Genetics: The Genetic Index Overview
Genetic Index – Cattle breeders have been studying genetic indexes for over 25 years, but for those who are not familiar with how they work and for whom some of the most commonly used terms are alien, we have put together a useful overview as a starting point in understanding this interesting science.
Passing good genes to the next generation
A genetic index is a means of measuring how well an animal will pass on its genes to the next generation. A particular genetic make-up can be important for a number of reasons, such as for conformation, production and for other measurable heritable traits. Physical traits of stature, colour of coat and milk production, known as an animal’s phenotype, are not included in the indexing process, but its genotype, or genetic make-up, is. This distinction is important because an animal’s high milk yield, for example, may be the result of the animal’s management rather than because of its genetic composition. The genetic reference point, therefore, is used to establish an animal’s likely quality outside of any controlled management regimen.
Calculating a genetic index
The performance of their daughters is usually used to calculate the genetic indexes of male dairy cattle, but other factors such as the genetic scoring of other relatives, and in particular the bull’s sire, are also used. Calculations for female dairy cows generally follow the same lines but they can obviously have their own performance added to the indexing process along the way. As an animal ages, and more data is collected from the animal itself, its ancestry becomes less important in indexing.
The key pieces of information used to calculate a genetic index focus on the performance of the animal itself and the performance of its offspring, coupled with any indices of all known relatives, the sire and that of the dam.
Predicted Transmitting Ability
The most common genetic index is the Predicted Transmitting Ability (PTA) which is the bedrock of other UK indices concerned with production characteristics. It looks specifically at three areas, the weight of milk, the weight of fat and the percentage and weight of protein. These characteristics are calculated for both male and female animals.
An example of a genetic index PTA for a bull might be 384kg milk, indicating that a daughter should be giving 384kg more milk during its first lactation than a daughter from a bull with a milk PTA of 0kg. And this is the same for the calculation for fat and protein. A reliability percentage is also given within the PTA reflecting the number of daughters from a particular bull which have contributed to the proof of the bull. Reliability ranges published for bulls should be between 50 per cent and 99 per cent with 99 per cent suggesting that the figures are an accurate indication of the bull’s ability to transmit its genes. It is important to say that low reliability scoring in a bull would indicate that its use should be limited within a herd.