Haplotype Analysis and Recessive Traits: What Holstein Breeders Need to Screen For

Why this matters even when no calf is ever born

Most breeders think of genetic defects as deformed calves. In Holsteins, the bigger cost often hides earlier. Several recessive haplotypes cause embryonic or fetal death when an animal inherits the same copy from both parents, which looks on the farm like cows that fail to conceive, return to heat late, or abort. A review in Genes notes that individual mutations can cause embryo death at any stage of pregnancy. The result shows up in days open and conception rate, not in a necropsy.

What a haplotype is

A haplotype is a block of DNA inherited together. Researchers found that for certain blocks, living animals that carry two copies were missing from the population, which suggested those embryos did not survive. Holstein Association USA describes six Holstein fertility haplotypes, HH1 to HH6, each treated as a separate trait, and its guidance says the haplotypes occur at low frequency, and no tested living animal has been found with two copies of one. Beyond these fertility haplotypes, breeders also watch for a short list of named recessive conditions. AHDB in the UK lists, in addition to HH1 to HH6, BLAD (bovine leukocyte adhesion deficiency), Brachyspina, citrullinemia (CIT), complex vertebral malformation (CVM), DUMPS and muscle weakness, and notes that they can affect development, fertility and production, and that some are lethal. The review above also lists Holstein cholesterol deficiency and HH7 among the conditions of concern.

How common are carriers?

It depends on the population and the haplotype, and the numbers shift as breeders respond. A survey of Polish Holstein Friesian cattle, based on genotypes from 78,884 cows and 691 bulls and published in Animals, found the highest carrier numbers for HH5, at about 6.7 percent of bulls and 5.4 percent of cows, and the lowest for DUMPS, factor XI and mule foot. That is one country at one time, so do not read it as the U.S. frequency. The practical point is that the HH5 pattern shows carriers are not rare enough to ignore, and that frequencies differ between haplotypes.

The mating math

Recessives show up only when an animal inherits two copies. Holstein USA's examples lay out the probabilities clearly:

Mating	What to expect
Carrier x carrier, same haplotype	25% of embryos have two copies and do not survive to birth. Of live offspring, two-thirds are carriers and one-third are clear.
Untested cow whose sire was a carrier, bred to a carrier bull	About a 12.5% chance the embryo has two copies.
Carrier cow x carrier bull of a different haplotype	25% clear of both, 25% carry the first only, 25% carry the second only, 25% carry both. No embryos lost to these haplotypes.

Carrier status is shown with a code, such as HH1C for carrier and HH1T for a tested non-carrier. The main recommendation is to avoid carrier-by-carrier matings for the same haplotype, since these show a significant drop in conception rate.

What not to do: panic-cull carriers

A carrier is a healthy animal. Holstein USA states valuable carrier animals can still be used, paired with non-carrier mates. It also makes a point many breeders miss: Daughter Pregnancy Rate already reflects haplotype effects, so selecting on fertility traits and then also avoiding every carrier counts the same effect twice. Dropping every carrier from your bull list shrinks your genetic pool, raises inbreeding and costs you real merit, in return for avoiding a risk you can already manage through matings.

A screening protocol that works

1. Know your sire status. Holstein USA says bull status is listed in the Reference Information section of its sire summaries, and the status of any genomically tested Holstein can be found through the Family Tree in its Animal Search tool. Your A.I. company can supply it too.
2. Genotype your females where it pays. Genomic testing exposes carrier status and corrects pedigree errors. Our article on when genomic testing pays covers the economics.
3. Use the mating program or checker. Many breeding software tools flag matings that could produce affected offspring. AHDB notes its inbreeding checker does this where information is available, and that its carrier calls are estimates inferred from genotypes, not direct tests. A direct test from a third-party provider takes priority.
4. Treat untested animals as unknown, not clean. The 12.5 percent example above shows that a cow with a carrier sire is at risk even without her own result.
5. Keep a log of unexplained fertility problems. If conception rates or abortions look worse in a family or sire group, check haplotype status before blaming nutrition or heat. See our guide to heat detection for ruling out management causes first.
6. Re-check as calls change. AHDB notes an animal's calls may change over time as new genotype data arrive or the reference population grows.

What to say to your A.I. rep and vet

• Ask for recessive and haplotype status on every bull you buy, and which haplotypes and diseases that status covers.
• Ask whether a carrier call is a direct test or an inference.
• If a calf looks abnormal or a pregnancy loss pattern appears, involve your veterinarian for diagnosis. Haplotype status can inform the investigation but does not replace a veterinary workup.

Takeaway

Screening for haplotypes and recessives is a mating-management task, not a culling task. Know your bulls' status, avoid carrier-by-carrier matings for the same condition, use fertility indexes as the main selection tool, and keep valuable carriers in the program with clear mates. For more resources see the Agriculture Directory and farm.fm's education page. This site is independent and not affiliated with any breed association; confirm current haplotype lists and codes with your association or A.I. provider.