livestock genetics. objectives explain how genetics relates to improvement in livestock production...
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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 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
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
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
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