Download Glossary of technical terms in animal genetics for course WAP 214

Glossary of Technical Terms in Animal Breeding and
Genetics for Module WAP 214
Compiled by Obert Tada
Accuracy -- A measure of the strength of the relationship between true values and
estimates of them, varying from 0 to 1. Breeding values of animals, for example, are
estimated from phenotypic records, and there is sometimes very little information (for
example a parent's performance record) available and thus the accuracy is low, e.g. .10.
Breeding values of some dairy sires are estimated with high accuracy, e.g. .99, because
there is substantial information (thousands of daughters with performance records) on the
"performance" of that sire's genes.
Additive genetic correlation (rBVx, BVy) -- The measure of association between the
breeding value for one trait (BVx) and the breeding value for another trait (BVy), ranging
from -1 to +1. The genetic basis for an additive genetic correlation between two traits is
pleiotropy and linkage. The additive genetic correlation between birth weight and
weaning weight of beef cattle is about +.6. See Pleiotropy and Linkage.
Additive genetic effects -- The effect of an allele on animal performance, independent of
the effect of the other allele at a locus. These effects of the two alleles at a locus add up
(thus "additive"). Alleles at a locus may have other effects (dominance, epistasis), so that
there are not genes that have just "additive" effects and other genes with only
"dominance" effects. Additive genetic effects can be inherited, other genetic effects such
as dominance and epistasis are the result of allele combinations that are lost between
generations. The additive genetic effect that an animal has for a trait is equal to its
breeding value.
Additive genetic variance -- A measure of the variability among animals in their
breeding values, e.g the additive genetic variance for weaning weight of beef cattle is 190
kg2.
Allele -- A member of a pair, or series of hereditary factors which may occupy a given
locus on a specific chromosome, e.g. the allele for white face coat pattern in Herefords,
and the allele for solid colour, are two alleles for coat pattern in cattle.
Allozymes -- Enzymes differing in electrophoretic mobility as a result of allelic
differences in a single gene. As with other markers (e.g. microsatellites) allozyme
variation in a population is an indication of genetic variation.
Artificial insemination (A.I.) -- The use of semen collected from males, either in fresh
or frozen form, to breed females (frozen semen is thawed before use!). A.I. is commonly
used in dairy cattle and is less commonly used with beef cattle. It is also used
increasingly in swine.
Artificial selection -- Selection of animals to become parents of the next generation
using criteria that are defined by humans. See also Natural selection.
Autosomes -- Chromosome pairs which are alike in both sexes.
Backcrossing -- Mating a hybrid to a breed that the hybrid is already composed of, e.g.
breeding an Angus x Hereford hybrid female to an Angus mate.
Breed -- A group of animals within a species having a common origin and identifying
characters that distinguish them as belonging to a breeding group.
Breed complementarity -- Breeds complement one another when they excel in different
traits, and their cross results in desired performance in a variety of traits, sometimes
expressed in different generations. In livestock used for meat production, for example,
breeds that excel in growth and carcass traits are often used as the male parent breed in a
crossbreeding program with a female parent breed that excels in maternal traits and
reproduction.
Breeding objective -- A general goal for a breeding program, a notion of what
constitutes the best animal. See also Selection criterion.
Breeding value -- The value of an individual as a parent. The effects of an animal's genes
that can be passed on to offspring. Because one-half of an animal's genes are passed to
offspring, one-half of an animal's breeding value is also passed on (see Transmitting
ability). Breeding value can be determined for a genotype at a single locus, but it is also
calculated for polygenic, quantitative traits (i.e. most economically important traits).
Breeding values are available for breeding stock of many of our agricultural species of
animals, and are used to make selection decisions. Breeding values are reported in units
of trait measurement and are often expressed as deviations from the current population
average. A dairy bull may have an Estimated Breeding Value (EBV) for milk production
of +1000 kg and is expected to have daughters which have the genetic capability to
produce 500 kg more milk in a lactation than the current population average. See also
Transmitting ability, and Expected Progeny Difference (EPD.
