Transcript Heritability Estimate

Livestock Genetics
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
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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
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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
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
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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
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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
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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
Cell Division (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
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
Meiosis
 When cells divide by mitosis the daughter cells contain two
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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
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
 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
 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 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 sexlinked
 Normal size in cattle is dominant to “snorter”
dwarfism
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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
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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
Male Gametes
 Female gametes on the
Female Gametes
left side
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
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
Codominance
R
R
W
RW
RW
W
RW
RW
R
W
R
RR
RW
W
RW
WW
 Occurs when neither allele
in a heterozygous
condition dominanates
the other and both are
fully expressed
 Example
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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
X
Y
X
XX
XY
X
XX
XY
 Sex of the offspring is
determined at fertilization
 Female mammals have two sex
chromosomes in addition to the
regular chromosomes.
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They are shown as XX
 Male mammals have only one
sex chromosome, the other
chromosome of the pair is
shown as Y
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Thus the male is XY
 Sex of offspring is determined
by the male
Sex Determination: Birds X
Z
Z
Z
ZZ
ZZ
W
ZW
ZW
 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
Sex Linked Characteristics
 Genes are only carried on
sex chromosomes
 Example is barred color in
chickens
ZB
 Barred is dominant to
black
B
 Result of
crossing a
b
b
barred female Z W with a
W
black male Z Z
Z
b
Z
b
ZB Z b
ZB Z b
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
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
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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
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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
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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