterozygotes are carriers of a recessive gene. Exercise; Work out the phenotypic and genotypic ratios of the outcome of mating Black polled Angus bulls to horned red Hereford cows in; i. First generation ii. Intra-mating F1  Solution; i. BP * bp = BbPp ii. Intra-mating Angus*Hereford BP Bp bP bp BP BBPP BBPp BbPP BbPp Bp BBPp BBpp BbPp Bbpp Angus* Hereford bP BbPP BbPp bbPP bbPp bp BbPp Bbpp bbPp bbpp  Incomplete or partial dominance – e.g. one allele is partial dominant over the other, which means it does not completely mask the other allele. For example in case of the Booroola gene litter size shows partial dominance although ovulation rate show additive gene action. In this case F+ is close to FF than ++. The Booroola gene was discovered in Australia in the 1970s in Merino sheep. Merino sheep normally have low fecundity (ovulation rate of 1.4) but animals carrying this gene were noticed to have high fecundity (ovulation rate of 4.38), resulting in litter sizes of 1.48 and 2.66 lambs respectively.  This is through an additive gene action, with the heterozygous having an intermediate ovulation rate of 2.82 and a litter size of 2.17. Another example is double muscling or muscular hypertrophy, which is common in Belgian Blue and Piedmontese breeds, where heterozygotes are closer to homozygous double-muscled animals.  Over dominance – occurs where the heterozygous shows extreme performance than either homozygotes. This is a rare gene action. For example Inverdale gene, which affects fertility in sheep. This gene was discovered in Romney sheep at the Invermay Agricultural Centre in New Zealand. Romney ewes carrying a single copy of the Inverdale gene have litter sizes about 0.58 higher than normal Romney ewes, because of higher neonatal lamb mortality, this figure is reduced to 0.17 extra live lambs at one day of age. However, Romney ewes homozygous of the Inverdale gene are completely infertile, they have undeveloped ovaries. B. Genes located on sex chromosomes Sex-linked genes – Genes located on a sex chromosome, E.g. the Inverdale gene is located on the X-chromosome Sex-limited genes – genes present in both sexes but phenotypically seen in one sex only. E.g. milk production, ovulation rate  Sex influenced genes – this is a situation where expression of genes is influenced by sex without being sex linked or sex limited. E.g. in some breeds of sheep such as Merino, Welsh Mountain, males are usually horned and females are usually polled even though they have the same genotype. The polled locus is not on the sex chromosomes, so it is not sex linked and other breeds have horns in both males and females. C. Other gene actions Genomic imprinting – a genotype/phenotype is only seen when a parent of certain sex passes the gene to the progeny e.g. Callipye gene, which causes extreme muscularity in sheep. This condition is seen when a progeny receives a copy from the sire only (not dam).  Pleiotropy – one allele at locus influence one or more traits e.g. a single gene is responsible for double muscling in several cattle breeds, and this condition is associated with reduced fertility, lower calf survival and sometimes with increased stress susceptibility. The homozygous polled condition in goats leads to development of intersex animals rather than females.  Epistasis – one allele at a single locus can be influenced by alleles at different loci e.g. colour in Labrador dogs where B-E- are black, bbE- are chocolate, B-ee are yellow with black nose and bbee are yellow with liver nose.  Variable expressivity – a genetic disorder is shown by all animals but to varying degrees e.g. a condition known as mulefoot, which occurs in cattle and pigs. This is believed to be caused by a single dominant gene in pigs and a single recessive gene in cattle, affected animal show solid hooves, rather than the usual cloven hooves. However, animals with mulefoot genotype can have one to four feet affected.  Cytoplasmic or mitochondrial inheritance – non-Mendelian heredity involving replication and transmission of extra chromosomal genetic information found in organelles such as mitochondria. Mitochondria are small structures in the cytoplasm of cells which are responsible for energy generation. These are only contained in eggs but not sperms. It is possible that mitochondrial DNA, which is passed from mother to daughter is responsible for the apparent superiority of some dam families. Although there is controversy surrounding this issue, up to 5% of the total performance could be due to cytoplasmic effects.