Categorical variables -- Variables which take on discrete values only. An example is
calving ease in cattle where the ease of calving is recorded as Unassisted, Easy pull, Hard
pull and Surgical. Though occurring in discrete categories the underlying genetic control
may be polygenic, as is the case for calving ease; or controlled by a single locus, as is the
case for a trait such as horns and polledness. See continuous variables.
Chi-square. A statistical test of goodness of fit of experimental data to expectation. Used
in genetics to determine if an observed distribution of phenotypes conform to theoretical
expectation, or to chance.
Chromosome -- one of a number of long strands of DNA and associated proteins present
in the nucleus of every cell. The nomenclature used for most species is numeric, with
chromosome number 1 being visually the largest chromosome, e.g. swine have 19 pairs
of chromosomes with 18 pairs of autosomes and one pair of sex chromosomes. The gene
for Porcine stress syndrome has been mapped to chromosome #6 (Grimm et al. 1994).
See autosomes and sex chromosomes.
Clones or clonal line -- A group of genetically identical individuals.
Coefficient of variation -- The phenotypic standard deviation of a trait expressed as a
percent of the mean, used to assess the relative variability of a trait, e.g. the coefficient of
variation for growth rate from 200 to 365 days of age in cattle is about 10%; the
coefficient of variation of litter size in swine is about 20%.
Composite (synthetic breed) -- A breed made up of two or more component breeds and
designed to benefit from hybrid vigour without crossing with other breeds.
Conformation -- Visual or measurable variation in shape or body proportions of animals.
Sometimes assessed with a subjective scoring system as in type scores for dairy cattle.
Contemporary group -- A group of animals that experience a similar environment with
respect to the expression of a trait. Contemporaries express their performance in the same
location and time, are of the same sex and similar age, and are managed alike. An
example of a contemporary group is the group of bulls of the same breed at a bull test
station in the province, tested in the 1997-98 test year.
Continuous variables -- Variables which vary continuously, e.g. animal weight, or milk
production. See Normal distribution.
Correlation (rxy ) -- A measure of the degree of association between two variables (x and
y), varying between -1 and +1. A correlation of 0 means there is no association. In animal
genetics one correlation of interest is the correlation between phenotypes for two traits
expressed by animals, i.e. the phenotypic correlation. The phenotypic correlation between
protein production and fat production in milk of dairy cows is about +.8. See also
additive genetic correlation, environmental correlation and phenotypic correlation.
Covariance -- The covariance between x and y, sometimes written as Cov(x,y). It is a
measure of how two variables vary together. The covariance is used to calculate
correlation and regression coefficients.
Crossbreeding -- Mating systems in which two or more breeds are combined, usually in
a systematic fashion to make use of heterosis and breed complementarity, e.g. three-breed
terminal crossbreeding systems are common in beef cattle and in swine production.
Crossing over -- A reciprocal exchange of DNA segments between members of a pair of
chromosomes.
Culling -- Removal of parent animals from the breeding herd.
Deleterious alleles -- Genes which in their homozygous state (and sometimes in their
heterozygous state as well) have undesirable effects on the individual's viability or
usefulness.
Direct genetic effects -- The effect of an individual's genes on its performance. This term
is used when referring to traits that have both direct genetic effects and maternal effects
on the expression of the trait, e.g. birth weight of mammals has a direct component (due
to growth capacity of the youngster) and a maternal component (due to the uterine
environment provided by the mother).
Diploid -- Organisms or cells which have two members of each type of chromosome. All
higher animals are diploid (the 2n condition). There is a variety of "ploidy" conditions in
plants and in lower animals -- some are haploid (one copy of each chromosome) while
others may have more than two members of each chromosome type, e.g. tetraploids have
four chromosomes of each type (4n).
DNA -- Deoxyribonucleic acid, the molecule that forms the genetic code.
