GENETICS TERMS EXPLAINED

Gene: The unit of inheritance. A gene is a sequence of molecules on the DNA strand that encodes the synthesis of a protein. In Sewall Wright and Kleberg’s time this was not known; geneticists of that era conceptualized genes as discrete particles, often pictured as beads.

Genotype or Genome: The genes of an individual (not visible to ordinary observation) that determine traits that can be observed; see phenotype.

Gene Pool: The stock of genes carried by a breeding population, including all possible allelic variants that can be found in that population. Note the difference between the genome (possessed by the individual) and the gene pool (representing the whole population).

Phenotype: All the factors about an individual that are observed or measured. The term covers an individual horse’s conformation, developmental process, biochemistry, physiology and behavior. A horse’s phenotype is determined by its genotype acting under the influence of the environment in which it lives.

Chromosome: The DNA molecule that carries genes. Chromosomes occur in pairs. Different organisms have different numbers of chomosome pairs, referred to as the diploid number or “2n”; domestic horses, for example, have

2n = 64. Individual chromosomes detach from their partner and segregate (a process called reduction division) during the formation of eggs and sperm. The foal receives one strand from each of the 64 chromosomes from its sire and one from its dam. When sperm fertilizes egg they join together, resulting in a unique combination with the correct number of chromosomes (and genes).

Allele: A variant form of a given gene which is carried at the same place (locus) on each chromosome of a pair. When two alleles at a locus are the same, the organism is said to be homozygous for whatever traits those genes govern. If they are different, the organism is said to be heterozygous.

Dominant and Recessive: A genetic trait is considered dominant if it is expressed in a horse who has only one copy of that gene, i.e. only one allele of that form. Such an individual is by definition heterozygous. A recessive trait can be expressed only when the horse has two similar alleles, i.e. is homozygous. Notice that “dominance” in this sense has nothing to do with physical strength or aggressive behavior.

Pleiotropy: An effect at the cellular and molecular level that occurs when one gene influences two or more seemingly unrelated phenotypic traits. Mutation in a pleiotropic gene affects multiple traits simultaneously usually because the gene codes for a protein used by a variety of different cells.

Heterosis (also called hybrid vigor or outbreeding enhancement) is the

improved or increased functioning of any phenotypic quality in a hybrid or outcrossed individual.

Recombination: The result of reduction division in the process of forming eggs and sperm. Recombination ensures the exchange of genetic material between sire and dam that leads to offspring with combinations of alleles—and thus traits—that differ from those found in either parent. It is thus an important source of phenotypic variability in a herd.

Mutation: Change in the molecular structure of a gene that affects either the structure of the protein it makes or else causes it to interact differently with other genes. Mutations may be caused by environmental stimuli such as radiation or toxins, or may arise spontaneously. Statistically and over the long term, they occur at random and are the ultimate source of new alleles and of allelic diversity in the herd.

Random Breeding or Panmixia: A breeding system in which every male has an equal chance of mating with every female. This rarely occurs in nature but is important in managing zoo populations of endangered species, especially where the number of surviving individuals is small. Panmixia maintains maximum possible allelic diversity and phenotypic variability, and is the antidote to close inbreeding.

Effective Population Size: The size of the population of animals within a species that can actually meet each other to mate. A wild population may be spread out over a wide geographic region, in which case panmixia of the whole population is unlikely because certain males and females will not be able to meet. In nature, the effective population size is usually small. Likewise the effective population size of a small horse breeder is his stallion, the half-dozen or so broodmares that he owns, plus however many mares come from other farms to be bred to his stallion. If the breeder belongs to a semen cooperative or a breed club that maintains an open registry, the effective population size is larger.

The King Ranch is historically unique in possessing such large numbers of horses that it could produce its own distinct, true-breeding bloodline (and potential breed) of sorrels.

Genetic Drift (also known as allelic drift or the Sewall Wright effect) is change in the frequency of alleles in a population due to chance. Genetic drift affects small populations (small gene pools with relatively low allelic diversity) more strongly than large ones. The alleles in any individual are a sample of those in its parents, and chance plays a role in determining whether a given individual survives long enough to reproduce. In a small population, unique alleles carried by only one individual will disappear from the gene pool if that individual dies; in this way, genetic drift reduces allelic diversity. It can also work the opposite way, causing initially rare alleles to become much more frequent.

Selection: Natural selection is the process whereby organisms better adapted to their environment tend to survive and produce more offspring. While natural selection is thought to be without directed purpose, selection in the breeding shed is carried out for the purpose of maintaining and improving strains of livestock. It does not involve merely the choice or “selection” of sire and dam, but also requires assessment, testing and culling which mimic the rigors of the wild and result in the survival of only a fraction of the bestadapted foals from each generation.

Inbreeding: The breeding of close relatives. Linebreeding and inbreeding are the same thing because they produce essentially identical results.

Outcrossing or Outbreeding: The mating of animals that are unrelated, that is, have no ancestors in common going back at least four generations.

Violation of Assumptions:

The mathematical models used by Sewall Wright and other geneticists and population biologists to predict relatedness, population structure and dynamics are all based on certain assumptions, including panmixia, the absence of selection and the functional neutrality of alleles. Assumptions make it possible to create formulas (models of population structure and dynamics) that are simple enough to be comprehensible, and that can be used to test predictions, but real-world populations—wild or domestic—almost always violate these assumptions.

There may be other problems too. As geneticist W. G. Hill observes, “In livestock improvement, the objective is to select animals expected to have the highest performing offspring, so that accurate prediction of breeding value is of fundamental importance. The problem is that selection may have to be practiced among animals that are not directly comparable on performance either in space or time, and have differing amounts of information available on them or their relatives.” Breeders have developed various ways to average performance scores or to weight the results of testing to develop “indexes” for the breeding value of individuals, but as Kleberg notes, none of these methods is perfect so that

“the skill of the breeder and his care in the choice of individuals as foundation stock and in the planning of individual matings remains the all-important part of any breeding program.”