Genomics and genotyping have allowed discovery of genetic variants on the bovine genome that are causal to the expression of specific phenotypic features. In some cases, this is a desired expression. In other cases, it is detrimental or even lethal. Multiple genetic variants have been identified in specific dairy breeds.
Official documentation from the breed associations and Council of Dairy Cattle Breeding contains codes that clarify the carrier status of the bull for these genetic variants.
“Recessives” are the variants that are only expressed when the individual carries two copies of the gene. These will only occur when a mating has taken place between two parents that are either affected or carriers of the variant. This can be prevented through avoiding mating of two carriers.
Because there have been different versions of DNA microarrays (chips) used to genotype animals over the years, not all chips contain the actual genetic variant. In that case, and when the exact variant is still unknown, a haplotype (larger strand of DNA bases that are inherited together) is used to identify the carriers or affected animals.
Below is a summation of the official genetic variants and haplotypes commonly reported by the Council for Dairy Cattle Breeding. Individual bull studs could also test and report variants that are not on this list.
Haplotype code
Description
AH1
Ayrshire haplotype affecting fertility
AH2
Ayrshire haplotype affecting fertility
Code
Type
Description
A
Recessive
Syndrome of arachnomelia and arthrogryposis (SAA; spiderleg)
A*
Recessive
Syndrome of arachnomelia and arthrogryposis (SAA) through DNA marker testing
TA
Tested free of SAA
D
Recessive
Spinal dysmyelination (SDM)
D*
Recessive
Spinal dysmyelination (SDM) through DNA marker testing
TD
Tested free of SDM
M
Recessive
Spinal muscular atrophy (SMA) through affected offspring or gene test
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