Livestock Genetics

Objectives

Explain how genetics relates to improvement in livestock production

Describe how cell division occursDiagram and explain how animal

characteristics are transmittedDiagram and explain sex determination,

linkage, crossover and mutation

Additive and Non-Additive Gene Effects

Two factors responsible for genetic variation in animals

Additive Gene Effects

Many different genes involved in the expression of the trait

Individual genes have little effect upon the trait

Effects of each gene are cumulative with very little or no dominance between pairs of alleles

Each member of the gene pair has equal opportunity to be expressed

Traits that Result from Additive Gene Effects

Most of the economically important traits Carcass traits Weight gain Milk production

All have moderate to high heritability

Quantative

Environment often influences expression

Difficult to classify phenotypes into distinct categories because they usually follow continuous distribution

Difficult to identify animals with superior genotypes

Non-Additive Gene Effect

Control traits by determining how gene pairs act in different combinations with one another

ObservableControlled by only one or a few pairs of genesTypically one gene pairs will be dominant if

the animal is heterozygous for the trait being expressed.

When combinations of gene pairs give good results the offspring will be better than either of its parents This called hybrid vigor or heterosis

Traits That Result From Non-Additve Gene Effects

QualitativePhenotype is easily identifiedLittle environmental effectGenotype can be easily determined

Heritability Estimates

Heritability: the proportion of the total variation (genetic and environmental) that is due to additive gene effects

Heritability Estimate: expression of the likelihood of a trait being passed from the parent to the offspring

Traits that are highly heritable show rapid improvement

Traits with low heritability make take several generations of animals for desirable characteristics to become strong

Selecting Breeding Stock

Selecting Breeding Stock

Computer programs and data bases developed by Universities available

Breed associations provide informationBreeding values and Expected Progeny

Difference (EPD) help producers make fast genetic decisions

Also 3 types of systems that producers can use to select breeding animals Tandem Independent Culling Levels Selection Index

Tandem

Traits are selected for one at a time and selection for the next trait does not begin until the desired level of performance is achieved with the first.

Animals with one desirable trait but with other undesirable ones may be kept for breeding

For the most profitable production, emphasis has to be placed on several traits when selecting breeding stock; Tandem selection does not do this!

Simple to use but not recommendedLeast effective of the selection methods

Independent Culling Levels

Establishes a performance level for each trait in the selection program. The animal must achieve that level to be kept for breeding stock.

Selection for the breeding program is based on more than one trait

Disadvantage to this type of selection is that superior performance in one trait cannot offset a trait that does not meet selection criteria

Most effective when selecting for only a small number of traits

Second most effective method of selection Most widely used

Selection Index

Index of net merit is established that gives weight to traits based on the economic importance, heritability and genetic correlations that may exists between the traits

Does not discriminate against a trait with only slightly substandard performance when it is offset by high performance in another trait

Provides more rapid improvement in overall genetic improvement in the breeding group

Extensive records are required to establish the index

Is the most effective method of achieving improvement in genetic merit

The Practical Viewpoint

Wise to use a combination of selection methods

Cell Division (Mitosis)

The division of cells in the animals bodyAllows animals (and us) to growReplaced old cells that die

Chromosomes

Occur in pairs in the nucleus of all body cells except the sperm and ovum

Each parent contributes to one-half of the pair

The number of pairs of chromosomes is called the diploid number

The diploid number varies species to species but is constant for each species of animal

Common Livestock Diploid Number

Cattle 30Swine 19Sheep 27Goat 30Horse 32Donkey 31Chicken 39Rabbit 22

So What Happens During Mitosis?

Chromosome pairs are duplicated in each daughter cell

What Causes Animals to Age

Ability of cells to continue to divide is limitedAt the end of each chromosome in the

nucleus there is specific repeating DNA sequence called a telomere

Each time the cell divides some the of telomere is lost

As the animal ages the telomere becomes shorter and eventually the cell stops dividing

Meiosis

When cells divide by mitosis the daughter cells contain two of each type of chromosome, they are diploid

Reproductive cells are called gametesThe male gametes is the sperm, the female gamete

is the eggWhen the sperm and egg unite they form a zygoteIf each gamete were diploid the zygote would have

twice as many chromosomes as the parents, since that can not be there is a mechanisms that reduces the number of chromosomes in the gametes by one-half

This specialized type of cell division is called meiosis.

What Happens During Meiosis?

Chromosome pairs are divided so that each gamete has one of each type of chromosome

The gamete cell has a haploid number of chromosomes

The zygote that results from the union of the gametes has a diploid number of chromosomes

Fertilization

Takes place when a sperm cell from a male reaches the egg cell of a female

The two haploid cells (the sperm and the egg) unite and form one complete cell or zygote

Zygote is diploid, it has a full set of chromosome pairs

This results in many different combinations of traits in offspring

Transmission of Characteristics

Genes

Pass heritable characteristics from one animal to another

Located on the chromosomes Occur in pairs just like the chromosomeGene pairs that are identical are homozygous and

they control the trait in the same wayIf the gene pairs code for different expression of the

same trait they are heterozygous and the genes are called alleles For example one gene may code for black and another for

red.The same trait is being affected but the alleles are

coding for different effectsGenotype is the combination of genes that an

individual poses

Genes

Provide the code for the synthesis of enzymes and other proteins that control the chemical reactions in the body

These reactions determine the physical characteristics

The physical appearance of an animal, insofar as its appearance is determined by its genotype, is referred to as its phenotype

Environmental conditions can also influence physical characteristics For example; the genotype of a beef animal for rate of gain

determines a range for that characteristic in which it will fall but the ration the animal receives will determine where it actually falls in that range.