Dominance -- An interaction between alleles at a single locus so that in heterozygous
animals the effect of one allele is greater than the other. The allele with the greater effect
is considered to be dominant over the other allele. See Partial dominance and
Overdominance.
Dominance genetic effects -- The effect that an allele has on animal performance which
depends upon the genotype at the locus. For example the "a" allele may have a different
effect on animal performance in "aa" animals than in "Aa" animals. See Additive genetic
effects.
Dominance genetic variation -- Variability among animals due to the dominance
genetic effects of their genes. See Additive genetic variation.
Effective population size (Ne) -- The size of a population as reflected in its rate of
inbreeding. This can be quite different from the actual population size, particularly when
the number of male parents or female parents is low in each generation (as in the case of
some agricultural animals where artificial insemination is common and the number of
male parents is very small).
Environmental correlation (rExEy) -- A measure of the strength of the relationship
between environmental effects on one trait (Ex) and environmental effects on another trait
(Ey), e.g. the environmental correlation between fat production and protein production by
dairy cows is .9, indicating that many of the same environmental factors (nutrition, etc.)
influence both traits.
Environmental effect (E) -- The effect of external, non-genetic factors on an animal's
performance. There are a number of types of environmental effects, for example,
permanent, temporary, systematic and contemporary group.
Environmental variance -- Variance among animals due to differences in animals in
their environmental effects. Depending on the trait there may be environmental variances
due to a various causes -- permanent, temporary, etc.
Epistasis -- The interaction among alleles at different loci such that the effect of an allele
at one locus depends upon the genotype at some other locus (or loci). Epistatic genetic
effects cannot be inherited because they are the result of allele combinations which are
broken up in the formation of sex cells.
Epistatic genetic variation -- Variation in animal performance as a result of epistatic
genetic effects.
Estimated Breeding Value (EBV) -- An estimate of an animal's breeding value, using
information on the animal's own performance, and often the performance of relatives,
primarily progeny. EBV's are usually less than perfect in accuracy (see Accuracy), except
for a number of dairy sires whose EBV's have an accuracy of .99 (nearly perfect),
because they have thousands of daughters whose performance records are used to
calculate the sire's EBV.
Expected Progeny Difference (EPD) -- A term used primarily by the beef cattle
industry to describe transmitting ability or one-half of an animal's breeding value for a
trait. A beef sire may have an EPD of -2 pounds (most beef data is reported using the
imperial system) for birth weight indicating that its calves are expected to be 2 pounds
lighter than the genetic average at birth. See this Canadian Web site for a listing of EPD's
for beef sires. (Choose "Beef Sires") EPD's for American beef cattle use a "zero" based
on historical base populations so require care in interpretation.
F1 -- the first generation of crosses between two unrelated populations.
F2 -- the offspring of mating F1 individuals among themselves.
Fitness -- The ability of an individual (and its corresponding genotype and phenotype) to
contribute offspring to the next generation. Refers to the ability to survive (or to be
selected as a parent) as well as the number of offspring.
Fixation -- The point at which the frequency of a particular allele becomes 1.
Gamete -- A sex cell, an egg or sperm cell.
Gene -- The basic physical unit of heredity consisting of a DNA sequence at a specific
location on a chromosome. The product of the gene is usually a polypeptide of some form
involved with structural or regulatory activities in cells.
Gene frequency or allele frequency -- The relative frequency of a particular allele in a
population.
Gene map (or linkage map or chromosome map) -- A diagram showing the
chromosomal location of specific genetic markers and genes of interest.
Generation length (L) -- The amount of time required to replace one generation with the
next. In some populations generations occur at discrete intervals of time, e.g. the annual
weeds in my garden go through one generation each year. In most farm animal
populations the generations overlap so that parents may coexist in the same herd with
their offspring who are also parents. Another definition is "the average age of parents
when the offspring which will replace them are born". Generation lengths are typically 46 years in beef cattle as an example. Generation lengths are often different for males and
females.
Genetic correlation -- See Additive genetic correlation.
