Glossary Of Sheep Breeding Terminology
There are certain genetic terms used to describe genetic concepts. This
glossary will help you understand them.
A B C
D E F G
H I J K L M
N O P Q
R S T U V
W X Y Z
A measure of how good and close a calculated estimate of an animals
genetic value is compared to the unknown true genetic value.
Across Breed Comparison (ABC)
An estimate of genetic merit. It indicates the expected difference between
the performance of progeny of an individual ram and the average of all
rams tested regardless of breed.
Additive Genes Effects
Members of a gene pair that have the ability to be expressed equally
are additive genes. The gene pair is expressed as the sum of the individual
effects of the genes in the pair.
Adjusted 50-Day Weight
This weight is calculated from an actual weight taken between 35 and
65 days of age. All lamb weights are then adjusted to the equivalent of
a 50day old ram lamb raised as a single from a mature ewe (45
years of age).
Adjusted 100-Day Weight
This weight is the 50day adjusted weight plus the postweaning 50-day
weight gain (postweaning ADG x 50).
Average Daily Gain (ADG)
ADG is the amount of body weight change of an animal per day. For example,
between the 50 and 100day weighings:
(100 Day Weight 50 Day Weight) ÷ 50 Days or ADG =
Backcross is the mating of a twobreed animal back to one of the
two parental breeds.
A heterozygous animal with one recessive and one dominant gene for a
particular gene pair.
A central testing location where animals from different flocks are gathered
to evaluate differences in performance traits under uniform management
and environmental conditions.
A DNA molecule. Genes are located on the chromosomes. Sheep have 27 pairs
An existing flock that does not introduce any outside breeding stock.
Relatives that are not progeny or parents (descendants or ascendants).
For example, brothers.
The number of ewes that lamb compared to the number of ewes exposed for
breeding. This is usually expressed as a percentage.
A condition that exists at birth.
A genetically similar group of sheep born in a particular time period
and raised under the same management and environmental conditions.
A measure of how two traits relate to each other. Correlation coefficients
are expressed in the range 1.00 to +1.00.
Traits which are negatively correlated act opposite, as one trait increases
the other decreases. Positively correlated traits, both traits increase
or decrease together. A zero correlation means as one trait changes, there
is no consistent response of the other trait.
Mating of different breeds to produce multi-breed offspring.
The difference between the average performance of the group and the performance
of an individual in the group.
That portion of a trait due to an individuals genotype.
A dominant gene affects the appearance of the sheep (phenotype). A dominant
gene from only one parent will change the phenotype, even if the corresponding
gene from the other parent is different.
All conditions that affect an animals performance that are not
Estimated Breeding Value (EBV)
An estimate of an animals genetic merit for a particular trait
based on the individuals performance and the performance of relatives.
This value is expressed as a difference from the breed average.
Expected Progeny Difference (EPD)
The expected difference between the progeny performance of a breed average
animal and the performance of an individuals progeny for any given
An animals EPD is ½ of its EBV and is equal to ½
(Sire EPD + Dam EPD).
Progeny resulting from the mating of a purebred ram and a purebred ewe
of different breeds.
The basic genetic unit of heredity that occurs in pairs in an individual,
but is passed to progeny from one parent as a single unit.
Average age of parents when the animals that will replace them in the
flock are born. This represents the average turnover rate of a flock.
Relationships between traits that arise because some of the same genes
affect both traits.
The genetic worth of an animal for a particular trait can be expressed
as an EPD, EBV or ABC.
An increase in the average genetic merit of a population for a particular
trait as a result of selective breeding.
The actual genetic make-up of an individual as determined by its genes.
Genotype Environment Interaction
Changes in the ranking of performance of genotypes in different environments.
For example, one genotype may perform the best in one environment and
only average in another environment.
Individuals who have the same sire or dam (i.e. half brothers and half
The transmission of characteristics from parents to their offspring through
Is the proportion of a measured or observed trait that is transmitted
to the offspring by genes. Therefore, the higher the heritability, the
more likely the individuals actual performance will be passed to
offspring and response due to selection for that trait will be faster.
Heterosis (Hybrid Vigour)
The increase in performance observed for a specific crossbred above the
average performance of the parental lines.
Each gene of a specific gene pair is different in an individual (i.e.
Each gene of a specific gene pair is the same in an individual (i.e.
Production of progeny from closely related parents. Inbreeding increases
the number of homozygous gene pairs and decreases the number of heterozygous
gene pairs. Inbreeding increases prepotency and the expression of undesirable
A form of inbreeding which increases the average relationship of the
individuals in a flock to an outstanding ancestor or line of ancestors.
Progeny produced by crossing two or more inbred lines.
The effect that a dams maternal ability may have on a trait. For
example, the effect of milk yield on weaning weight.
Sires that are used in a crossbreeding program to sire replacement females
for the flock. These sires have strong maternal traits to produce the
next generation of productive ewes.
The average value for a trait of a group.
Multiple Trait Selection
Selection for more than one trait at the same time.
Nonadditive Gene Effects
Favourable effects produced by specific gene combinations in which the
members of the gene pairs are not equally expressed. This is the primary
cause of heterosis.
Normal Distribution (Bell Curve)
A graph of all possible performance levels for a trait. The number of
individuals displaying each performance level usually forms a normal distribution.
Mating of individuals within a breed that are not closely related.
A list of an individuals ancestors.
Systematic collection of performance information for use in decision
making to improve genetic merit, productivity, efficiency and profitability.
The observed or measured expression of a trait for an individual (i.e.
weaning weight, birth type, etc.). Phenotype is equal to genotype plus
A correlation between two traits caused by genetic plus environmental
factors influencing both traits.
The ability of a parent to pass traits to its progeny so that they
resemble the parent more than usual. Inbred sheep are usually more prepotent
than outbred sheep because more homozygous gene pairs are present.
The evaluation of an individuals genotype using the performance
records of its progeny.
Traits which show a sharp distinction between phenotypes. For example,
polled and horned. Usually only one or few gene pairs are involved.
Traits that do not show a sharp distinction between phenotypes. There
is a gradual variation from one phenotype to another, for example, birth
weight. Usually, environmental effects as well as many gene pairs are
A mating system where all ewes have the same chance of being mated to
any ram used.
Rate of Genetic Improvement
- Number of traits being selected at one time
- Heritability of the traits
- Genetic correlations between the traits selected
- Selection differentials
- Generation interval in the flock. The rate of improvement is usually
expressed per year.
Recessive genes only affect the phenotype when present in a homozygous
condition. Therefore, the recessive gene must be received from both parents
before the recessive phenotype will be observed.
Systematic crossing of two or more breeds in which the crossbred ewes
are mated to rams of the breed contributing the least genes to that ewes
The process of deciding which animals will be parents of the next generation.
The difference between the average for a trait in replacement animals
and the average of the group from which the replacements were chosen.
The expected response from selection equals selection differential times
heritability of a trait.
Sibs (Full Sibs)
Brothers and sisters from the same sire and dam.
Sires that are used in a crossbreeding program where all offspring are
marketed. This system allows for maximum heterosis and breed complementarity.
Usually an F1 female with strong maternal traits is bred to a terminal
sire that imparts carcass quality and growth ability to the offspring
to be marketed. But, a supply of F1 females is necessary.
Any measurable or observable characteristic of an animal.
The amount of difference observed or measured for a trait in a group