Genes

Some traits controlled by a singe pairMost traits however are controlled by many

pairs Carcass traits, growth rate, feed efficiency are all

controlled by many gene pairs

Dominant and Recessive Genes

In a heterozygous pair the dominant gene hides the effect of its allele

The hidden allele is called a recessive geneWhen working problems involving genetic

inheritance the dominant gene is usually written as a capital letter and the recessive gene is written as a lowercase letter

For example the polled condition in cattle is said to be dominant so it would be written as Pp

Example Dominant & Recessive Traits

Black is dominant to red in cattleWhite face is dominant to color face in cattleBlack is dominant to brown in horsesColor is dominant to albinismRose comb is dominant to single comb (chicken)Pea comb in chickens is dominant to single combBarred feather pattern in chickens is dominant

to nonbarred feather—the dominant gene is also sex-linked

Normal size in cattle is dominant to “snorter” dwarfism

Homozygous Gene Pairs

Homozygous gene pair carries two genes for a trait For example a polled cow might carry a gene pair PP

or a horned cow must carry the gene pair pp For a cow to have horns she must carry two recessive

genes

Heterozygous Gene Pairs

Carry two different genes (alleles)For example a polled cow may carry the gene

pair Pp

Six Basic Crosses

Homozygous x Homozygous (PP x PP) (Both Dominant)

Heterozygous x Heterozygous (Pp x Pp)Homozygous x Heterozygous (PP x Pp)Homozygous (dominant) x Homozygous

(recessive) (PPxpp)Heterozygous x Homozygous (recessive) (Pp

x pp)Homozygous (recessive) x Homozygous

(recessive) (pp x pp)

Predicting Results

Punnett SquareMale gametes on topFemale gametes on

the left sideP P

P PP PP

P PP PP

Male Gametes

Fem

ale

Gam

etes

Multiple Gene Pairs

When you have more than 1 gene combination you must account for all the possible combinations

For example you are crossing a polled black bull (PpBb) and a polled black cow (PpBb) both are heterozygous for polledness and color

Multiple Gene Pairs

PB Pb pB pb

PB PPBB PPBb PpBB PpBb

Pb PPBb PPbb PpBb Ppbb

pB PpBB PpBb ppBB ppBb

pb PpBb Ppbb ppBb ppBb

MALEFE

MAL

E

Incomplete Dominance

Occurs when the alleles at a gene locus are only partially expressed

Usually produces a phenotype in the offspring that is intermediate between the phenotypes that either allele would express

CodominanceOccurs when neither

allele in a heterozygous condition dominanates the other and both are fully expressed

Example Roan color in Shorthorn

Cattle

R R

W RW RW

W RW RW

R W

R RR RW

W RW WW

Sex-Limited Genes

The phenotypic expression of some genes is determined by the presence or absence of one of the sex hormones

Limited to one sexExample: Plumage patterns in male and

female chickens Males neck and tail feathers are long, pointed and

curving

Sex-Influenced Genes

Some traits are expressed in one sex and recessive in the other

In humans male pattern baldness is an example

In animals horns in sheep and color spotting in cattle Horns are dominant in male sheep and recessive in

females

Sex Determination: Mammals

Sex of the offspring is determined at fertilization

Female mammals have two sex chromosomes in addition to the regular chromosomes. They are shown as XX

Male mammals have only one sex chromosome, the other chromosome of the pair is shown as Y Thus the male is XY

Sex of offspring is determined by the male

X Y

X XX XY

X XX XY

Sex Determination: Birds X

Female determines the sex of the offspring

Male carries two sex chromosomes

Female carries oneAfter meiosis all the

sperm cells carry a Z chromosome and only one-half of the egg cells carry a Z, the other half carry a W

Z Z

Z ZZ ZZ

W ZW ZW

Sex Linked Characteristics

Genes are only carried on sex chromosomes

Example is barred color in chickens

Barred is dominant to black

Result of crossing a barred female Z W with a black male Z Z

Z Z

Z Z Z Z Z

W Z W Z W

B

b b

b b

B b b

b b

B B

Linkage

Tendency for certain traits to stay together in the offspring

The closer the genes are located together on a chromosome the more likely they are to stay together

Crossover

May result in the predictions of mating not always happening

During one stage of meiosis the chromosomes line up very close together. Sometimes the chromosomes cross over one another and split

This forms new chromosomes with different combinations of genes

The farther apart two genes are on a chromosomes the more likely they are end up in new combination

Mutation

Generally genes are not changed from parent to offspring

However, sometimes something happens that causes genes to change

When a new trait is shown which did not exist in either parent is called mutation

Radiation will cause genes to mutateSome mutations are beneficial, some harmful and

other are of no importanceVery few mutations occur and are not depended on

for animal improvementPolled Hereford cattle are thought to be the result

of a genetic mutation

Summary

Livestock improvement is the result of using the principles of genetics

Gregor Mendel is considered the father of genetics The amount of difference between parents and offspring is

caused by genetics and the environment Heritability estimates are used to show how much of a

difference in some traits might come from genetics Animals grow by cell division Ordinary cell division is called mitosis During mitosis each new cell is exactly like the old cell Reproductive cells are called gametes Gametes divide by meiosis Male gamete is the sperm Female gamete is the egg

Summary

Fertilization occurs when the sperm cell penetrates the egg and the chromosome pairs are formed again when fertilization takes place

Genes control an animals traits Some genes are dominant and some are recessive Animals may carry two dominant or two recessive genes for a

trait. They are called homozygous pairs Animals may also carry a dominant and recessive gene pair.

They are called heterozygous pairs Sex of mammals is determined by the male Sex of birds is determined by the female Some characteristics are sex linked and are located on the sex

chromosome Crossover occurs when chromosomes exchange genes Genes are sometimes changed by mutation and they are of little

value in improving livestock