Genetic marker -- A gene or DNA fragment used to identify alleles at linked loci. See
also Allozymes, Haplotypes and Microsatellites.
Genotype -- This can be used in two different ways: First, it may be used to indicate the
total genetic make-up of an individual. OR Second, it may refer to the combinations of
alleles present at one, or a few loci in an individual.
Genotype by environment (G x E) interaction -- The situation where the performance
of genotypes changes from environment to environment. The most serious G x E
interaction to concern animal breeders is where the change in the performance of a set of
genotypes is such that they change in rank from one environment to another, e.g. the best
performing genotype in environment 1 is the poorest performing genotype in
environment 2.
Genotypic value (G) -- The total effect of an animal's genes (additive, dominance and
epistasis) on its performance for a trait.
Haplotype -- a set of alleles at a closely linked group of loci, so closely linked that the
allelic set behave almost as one allele in terms of inheritance. The Rh blood group system
in humans, for example has three tightly-linked loci, C, D, and E, with two or more
alleles (C, c, etc.) at each locus; common haplotypes are CDe, cde and cDE. The CDE
and cdE haplotypes are rare. Individuals with a genotype, CDE/cdE, are thus very
uncommon. Individuals who are cde/cde are the commonest Rh negative genotype.
Heritability (h2) -- Two definitions; narrow and broad sense. 1. The narrow sense
definition of h2 is the most commonly used with agricultural animals: A measure of the
strength of the relationship between performance (phenotype), and breeding values for a
trait in a population, ranging from 0 to 1. This is often measured as the ratio of additive
genetic variance to phenotypic variance, or as twice the regression coefficient relating
offspring performance to parent performance. The heritability of milk production in dairy
cattle is about .25 or so. 2. The broad sense definition of h2 is as follows: the proportion
of phenotypic variation that is of genotypic origin (i.e. due to all genetic effects, i.e.
additive, dominance and epistatic effects).
Heterosis or hybrid vigour -- The difference in performance of crossbred animals
relative to the average performance of the parental breeds. The genetic basis for hybrid
vigour is heterozygosity, and the dominance effects associated with heterozygous loci;
hybrid animals have more heterozygous loci than their purebred parents. Heterosis is
often reported as a percentage, e.g. heterosis for growth traits seldom exceeds 5%,
indicating that the crossbred group will exceed the average performance of the parental
breeds by 5%.
Heterozygote or heterozygous genotype -- Considering one locus, an animal that has
functionally different alleles at the locus. Hybrid animals tend to have a higher proportion
of their loci in a heterozygous condition than their purebred parents.
Homozygote or homozygous genotype -- Considering one locus, an animal that has
functionally identical alleles at the locus. Inbred animals tend to have a higher proportion
of their loci in a homozygous condition than non-inbred animals.
Inbreeding -- A mating system or process in which related parents are mated to produce
offspring.
Inbreeding coefficient (Fx) -- A measure of the level of inbreeding in an individual, x: 1.
The probable proportion of an individual's loci that contain alleles that are identical by
descent (originated in the same ancestor), and thus in the homozygous state. Used in this
sense the inbreeding coefficient is not an absolute measure of homozygosity, but will be
relative to the base group of animals used in calculation of the coefficient. 2. The
probability that both alleles at a locus are identical by descent.
Inbreeding depression -- The reverse of hybrid vigour -- a decrease in the performance
of inbreds, most noticeably in traits like fertility. The genetic basis for this is the
increased homozygosity of such individuals, and the resultant loss in dominance effects,
and possibly the expression of deleterious recessive alleles in their homozygous state.
Independent culling levels -- In selecting animals for a number of traits, minimum
standards are set for each trait. Selected animals must have trait levels above all
minimums. This is not as effective as using a selection index of multiple traits for
selecton.
Indicator trait -- A trait that may not be of importance in itself, but is used in selection
to improve some genetically correlated trait which may be much more difficult, or
expensive to measure, e.g. type or conformation traits in dairy cattle are used as
indicators of longevity.
