I recently had a conversation with a gentleman contemplating purchasing a bull who was concerned that the prospective bull may be a carrier for a genetic defect. This potential bull buyer had studied the bull's pedigree and noticed several relatives were carriers or potential carriers. So, his question to me was, "Is this bull a carrier?" This led to a conversation on simply inherited traits and how DNA testing plays a big role in selecting against genetic defects.
To understand the role DNA plays in selecting simply inherited traits, it is important to first understand simple inheritance. Simply inherited traits are defined as traits affected by one or only a few genes. Examples include red/black coat color, horns and genetic defects such as arthrogryposis multiplex (AM or curly calf syndrome). Simply inherited traits have two common characteristics. First, phenotypes for these traits are expressed as "either/or." For example, a bull is either red or black. And second, environment has very little effect on the expression of these traits. A drought or harsh winter will not change an animal's coat color. In some cases, animals with a black coat that spend a lot of time in the sun may result in a bleached-out coat, but at the end of the day, that animal still has a black coat. The advantage of simply inherited traits is that they are relatively straightforward to select for or against, depending on the trait.
A gene is the basic unit of inheritance and is a segment of DNA. An allele is an alternative form of a gene. For example, the allele for black coat color can be represented by B, and the alternative allele for red can be represented by b. Cattle, and most living organisms, inherit one allele from the sire and one from the dam. A bull can inherit a B from his sire and a b from the dam. The animal’s genotype is the combination of these genes or alleles. Therefore, the genotype for our hypothetical bull would be Bb. The interaction of these two alleles determines the gene's expression. If an allele is dominant, it will mask its recessive partner. In our example, black is dominant over red. Based on the genotype of Bb for our hypothetical bull, he would have a black coat color because the black allele (B) would be dominant over the red allele (b). For a recessive allele to be expressed as a phenotype, two copies of the recessive allele must be present because it cannot be expressed with only one copy. If our bull were red, we would know his genotype would be bb and considered homozygous (when two identical copies of the allele are present). Instead, our bull’s genotype is heterozygous because the copies of the alleles are different, i.e., Bb. If this bull inherited a black allele (B) from his sire and dam, his resulting genotype would be homozygous black (i.e., BB).
Before DNA testing, an animal's genotype was determined using test matings. Using our black/red coat color example, if you have a herd of black-hided cattle and a red calf hit the ground, you know that the cow and bull were carriers of the recessive red gene (this is assuming the neighbor’s bull didn't jump the fence.) You must cull the red calf's sire and dam to remove the red allele from your herd. You would also have to consider not retaining any of the bull’s calves as replacements since there would be a percentage of calves carrying the recessive red allele. The challenge is that the recessive allele is hidden (not expressed) and would remain at a certain percentage in your herd.
Test matings were designed breeding schemes to determine the genotype of an animal. To test a black bull to see if his genotype is BB or Bb, we could breed him to red cows whose genotypes we know are bb. If the bull were heterozygous (Bb), roughly half of his calves from red cows would be black, and the other half would be red. If the bull were homozygous (BB), all his calves would be black. Your confidence in test matings depends on the number of matings made. The more cows you breed to your bull, the more confidence you can have that he is or is not a carrier. In the case of our example, we would have to breed the black bull to at least seven red cows to have a 99% confidence of the bull’s genotype. If the black bull were bred to a random population of black cows, it would take more than 367 matings to gain a 99% confidence that the bull was not a carrier of the recessive red gene. In cattle, test matings are expensive, complicated to implement and time-consuming. In addition, cattle would have to be at least two years of age before you could draw any conclusion for a genotype from test matings.
What about testing the cows in your herd? Test matings can work well for bulls since they can breed more than one cow, but discovering if a cow is a carrier is more complicated when using test matings. Suppose you designed a test mating to breed your black cow to a homozygous red bull (i.e., bb). In that case, the probability of her having a red calf is 50% if her genotype is Bb. Since cows only have one calf per year, there is a chance she would never have a red calf throughout her lifetime (ignoring the use of embryo transfer). There is also an increased risk of the recessive allele remaining in the herd if you keep her daughters as replacements. For this reason, eliminating a recessive allele from a cattle population can take multiple generations and possibly several generations of the farmers themselves.
Genomic testing is a game changer in the selection of simply inherited traits and has replaced the need for test matings. Since simply inherited traits involve only a few genes, they are straightforward to test and select against or for once the gene has been identified. A DNA test returns the animal’s genotype and tells us which alleles it has in its genome. When an animal's genotype is known, this eliminates the need for test matings and allows breeders to make more accurate culling decisions sooner in the animal’s life. So rather than discovering your bull was a carrier of a recessive allele during his first, second, third or later calving season, you would know what his genotype is before breeding him for the first time. If you ran a genomic test on a bull calf, you could know whether he was a carrier before weaning. DNA testing also allows you to test your cow herd, allowing you to know if heifers are carriers or not.
Knowing the genotype of your bulls and cow herd can be extremely important when considering genetic defects such as rat tail (hypotrichosis) or curly calf syndrome. Most genetic defects develop due to a genetic mutation, which is a change to the genetic code from the previous version. Most lethal genetic defects are recessive. Being aware of the carrier status of your bulls and cow herd can be crucial for preventing these defects. If you found some of your cows are carriers for curly calf syndrome, should you cull them? Whether to cull or not is always up to farm management, but the risk of having affected calves is greatly reduced if DNA testing is used. Breeding cows that are known carriers to a bull who has tested free reduces the risk of having affected calves to essentially zero. You can also test the progeny from these matings and know what their carrier status is as well.
Knowing all of this, here was my response to the prospective bull buyer: "Did the bull have a DNA test?" That DNA test removes a lot of uncertainty and can greatly improve a buyer’s confidence in what he or she is buying. In this day and age, genomic testing is commonplace and is well worth the investment to pay for a bull who has been tested to prevent genetic defects in your herd.
This article was originally published in the July 2023 issue of the Iowa Cattlemen's Association's Iowa Cattleman magazine and is used here with permission.
