Genetic defects

There are several common genetic defects that cause problems with dairy breeding, which can be identified through testing. Find out about the common genetic defects affecting dairy breeding in the UK.

Many defects have been known and understood for several years, but some have only been discovered since the introduction of genomic evaluations. Such discoveries have helped solve some problems that have perplexed dairy farmers, vets and scientists for many years.

• Complex vertebral malformation (CVM): this causes stillbirths or, more commonly, abortion or foetal reabsorption before 260 days of gestation
• Brachyspina (BY): this causes abortion and stillbirth, shortened spinal cord, long legs and abnormal organs
• Fertility haplotypes (e.g. HH1, HH2, HH3, HH4, HH5 and HH6): these cause lower fertility, associated with early embryonic death or abortions. There are fertility haplotypes in the Holstein, Jersey, Ayrshire and Brown Swiss breeds
• Holstein cholesterol deficiency (HCD): this causes a lack of cholesterol in the animal’s cells, usually leading to death within months. Heterozygous animals (those that inherit only one copy of HCD) also have reduced levels of cholesterol and compromised health

These conditions are mostly caused by recessive genes, so they tend to be expressed only when two carriers of the defective gene are mated together. The genetic defect occurs in about one in four of the offspring.

Most AI sires are tested, and any found to be carrying the defective gene are identified by suffixes, such as *CVC, *BYC, *HH1C and *CDC. Avoid matings between carrier parents to prevent any risk of genetic defect.

Only a tested free coding (*CVF, *BYF, *HH1T and *CDF) indicates that the animal is definitely not a carrier. No coding simply means that no notification of a test result has been received, so the animal may or may not be carrying the defect.