Karyotype -- The chromosomal constitution of an individual, e.g. for a pig.
Key equation --The equation relating the rate of genetic change resulting from selection
to four factors: Accuracy of selection, selection intensity, genetic variation and
generation length.
Line -- A group of related animals within a breed.
Line breeding -- The mating of individuals within a particular line; a mating system
designed to maintain a high degree of relationship to a highly regarded ancestor without
causing high levels of inbreeding.
Linkage -- The occurance of two or more loci of interest on the same chromosome.
Linkage is a very important consideration in the use of genetic markers in selection
programs. A genetic marker must be fairly tightly linked to a locus that affects a trait, or
the pairing of the marker allele at the linked locus and the desired allele at the locus of
interest will be lost quickly (in a few generations). Linkage can be a reason for a spurious
genetic correlation between traits, which disappears when linkage groups break up
through recombination.
Locus -- The specific location of a gene on a chromosome.
Major gene -- A gene that has a major influence upon a trait. There are several instances
of this discovered so far for economically important traits in farm animals. See, for
example, the paper by Rothschild et al. (1994) where a major gene affecting litter size in
pigs (the Estrogen Receptor gene (ESR)) is discussed.
Marker assisted selection -- Selection for specific alleles (which affect a trait of interest)
using genetic markers.
Maternal effects -- The effect of a dam of an individual on its performance through the
dam's provision of an environment in which the offspring performs. This is a very
important component in the early growth of mammalian species because of the early
dependence of the offspring on its mother for nutrition, among other things. There are
also genetic and environmental determinants of maternal ability, e.g. milk production by
beef cows is a component of the maternal enviroment provided to the calf; milk
production is affected by the cow's genes as well as the environment in which she lives.
Even non-mammalian species may have important maternal effects. In poultry, the size of
the egg (which is determined by the hen) plays a role in the early growth and survival of
the chick.
Mean --An arithmetic average which can be calculated from continuous variables by
dividing the sum for a set of data by the number of observations. For categorical data
where the categories have numeric values, the mean can be easily calculated by taking a
weighted average of the categories (weighting each category's value by the number of
observations in the category).
Meiosis -- One of the steps in sex cell formation in which the nucleus divides, leaving
one copy of each chromosome in each sex cell.
Mendelian sampling -- The random sampling of parental genes caused by segregation
and independent assortment of alleles during germ cell formation.
Microsatellite -- Very small satellites! (NOT) ... Seriously ... DNA or genetic markers of
a specialized nature where the DNA sequence is highly repetitive. A sequence repeat of a
very simple sequence such as AG may exist as AGAGAGAGAGAG, etc.; or a threenucleotide series such as AAG as AAGAAGAAGAAGAAGAAG, and so on. These are
called satellite DNA because of how the DNA appears in a density gradient while being
analysed. Microsatellites are widely used as markers in agricultural animals. One of the
advantages of microsatellites is that at a given locus they will be highly polymorphic,
with many alleles (more than 10 in some cases); they are thus useful in assessing genetic
diversity of livestock.
Migration -- The movement of individuals into or out of a population. One of the forces
responsible for change in gene frequency.
Most Probable Producing Ability (MPPA) -- A prediction of producing ability. See
Producing ability.
Multiple Ovulation and Embryo Transfer (MOET) -- Production of multiple eggs by
a female (usually in cattle), as a result of hormonal injection, followed by transfer of eggs
into recipient females. This process is part of a technique to increase the rate of genetic
change in populations.
Mutation -- The ultimate source of genetic variation. In a general sense it could include
all genetic changes, including visible chromosomal abnormalities (inversions,
translocations), polyploidy (multiple sets of chromosomes), and changes in DNA at the
molecular level. In a restricted sense mutation refers to changes in DNA at the molecular
level. This is a force which is responsible for change in gene frequency, though the
mutation rate is very low. The mutation rate in polypeptides in humans has been
estimated to be of the order of 10-6, i.e. one in 1,000,000. At the DNA level the mutation
rate would be about 10-8 changes per nucleotide per generation. If the haploid genome in
man contains 3 x 109 nucleotides, this implies that there are about 30 new nucleotide
mutations per human gamete (Hartl and Clarke 1989).
