Glossary Of Sheep Breeding Terminology

Agdex#:	430
Publication Date:	October 2000
Order#:	00-079
Last Reviewed:	October 2000
History:	New
Written by:	Delma Kennedy - Sheep Specialist - Genetics, Reproduction and Performance Programs/OMAFRA

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

Accuracy

A measure of how good and close a calculated estimate of an animal’s 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 50–day old ram lamb raised as a single from a mature ewe (4–5 years of age).

Adjusted 100-Day Weight

This weight is the 50–day 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 100–day weighings:

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B

Backcross

Backcross is the mating of a two–breed animal back to one of the two parental breeds.

C

Carrier

A heterozygous animal with one recessive and one dominant gene for a particular gene pair.

Central Test

A central testing location where animals from different flocks are gathered to evaluate differences in performance traits under uniform management and environmental conditions.

Chromosome

A DNA molecule. Genes are located on the chromosomes. Sheep have 27 pairs of chromosomes.

Closed Flock

An existing flock that does not introduce any outside breeding stock.

Collateral Relatives

Relatives that are not progeny or parents (descendants or ascendants). For example, brothers.

Conception Rate

The number of ewes that lamb compared to the number of ewes exposed for breeding. This is usually expressed as a percentage.

Congenital

A condition that exists at birth.

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Contemporary Group

A genetically similar group of sheep born in a particular time period and raised under the same management and environmental conditions.

Correlation

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.

Crossbreeding

Mating of different breeds to produce multi-breed offspring.

D

Deviation

The difference between the average performance of the group and the performance of an individual in the group.

Direct Effect

That portion of a trait due to an individual’s genotype.

Dominance

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.

E

Environment

All conditions that affect an animal’s performance that are not genetic.

Estimated Breeding Value (EBV)

An estimate of an animal’s genetic merit for a particular trait based on the individual’s performance and the performance of relatives. This value is expressed as a difference from the breed average.

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Expected Progeny Difference (EPD)

The expected difference between the progeny performance of a breed average animal and the performance of an individual’s progeny for any given trait(s).

An animal’s EPD is ½ of it’s EBV and is equal to ½ (Sire EPD + Dam EPD).

F

F1

Progeny resulting from the mating of a purebred ram and a purebred ewe of different breeds.

G

Gene

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.

Generation Interval

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.

Genetic Correlation

Relationships between traits that arise because some of the same genes affect both traits.

Genetic Merit

The genetic worth of an animal for a particular trait can be expressed as an EPD, EBV or ABC.

Genetic Progress

An increase in the average genetic merit of a population for a particular trait as a result of selective breeding.

Genotype

The actual genetic make-up of an individual as determined by its genes.

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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.

H

Half-sibs

Individuals who have the same sire or dam (i.e. half brothers and half sisters).

Heredity

The transmission of characteristics from parents to their offspring through genes.

Heritability

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 individual’s 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.

Heterozygous

Each gene of a specific gene pair is different in an individual (i.e. Aa).

Homozygous

Each gene of a specific gene pair is the same in an individual (i.e. AA).

I

Inbreeding

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 recessive genes.

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L

Linebreeding

A form of inbreeding which increases the average relationship of the individuals in a flock to an outstanding ancestor or line of ancestors.

Linecross

Progeny produced by crossing two or more inbred lines.

M

Maternal Effect

The effect that a dam’s maternal ability may have on a trait. For example, the effect of milk yield on weaning weight.

Maternal Sires

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.

Mean

The average value for a trait of a group.

Multiple Trait Selection

Selection for more than one trait at the same time.

N

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.

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O

Outcrossing

Mating of individuals within a breed that are not closely related.

P

Pedigree

A list of an individual’s ancestors.

Performance Test

Systematic collection of performance information for use in decision making to improve genetic merit, productivity, efficiency and profitability.

Phenotype

The observed or measured expression of a trait for an individual (i.e. weaning weight, birth type, etc.). Phenotype is equal to genotype plus environment effects.

Phenotypic Correlation

A correlation between two traits caused by genetic plus environmental factors influencing both traits.

Prepotent

The ability of a parent to pass traits to it’s 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.

Progeny Testing

The evaluation of an individual’s genotype using the performance records of it’s progeny.

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Q

Qualitative Traits

Traits which show a sharp distinction between phenotypes. For example, polled and horned. Usually only one or few gene pairs are involved.

Quantitative Traits

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 involved.

R

Random Mating

A mating system where all ewes have the same chance of being mated to any ram used.

Rate of Genetic Improvement

Depends on:

1. Number of traits being selected at one time
2. Heritability of the traits
3. Genetic correlations between the traits selected
4. Selection differentials
5. Generation interval in the flock. The rate of improvement is usually expressed per year.

Recessive Genes

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.

Rotational Crossbreeding

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 ewe’s phenotype.

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S

Selection

The process of deciding which animals will be parents of the next generation.

Selection Differential

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.

T

Terminal Sires

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.

Trait

Any measurable or observable characteristic of an animal.

V

Variation

The amount of difference observed or measured for a trait in a group of animals.

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