1. Livestock Genetics
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2. 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

3. Additive and Non-Additive Gene Effects
Two factors responsible for genetic variation in animals

4. 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
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5. 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

6. 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

7. Traits That Result From Non-Additve Gene Effects
Qualitative
Phenotype is easily identified
Little environmental effect
Genotype can be easily determined

8. 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

9. Selecting Breeding Stock

10. 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

11. 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

12. 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

13. 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

14. The Practical Viewpoint
Wise to use a combination of selection methods

15. Cell Division (Mitosis)
The division of cells in the animals body
Allows animals (and us) to grow
Replaced old cells that die

16. 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

17. Common Livestock Diploid Number
Cattle 30
Swine 19
Sheep 27
Goat 30
Horse 32
Donkey 31
Chicken 39
Rabbit 22

18. So What Happens During Mitosis?
Chromosome pairs are duplicated in each daughter cell

19. 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

20. 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.

21. 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

22. 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

23. Transmission of Characteristics

24. Genes Pass heritable characteristics from one animal to another
Located on the chromosomes
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

25. 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.

26. 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

27. 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

28. 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

29. 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

30. Heterozygous Gene Pairs
Carry two different genes (alleles)
For example a polled cow may carry the gene pair Pp

31. 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)

32. Predicting Results Male Gametes Female Gametes P Punnett Square
Male gametes on top
Female gametes on the left side
Male Gametes P PP
Female Gametes

33. 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

34. Multiple Gene Pairs MALE FEMALE PB Pb pB pb PPBB PPBb PpBB PpBb PPbb

35. 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

36. Codominance
Occurs when neither allele in a heterozygous condition dominanates the other and both are fully expressed
Example
Roan color in Shorthorn Cattle R W RW R W RR RW WW

37. 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

38. 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

39. 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 XX XY

40. 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 ZZ W ZW

41. 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 W
Z W B B b B b B b b b b

42. 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

43. 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

44. 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

45. 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

46. 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