(CD) have more serious consequences and are often referred to as lethal recessives. These are obviously more likely to be researched and have carriers identified than is the case for less harmful alleles.
What about fertility Haplotypes such as HH1 to HH5?
These haplotypes are chunks of genes, among which are suspected to be recessive alleles affecting embryo survival. This is assumed because homozygous animals have not been found in the population via genomic testing and so are presumed to have been non-viable as embryos. There is as yet no direct gene test for these recessives, as opposed to BLAD and CVM which are identified at exactly known chromosomal regions.
Is this also the case for the new haplotype, HCD? Yes, HCD carriers have been identified through genomic testing and pedigree analysis, as carrying the Haplotype which likely contains the Cholesterol Deficiency (CD) causing recessive allele.
HCD1 coded bulls are considered as having a high probability of carrying the CD allele. Maughlin Storm is the bull from which HCD1 carriers are descended. Goldwyn has been identified as a carrier and breeders using red factor descendants of Storm should be particularly wary of HCD as several prominent RC bulls are carriers.
Sires coded HCD3 probably have the CD allele as well, so it is best to consider them also as carriers for now. Researchers have only recently found the exact allele causing CD, so in the future animals will be more precisely coded, in the same way that BLAD and CVM carriers are.
Should breeders avoid line breeding now? Yes. Holsteins are more closely related now than for example in the 1950's when AI use first became widespread. Nowadays the chance of mating two descendants of any particular individual AI bull is much greater and thus so also is the risk of breeding homozygous carriers of undesirable recessive genes. Line breeding, where inbreeding coefficients above
6% are predicted is not advisable. The risk can be mitigated by using inbreeding calculators provided in mating programs such as Holstein UK's WEBMATE. If in doubt, line breeding should not be practised at all with common ancestors in the first four generations (i.e. no animal should appear on both the paternal and maternal sides of a conventional Holstein printed or web pedigree).
59
The mighty Roybrook Tempo (born in 1973)
FIG 1. ROYBROOK TEMPO'S PEDIGREE. If you have more questions please contact Darren by email:
darrentodd@holstein-uk.org
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