Darrh Bullock from the University of Kentucky gives a brief introduction to genetics in the cattle industry
Genetics is used in the cattle industry to get a desired trait: for example - carcase yield or feed efficiency. This trait is influenced by two things. Firstly the genetic makeup of the animal, but equally importantly, by the environment in which it is reared.
It is safe to say that each animal receives half of its genes from its sire, and the other half from its dam. However the random combination of these genes means there is a widespread difference in the offspring. Some offspring may be exceptional, some average and some below average.
There are additive genetics and non-additive genetics.
Additive: These are predictable effects that get passed on from generation to generation.
Non-additive: These are dominance and genetic interactions, and are influenced by how the genes match up.
Crossbreeding has occurred for centuries. By crossbreeding, producers can give the offspring an advantage over the average of the parent breeds, because of non-additive genes and the way they match up. This advantage is known as heterosis.
There are two types of traits, qualitative and quantitative.
Qualitative traits are usually controlled by one pair of genes, and are not easily influenced by the environment.
An example of a qualitative trait is coat colour. The coat colour of the animal will be determined by the dominant/ or recessive genes carried by its parents.
For example, black (B) is dominant to red (b). Animals with genes BB would be a homozygous dominant. Animals with Bb would be known as a carrier, or a heterozygous.
Red coated animals would be homozygous recessive - bb.
If two carriers mated, there is a 25 per cent chance that the offspring would be homozygous recessive, and 25 per cent chance it would be homozygous dominant. There is also a 50 per cent chance it would be a carrier.
However, if a homozygous dominant was bred with a carrier, there is a 50 per cent possibility that the offspring will be dominant (BB), and 50 per cent chance it will be a carrier (Bb).
Quantitative traits are controlled by many gene pairs, and the environment will play a major role in the outcome. Quantitative traits are measured by heritability, the proportion of the trait which is controlled by additive gene action.
The majority of production traits are quantitative traits.
Because of the numerous possibilities with quantitative traits, due to environmental affects, they can be difficult to improve genetically. More and more research is being done to improve these traits, as they are often economically significant ones such as carcass yields, fertility, feed efficiency, etc.
Heritability measures the proportion of a trait that is controlled by additive gene action. It is the resemblance between parents and offspring, for a particular trait, due to genetics.
If a trait has low heritability (eg reproduction) this indicates that there is a low involvement of additive genes and/or that the environment has a much larger influence on the trait. High heritability indicates that additive genes play a relatively large role in the trait change, and so these traits are less affected by the environment.
Whilst fertility does have low heritability, traits such as birth and weaning weight, carry high heritability, as does yield per cent of carcass weight. The heritability of the traits does vary between breeds.
Selection is the process of introducing the most desirable genotypes that will leave offspring that are beneficial to the herd and enhance business productivity and profitability. After several generations of selection for certain traits the more desirable genes become more frequent and the less desirable genes become rarer with the result that the overall genetic and phenotype merit of the herd increases.
Selection method indexes, such as estimated breeding values (EBVs/ EPDs) can result in rapid genetic improvement. Traits can be prioritised depending on economic performance, and single traits can be compared. The animals selected for breeding are those with the highest scores based on prioritised traits.
Whilst this is a reliable methods, visual assessment is still required, as not all traits can be measured.
Over time and before selection indexes, selection methods have definitely made a difference. Looking across the breeds, there have been drastic changes in the genetic merit of most traits. This indicates that simultaneous selection for antagonistic traits is possible.
Measuring genetic merits
Measuring genetic merits is a complex task. There are number of different methods to help selection, including visual, estimated progeny differences, genomics and selection indices.
A trait is measured by taking the genetic influences and adding the environmental factors. However there is always an unexplained variation. This variation cannot be measured. For example, if breeding with a bull that was born at a weight 75 lbs, his progeny's birth weight could vary between 55 lbs and 95 lbs.
The more genetic data available, the less the unexplained variation will be. Selection should be made on traits that economically impact individual herds.