Natural selection -- Selection that occurs in nature independent of human control.
Natural service -- Natural mating.
Normal distribution -- The distribution of observations that appears graphically as a
bell-shaped curve. The horizontal axis represents the level of performance of animals, or
their breeding values, etc., and the height of the curve represents the relative frequency of
observations. What this means is that for a normal distribution, we expect most
observations to cluster near the mean and extreme values far away from the mean are rare
(compare this to the case of a uniform distribution, where the observations are equally
likely over the relevant range, e.g. in a wheel of fortune, all positions are equally likely).
Most continuous variables that we measure on animals are normally-distributed. See
Continuous variables.
Outbreeding -- The mating of unrelated individuals.
Overdominance -- A form of dominance where the performance of the heterozygote
exceeds that of the best homozygote.
Partial dominance -- A form of dominance where the performance of the heterozygote
is intermediate between the two homozygotes, but more closely resembles the
performance of the homozygous dominant type.
Permanent environmental effect (Ep) -- An environmental effect unique to each animal
which permanently affects the expression of a repeated trait.
Phenotype -- The observed appearance or performance of an individual for a trait.
Phenotypic correlation (rPx,Py) -- A measure of the association between performance in
one trait (Px) and performance in another trait (Py). See also Correlation, Additive genetic
correlation and Environmental correlation.
Phenotypic selection -- Selection based on phenotypes of individuals.
Phenotypic selection differential (S) -- The difference between the average performance
of individuals selected to be parents and the average performance of all potential parents,
expressed in units of the trait.
Phenotypic value (P) -- A performance record; a measure of an animal's performance for
a trait.
Phenotypic variance -- The variance of individual performance for a trait in a
population, e.g. the phenotypic variance of weaning weight in beef cattle is about 625 kg2
.
Pleiotropy -- Where a gene or genes affect more than one trait. This is the basis for the
genetic correlation between traits.
Polled -- Without horns.
Polygenic trait -- A trait affected by many genes, with no one gene having a large
influence.
Polymorphic -- Where DNA or genes have at least two forms or alleles in the
population.
Population -- A group of intermating individuals. This term can be used to refer to all
animals in a breed, an entire species or even a single herd or flock.
Population genetics -- The study of factors affecting gene and genotype frequencies in a
population.
Producing ability (PA) -- The performance potential of an individual for a repeated trait.
PA includes the effects of the animal's genotype, as well as the animal's permanent
environmental effect. See Most Probable Producing Ability.
Progeny -- Offspring.
Progeny test -- Records on a number of progeny of an individual are used to estimate the
breeding value of an individual.
Punnett square -- A two-dimensional square used to determine the possible zygotes
from a mating.
Qualitative trait -- A trait in which the phenotypes show discontinuous expression, e.g.
presence or absence of horns; red or black coat colour. See Categorical variables.
Quantitative trait -- A trait in which the phenotypes show continuous (numerical)
expression. See Continuous variables.
Quantitative trait locus (QTL) -- A locus that affects a quantitative trait.
Recombination -- The formation of a new combination of alleles at linked loci as a result
of crossing over.
Recombination loss -- A loss in epistatic effects as a result of breaking up of the linked
loci in advanced generations in interbreeding hybrids (i.e. F2, F3, etc.).
Regression or Regression coefficient (by.x) -- The average change in one variable, y, per
unit change in another variable, x.
Relationship coefficient -- The probable proportion of one individual's genes that are
identical by descent to genes of a second individual.
