Download Genetics of Animal Breeding

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
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Additive and Non-Additive Gene
Effects
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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
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Traits that Result from Additive
Gene Effects
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Most of the economically important traits
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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
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Traits That Result From NonAdditve Gene Effects
Qualitative
 Phenotype is easily identified
 Little environmental effect
 Genotype can be easily determined
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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
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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
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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
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Independent Culling Levels
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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
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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
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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
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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
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Mitosis
The division of cells in the animals body
 Allows animals (and us) to grow
 Replaced old cells that die
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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
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Common Livestock Diploid Number
Cattle 30
 Swine 19
 Sheep 27
 Goat 30
 Horse 32
 Donkey 31
 Chicken 39
 Rabbit 22
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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
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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
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Meiosis
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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
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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
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Transmission of
Characteristics
Genes
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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
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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
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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
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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
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Carcass traits, growth rate, feed efficiency are all controlled by
many gene pairs
Coding Genetic Information
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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
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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
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Homozygous Gene Pairs
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Homozygous gene pair carries two genes for a trait
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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
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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)
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Predicting Results
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Punnett Square
Male gametes on top
Female gametes on the left
side
Male Gametes
Female Gametes
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P
P
P
PP
PP
P
PP
PP
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
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Multiple Gene Pairs
FEMALE
MALE
PB
Pb
pB
pb
PB
PPBB
PPBb
PpBB
PpBb
Pb
PPBb
PPbb
PpBb
Ppbb
pB
PpBB
PpBb
ppBB
ppBb
pb
PpBb
Ppbb
ppBb
ppBb
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
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Codominance
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Occurs when neither allele in a
heterozygous condition
dominanates the other and
both are fully expressed
Example
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R
R
W
RW
RW
W
RW
RW
R
W
R
RR
RW
W
RW
WW
Roan color in Shorthorn Cattle
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
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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
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Horns are dominant in male sheep and recessive in females
Sex Determination: Mammals
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Sex of the offspring is determined at
fertilization
Female mammals have two sex
chromosomes in addition to the
regular chromosomes.
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Y
X
XX
XY
X
XX
XY
Male mammals have only one sex
chromosome, the other chromosome
of the pair is shown as Y
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They are shown as XX
X
Thus the male is XY
Sex of offspring is determined by the
male
Sex Determination: Birds X
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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
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Genes are only carried on sex
chromosomes
Example is barred color in
chickens
Barred is dominant to black
Result of crossing a barred
female ZB W with a black male
b
b
Z Z
Z
b
Z
b
ZB
ZB Z b
ZB Z b
W
Z bW
Zb W
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
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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
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Mutation
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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
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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
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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