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
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
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
See Table 9-1,2,3 and 42-4 to see the heritability estimates
for several species of livestock
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
15. 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
16. 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
17. Mitosis
The division of cells in the animals body
Allows animals (and us) to grow
Replaced old cells that die
18. 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
20. 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
21. 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
22. 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.
23. 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
24. 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
26. 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
27. 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.
28. 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
30. 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
31. 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
32. 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
33. Heterozygous Gene Pairs
Carry two different genes (alleles)
For example a polled cow may carry the gene pair Pp
34. 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)
35. Predicting Results
Punnett Square
Male gametes on top
Female gametes on the left Male Gametes
side
P P
Female Gametes
P PP PP
P PP PP
36. 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
38. 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
39. Codominance
Occurs when neither allele in a R R
heterozygous condition
dominanates the other and W RW RW
both are fully expressed
Example W RW RW
Roan color in Shorthorn Cattle
R W
R RR RW
W RW WW
40. 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
41. 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
42. Sex Determination: Mammals
Sex of the offspring is determined at X Y
fertilization
Female mammals have two sex
chromosomes in addition to the
regular chromosomes.
They are shown as XX X XX XY
Male mammals have only one sex
chromosome, the other chromosome
of the pair is shown as Y
Thus the male is XY
X XX XY
Sex of offspring is determined by the
male
43. Sex Determination: Birds X
Female determines the sex of Z Z
the offspring
Male carries two sex
chromosomes
Female carries one Z ZZ ZZ
After meiosis all the sperm
cells carry a Z chromosome
and only one-half of the egg
W ZW ZW
cells carry a Z, the other half
carry a W
44. Sex Linked Characteristics
Genes are only carried on sex b b
Z Z
chromosomes
Example is barred color in
chickens
Barred is dominant to black ZB ZB Z b ZB Z b
Result of crossing a barred
female ZB W with a black male
b b
Z Z
W Z bW Zb W
45. 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
46. 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
47. 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
48. 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
49. 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
