Heterosis (hybrid vigor)--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.

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).

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.

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.