Repeatability (r) -- There are at least two usages for this term that are quite different so
be careful that you know how it is being used. 1. A measure of the strength of association
between repeated records (phenotypes)of an animal, ranging from 0 to 1. The basis for
this association is that each record that is expressed by an animal will have the same
genotypic effects and permanent environmental effects; this makes them similar.
Repeated records will not be identical, however, because of temporary environmental
effects and herd environmental effects which will be different for each expression of the
trait by an animal. Repeatability is used in various formulae, including formulae to
estimate Producing Ability and formulae to estimate Breeding Values. The repeatability
of milk production is about .50 or so. 2. It is used to refer to the accuracy of EBV's or
EPD's in publications of dairy sire genetic merit.
Repeated trait -- A trait which can be expressed repeatedly by an animal throughout its
life, e.g. milk production in a lactation expressed by dairy cattle.
Replacement rate -- The rate at which newly selected individuals replace existing
parents in a population. This can be between .1 and .2 for females in beef cattle herds.
Retained heterosis -- Hybrid vigour remaining in later generations of hybrids (F2, F3,
etc.). Retained hybrid vigour is commonly expressed as a proportion of the F1
(maximum) hybrid vigour. This is important in the development of composite breeds. See
Composite.
Seedstock -- Breeding stock.
Segregation -- The separation of paired alleles at loci during germ cell formation.
Selection -- The process which determines which individuals become parents, how many
offspring they produce, and how long they remain in the breeding population.
Selection criterion -- An EBV, EPD, phenotypic value, selection index, or other piece of
information forming the basis for selection decisions. See Breeding objective.
Selection differential -- The difference between the average selection criterion of those
individuals selected to be parents and the average selection criterion of all potential
parents, expressed in units of the selection criterion.
Selection index -- A linear combination of phenotypic information and weighting factors
used for genetic prediction.
Selection intensity (i) -- The selection differential expressed in standard deviation units.
Sire -- Male parent.
Standard deviation -- The square root of variance, and just like variance, gives a
measure of population diversity. It is expressed in the same units as the trait is measured
in.
Temporary environmental effect (Et) -- An environmental effect that influences a
single performance record of an animal but does not permanently influence the animal's
performance potential for the repeated trait.
Test cross -- A mating designed to reveal the genotype of an individual for a locus or a
small number of loci.
Trait -- Any observable or measurable characteristic of an animal.
Transgenic -- An individual that has received genetic material by gene transfer.
Transmitting ability -- One-half of an animal's breeding value.
Truncation selection -- Selection of animals, where animals above a given level in the
selection criterion are selected and those below it are rejected.
True value -- An unknown underlying attribute which affects animal performance, e.g.
breeding value, dominance genetic effect, epistatic genetic effect, permanent
environmental effect, temporary environmental effect, etc. We are interested in
estimating some of these for the animals that are candidates for selection (such as
breeding value and producing ability).
Variable -- Any quantity that can take on different numerical values. See Continuous
variables and Categorical variables.
Variance -- A measure of variation in a population. See Phenotypic variance and
Additive genetic variance.
Zygote -- A cell formed from the union of male and female gametes, thus having the full
complement of genes, half from the female parent, half from the male parent.
References
Many of the definitions in this glossary are based on the glossary in the text by
Bourdon.
Bourdon, R.M. 1997. Understanding animal breeding. Prentice Hall, New Jersey.
Grimm, D.R., Golman, T., Holley, R. and Lunney, J.K. 1994. Characterization of a
porcine chromosome 6 specific library. Pages 75-78 in Volume 21 of the Proceedings of
the 5'th World Congress on Genetics Applied to Livestock Production (WCGALP).
Hartl, D.L. and Clark, A.G. 1989. Principles of population genetics. Sinauer
Associates, Inc., Massachusetts.
Rothschild, M.F., Jacobson, C., Vaske, D.A., Tuggle, C.K., Short, T.H., Sasaki, S.,
Echardt, G.R. and McLaren, D.G. 1994. A major gene for litter size in pigs. Pages
225-228 in Volume 21 of the Proceedings of the 5'th WCGALP.