Beef Cattle Terminology continued

Heritability estimate--An estimate of the proportion of the total phenotypic variation between individuals for a certain trait that is due to heredity. More specifically, hereditary variation due to additive gene action.

Heterosis -- Heterosis is the difference in average performance for a trait between crossbred individuals and the average performance of parent breeds contributing to the cross. It frequently is economically beneficial, particularly for traits that contribute to reproduction, longevity and health. Amount by which measured traits of the crossbreds exceed the average of the two or more purebreds that are mated to produce the crossbreds.

Heterozygous--Genes of a specific pair (alleles) are different in an individual.

Homozygous--Genes of a specific pair (alleles) are alike in an individual.

Inbreeding-Production of offspring from parents more closely related than the average of a population. Inbreeding increases the proportion of homozygous gene pairs and decreases the proportion of heterozygous gene pairs. Also, inbreeding increases prepotency and facilitates expression of undesirable recessive genes.

Inbreeding Coefficient -- The inbreeding coefficient is a number between zero and one that quantifies the expected reduction in proportion of heterozygous loci in the inbred individual, compared to the proportion of heterozygous loci in a typical individual from the noninbred population from which the individual descended.

Inbreeding Depression -- Inbreeding depression is the average change in phenotypic value for a trait that accompanies each unit of change in inbreeding coefficient within a population. Generally it is economically detrimental, particularly for traits that contribute to reproduction, longevity and health.

Independent culling levels--Selection of culling based on cattle meeting specific levels of performance for each trait included in the breeder's selection program. For example, a breeder could cull all heifers with weaning weights below 400 pounds (or those in the bottom 20 percent on weaning weight) and yearling weights below 650 pounds (or those in the bottom 40 percent).

Indicator Trait -- An indicator trait is one that does not directly influence net profit of commercial livestock production but which is genetically correlated with one or more traits that do. For example, larger scrotal circumference of yearling bulls does not increase revenue or reduce cost of production, but it is predictive of a bull's genetic merit for age at puberty, an economically important trait in many instances.

Involution--The return of an organ to its normal size or condition after enlargement, as of the uterus after parturition. A decline in size or activity of other tissues; the mammary gland tissues normally involute with advancing lactation.

Linebreeding--A form of inbreeding in which an attempt is made to concentrate the inheritance of some one ancestor, or line of ancestors, in a herd. The average relationship of the individuals in the herd to this ancestor (outstanding individual or individuals) is increased by linebreeding.

Linecross--Offspring produced by crossing two or more inbred lines.

Marbling--The specks of fat (intramuscular fat) distributed in muscular tissue. Marbling is usually evaluated in the ribeye between the 12th and 13th rib.

Marker-Assisted Selection -- In marker-assisted selection, DNA markers are used to predict genotypes of candidates for selection at loci associated with merit for an economically important trait. Such information may then be used in breeding value estimation for the trait.

Metabolic body size--The weight of the animal raised to the 3/4 power (W0.75); a figure indicative of metabolic needs and of the feed required to maintain a certain body weight.

Metabolism--The transformation by which energy is made available for body uses.

Most probable producing ability (MPPA)--An estimate of a cow's future productivity for a trait (such as progeny weaning weight ratio) based on her past productivity. For example, a cow's MPPA for weaning ratio is calculated from the cow's average progeny weaning ratio, the number of her progeny with weaning records, and the repeatability of weaning weight.

National sire evaluation--Programs of sire evaluation conducted by breed associations to compare sires on a progeny test basis. Carefully conducted national reference sire evaluation programs give unbiased estimates of expected progeny differences. Sire evaluations based on field data rely on large numbers of progeny per sire to compensate for possible favoritism or bias for sires within herds.

Nonadditive gene effects--Favorable effects or actions produced by specific gene pairs or combinations. Nonadditive gene action is the primary cause of heterosis. Nonadditive gene action occurs when the heterozygous genotype is not intermediate in phenotypic value to the two homozygous genotypes.

Number of contemporaries--The number of animals of similar breed, sex, and age, against which an animal was compared in performance tests. The greater the number of contemporaries, the greater the accuracy of comparisons.

Open--A term commonly used to indicate a non-pregnant female.

Optimum level of performance--The most profitable or favorable ranges in levels of performance for the economically important traits in a given environment and management system. For example, although many cows produce too little milk, in every management system there is a point beyond which higher levels of milk production may reduce fertility and decrease profit.

Outcrossing--Mating of individuals that are less closely related than the average of the breed. Commercial breeders and some purebred breeders should be outcrossing by periodically adding new sires that are unrelated to their cow herd. This outcrossing should reduce the possibility of loss of vigor due to inbreeding.