How Genetic Recessives Work

Recently the American Angus Association announced the discovery of a genetic condition known as Developmental Duplication (DD). The condition is thought to be inherited as a simple recessive. In the case of DD, animals that are born affected by the condition are relatively rare as most DD affected pregnancies result in early embryonic death. Developmental Duplication is not the first genetic condition to be discovered in recent years, nor will it be the last. The discovery of DD has caused quite a lot of press in the past couple of weeks, and even some panic among breeders. For this reason I think it is important to step back and review how simple recessives work and how with a carefully managed breeding program you can avoid seeing affected animals in your herd.

Source:  http://www.flockandherd.net.au/cattle/reader/polymelia.html

             

A calf exhibiting the affects of Developmental Duplication.

         

Before we begin a discussion of genetic recessives it is important to understand a little bit about genetics and inheritance of genes. Phenotype is the external appearance of an animal. For instance, an animal’s phenotype would include coat color. Genotype refers to the actual genes that an animal has or the animal’s genetic make-up. Traits are the result of at least one pair of genes, one of the genes being inherited from an individual’s mother; the other is inherited from an individual’s father. When both genes in a pair are the same, they are called homozygous genes. If they are different they are known as heterozygous genes. Sometimes one of the genes in a pair is dominate over the other gene (recessive). When this happens the gene that is dominate is the gene that is expressed as an individual’s phenotype, even if the individual’s genotype is heterozygous for that trait/condition. The only way the recessive gene is expressed is if an individual has two copies of the recessive gene. It is important to note, that even though we often talk about genetic recessives in a negative context, such as the case with DD, a recessive is not necessarily negative or lethal. In cattle other examples of recessives include coat color (black being dominate to red) and horned/polled condition (polled being dominate to horned).

In the case of developmental duplication, and other simple recessive conditions, there are two possible phenotypes that an animal can have and three possible genotypes. The two possible phenotypes are a completely normal looking animal not affected by the condition and an animal that is affected by the condition resulting in either early pregnancy loss or possibly a calf born with extra limbs. The genotypes that can be expected are:

• DD – or an animal that receives a copy of the gene from both its mother and its father that is free of the mutation
• Dd – this animal’s outward appearance is totally normal and healthy, however they carry one copy of the gene that carries the mutation, an animal with this genotype is referred to as a carrier as they can pass on the mutated gene to their offspring without being affected themselves

• dd – this is an affected animal, the copies of the gene that it received from both its mother and its father carry the developmental duplication mutation.

The question becomes how can you best manage developmental duplication in your herd? The first option is going to be the most time consuming and costly. You can genetically test all the cows in your herd that are potential carriers for DD (the American Angus Association recently announced the release of a commercially available test for DD by Zoetis, read about it here) and cull those cows from your herd. Then make sure that all A.I. and herd sires you use are free of the genetic defect as well. For many people this will be the way they choose to manage and eliminate the developmental duplication gene from their herd.

The mating of a bull with a genotype of DD to a cow with a genotype of DD results in all potential offspring having the DD genotype.

The other option is a carefully managed breeding program. Since we know that in order to get an affected calf both the mother and father must be carriers you can choose to not test any of your cows that are potential carriers if you only use bulls that are non-carriers. If you would still like to use bulls that are carriers, you need to avoid breeding a carrier bull to any cows that are potential carriers or test your cows and avoid breeding carriers to carriers.

The mating of a bull with a genotype of DD to a cow that is a carrier of the condition (genotype Dd) will result in 50% of their potential offspring having a genotype of DD and 50% of their potential offspring being carriers for the condition with the genotype Dd.  None of their potential offspring will be affected by the condition.

The mating of a carrier bull (genotype Dd) to a carrier cow will result in 25% of their potential offspring having a genotype of DD, 50% of their potential offspring being carriers for the condition with the genotype Dd, and 25% of their potential offspring being affected by the condition with a genotype of dd.  This mating should be avoided.

Whatever your method of managing developmental duplication or other genetic conditions in your herd you can be assured that Genex will continue provide you with high quality semen, from bulls whose carrier status we know, so that you can make informed breeding decisions. In this age of genomic testing this is certainly not the last new genetic condition that will be documented so knowing all your options for managing them in your herd is essential.

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Author, Sarah Thorson, is our Beef Education Manager. Sarah grew up in Eastern Montana on her family's ranch. She is a graduate of Montana State University and has been a member of the Genex team since 2004. Sarah works closely with the Genex Beef Marketing Staff, providing training to the cooperative's nearly 200 Independent Contractors. She also provides Artificial Insemination training for Genex members and customers and reproductive consulting.