Genetics of Animal Breeding Animal Science II Unit 9.

Genetics of Animal Breeding

Animal Science II

Unit 9

Objectives Explain how genetics relates to improvement in livestock

production Describe how cell division occurs Diagram and explain how animal characteristics are

transmitted Diagram 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 Observable Controlled by only one or a few pairs of genes Typically 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 Qualitative Phenotype is easily identified Little environmental effect Genotype 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 See Table 9-1,2,3 and 42-4 to see the heritability estimates

for several species of livestock

Selecting Breeding Stock

Selecting Breeding Stock Computer programs and data bases developed by

Universities available Breed associations provide information Breeding 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 recommended Least 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

The Cell and Cell Division Body is made up of millions of cells Cells are the most basic and the smallest part of the body

that are capable of sustaining the processes of life Fig 9-1

The Parts of Cell Protoplasm- makes up most of the cell Nucleus- contains the chromosomes that contain the

genes, it also controls the cells metabolism, growth and reproduction

Cytoplasm- surrounds the nucleus and contains mitochondria, lysosmes, Golgi apparatuses, ribosomes

Cell membrane- semipermeable, surrounds the nucleus and cytoplasm

Mitosis The division of cells in the animals body Allows animals (and us) to grow Replaced 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 30 Swine 19 Sheep 27 Goat 30 Horse 32 Donkey 31 Chicken 39 Rabbit 22

So What Happens During Mitosis? Chromosome pairs are duplicated in each daughter cell Figure 9-2 p. 196 shows a cell going through the 4 typical

stages of cell division

What Causes Animals to Age Ability of cells to continue to divide is limited At 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 This causes the animal to eventually die of old age if it

doesn’t die of some other cause first

Meiosis When cells divide by mitosis the daughter cells contain two of each

type of chromosome, they are diploid Reproductive cells are called gametes The male gametes is the sperm, the female gamete is the egg When the sperm and egg unite they form a zygote If 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 Composed of DNA Occur in pairs just like the chromosome Gene pairs that are identical are homozygous and they control the

trait in the same way If 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 effects

Genotype 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 pair Most traits however are controlled by many pairs

Carcass traits, growth rate, feed efficiency are all controlled by many gene pairs

Coding Genetic Information Read p. 199-200 stop at Dominant and Recessive Genes

Dominant and Recessive Genes In a heterozygous pair the dominant gene hides the effect

of its allele The hidden allele is called a recessive gene When 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 cattle White face is dominant to color face in cattle Black is dominant to brown in horses Color is dominant to albinism Rose comb is dominant to single comb (chicken) Pea comb in chickens is dominant to single comb Barred 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 Square Male gametes on top Female 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

Codominance Occurs 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 sex Example: 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 one After 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

B

b

Z Z

Z Z Z Z Z

W Z W Z W

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 mutate Some mutations are beneficial, some harmful and other are of no

importance Very few mutations occur and are not depended on for animal

improvement Polled 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