Dairy@Purdue - Genetics and Breeding: Genetic Evaluations

Genetic Evaluations for dairy cattle are calculated routinely for cows and bulls in the six major breeds (Ayrshires, Brown Swiss, Guernseys, Holsteins, Jerseys, and Milking Shorthorns ) here in the United States. Genetic evaluations for production traits, including milk, fat, and protein yield, fat and protein percentage, productive life, and somatic cell scores (an indicator of mastitis) are calculated and released to the public four times per year by the Animal Improvement Programs Laboratory of the United States Department of Agriculture. Genetic Evaluations for individual bulls may be obtained from them on-line.

Genetic evaluations are also performed by breed associations for type or conformation traits of cattle. Type evaluations of Holsteins are produced by the Holstein Association, while type evaluations for other breeds are calculated by the Animal Improvement Programs Laboratory . Dairy cattle are genetically evaluated for numerous type traits. Currently, Holsteins are genetically evaluated for stature, strength, body depth, dairy form, rump angle, rump width, rear leg side view, rear leg rear view, foot angle, foot and leg score, fore udder attachment, rear udder height, rear udder width, udder cleft, udder depth, front teat placement, teat length, and final score.

Genetic Terminology: Breeding Value, Predicted Transmitting Ability, Heritability, Genetic Correlation, Reliability

Genetic Terminology

There are some terms that are often used in any discussions about genetic evaluation of dairy cattle. The following is intended to provide general definitions of the most common terms.

Breeding Value. Breeding value refers to the value of an animal in a breeding program for a particular trait. An animal's breeding value is estimated to be twice the expected performance of its progeny relative to a population mean when mated at random. The reason for doubling the expected progeny performance is that only half of the genes from the individual are transmitted to any offspring (with the remaining half coming from the other parent at random from the population). The expected progeny performance as a deviation from the population is called transmitting ability and is, therefore, half of the breeding value. In other words, transmitting ability is the genetic advantage an individual transmits to its offspring.

In practice, breeders want to know the level of performance that can be expected from progeny of certain individuals. True breeding values of individuals are not known because 1) most traits of interest are influenced by many genes, 2) only a sample half of the genes are transmitted at random to the offspring, 3) the number of possible combinations of genes in the offspring is large, and 4) performance of individuals is affected by environment. However, breeding values can be estimated based on the animal's own records and the performance of known relatives. These estimated breeding values divided by 2 may be used to predict the performance of future offspring relative to the population mean and are termed Predicted Transmitting Ability or PTA. For example, the daughters of a bull with a PTA of 1000 kg for milk yield would be expected to produce, on average, 200 kg more milk per mature lactation than the daughters of a bull with a PTA of 800 kg for milk yield if their dams have equal genetic merit. The actual difference will not be exact for comparing individual daughters because no two daughters would get exactly the same combination of genes or be exposed to exactly the same environment. Thus, daughters of the same sire may have widely varying performance.

Heritability. Heritability is the extent to which genetics influences a trait or characteristic. Unlike breeding values and predicted transmitting abilities, which are estimated for individuals, heritability is a population parameter. Strictly defined, heritability is the ratio of additive genetic variance to phenotypic variance. Additive genetic variance is the true variance among breeding values of animals in a population. Hence, heritability is a ratio of the variance of breeding values to the variance of phenotypes. The possible range of values for heritability is from 0 to 1.0, because additive genetic variance is a part of phenotypic variance. Phenotypes are what is observed or measured about a particular trait; phenotypes are influenced by genetic and environmental effects. In measuring heritability, phenotypic variances are taken to be the total of random sources of variation after adjusting for systematic sources of variability, such as herd-year, age, month of calving, or stage of lactation.

The extent of genetic control is different for each trait. Approximate heritabilities for several common traits of dairy cattle are in Table 1. The higher the heritability, the greater is the genetic control on the trait, and the more rapidly selection will result in genetic progress. In general, yield traits and overall type tend to be moderately heritable; fat and protein percentages, stature, and size have higher heritabilities, and reproductive efficiency has lower heritability. Mastitis resistance has a heritability of about .10. In other words, genetics accounts for 10% of the variation in cows' capacity to resist mastitis infection, and environment accounts for the remaining 90%.

Genetic Correlation. The correlation between breeding values for two traits is called genetic correlation and indicates to what extent the two traits are influenced by the same genes. For example, the genetic correlation between milk yield and protein yield (.9) is high. Many of the same genes that influence milk yield also influence protein yield, and a bull with daughters that have high mean milk yield almost always will sire daughters that have high mean protein yield. However, the genetic correlation between milk yield and fat percentage is -.3; therefore, bulls with daughters that have high milk yield often will have daughters with low fat percentage. As with any correlation, the larger the magnitude (i.e., the farther from 0), the greater is the relationship between the traits. For a heritable trait, selection of genetically superior animals to be parents (i.e., genetic selection) will produce offspring that are genetically better on average for that trait. This result is called response to selection. Genetic selection on such a trait will also affect any genetically correlated characteristics; this is called correlated response to selection.

Reliability. The measure of accuracy or degree of confidence in a PTA is called reliability (REL), which is defined as the squared correlation between an animal's true transmitting ability and PTA. Often, in practice, this value is approximated rather than calculated directly. Essentially, REL for PTA of a trait is a function of the heritability of that trait and the amount of information available. That information may come from the animal's own performance, from the performance of offspring, or from information for parents. As heritability and amount of information increase, REL also increases. Thus, an animal has a higher REL for milk yield than for reproductive efficiency (even if the same number of records are available from the animal and its relatives) because milk yield is under greater genetic control. Also, a bull with many daughters has a more reliable PTA for any given trait than a bull with few daughters.