Vet Science
Genetics
Definition of Genetics
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A branch of biology which studies heredity
and variation in organisms.
Genetics studies the transmission of genes
from one generation to another.
A blueprint of traits and characteristics is
established for the new offspring from the
genes transferred from both parents.
The genotype is the genetic makeup while
the phenotype is the physical makeup.
Genes and Chromosomes
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Chromosomes (found in pairs) are
contained in the nucleus of every cell.
Within the chromosomes are smaller
units called genes.
Genes contain the information that
control all of the biochemical processes
(life processes) of the cell.
DNA – Deoxyribonucleic Acid
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Genetic codes of species consist of
distinct DNA.
DNA is a complex molecule composed
of nucleotides joined together with
phosphate sugars.
Simply, it is a nucleic acid which
contains the genetic instructions used
in the development and functioning all
living organisms other than viruses.
Nucleotide is building block of nucleic
acids. Each nucleotide is composed of
sugar, phosphate, and one of the four
bases.
DNA Cont’d
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The main role of DNA molecules is long
term storage of information.
The segments of DNA which carry the
genetic information are called genes.
Other DNA sequences have structural
purposes or are involved with the use of
the genetic information.
Double Helix of DNA
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Consists of two long polymers of simple units called
nucleotides with backbones made of sugars and phosphate
groups held together by ester bonds.
The two strands run opposite directions to each other and
are said to be anti-parallel.
Attached to each sugar is one of 4 types of bases. It is
the sequence of these bases along the backbone of the
helix which encodes the information.
The 4 bases are divided into two groups: purines,
(adenine, A and guanine, G) and pyrimidines, (thymine,T
and cytosine, C)
Helix Cont’d
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The DNA is a sequence of the base pairs that
represents the code for a specific gene.
The sequence of the 4 bases arranged in
pairs with a the pairs of the allele provides
256 combinations.
Consider the many different traits and the
multiple genes and alleles make for
exponentially of the complexity of DNA.
Chromosomes
Organized structures of DNA.
 Contained in the nucleus of the cell.
 Found in pairs.
 Numbers of pairs are specie specific.
 Humans – 46, Cattle – 60, Swine – 38,
Sheep – 54, Goats – 60, Horses – 78
Dog – 78, Cats – 38,
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Genes
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Genes contain the information that
controls all of the biochemical processes
of the cell.
The gene codes are for the synthesis of
specific proteins of the cell.
Genome
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Definition - The complete genetic
material of an organism.
Genome bases are build to preserve
species in as pure a state as possible.
DNA Replication
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Definition – the process of making a
copy of the DNA molecule.
DNA Replication
Principles of Inheritance
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Gregor Mendel discovered the principles of
inheritance using peas as a subject.
Alleles affect the same trait, but each allele
causes the production of a different protein
and how the trait is expressed.
The locus of alleles on a chromosome is said
to be homozygous if the alleles are
identical. Unlike alleles at the locus on the
chromosome are called heterozygous.
Definition of Inheritance
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The method by which alleles are passed
from one generation to another is called
inheritance.
Gametes are produced from
reproductive cells by the parent.
Each gamete contains a single allele for
each gene or ½ of the genetic code of
the parent.
Genotypes and Phenotypes
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When gametes are combined during
fertilization, a complete set of genetic code is
present. This complete set contains the
genetic traits of the new individual is called
the genotype of the animal.
The physical traits which are expressed from
the genetic code present or the physical
appearance of the animal is called its
phenotype.
Two Principles of Genetics
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Principle of segregation – Mendel’s
first law states that when gametes are
formed, the genes at given locus
separate so that each is incorporated
into different gametes.
Alleles separate so that only one is
found in any particular gamete.
Principles Cont’d
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Principle of independent assortment –
states that in the formation of gametes,
separation of a pair of genes is independent
of the separation of other pairs.
When the two principles are applied together,
they provide a means for randomization of
alleles within the gametes. Thus, our
understanding of how variation exists within a
population.
Gametes
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Male gametes are the sperm cells.
Female gametes are the egg cells.
They are the germ cells of reproduction.
Each normal body tissue cell (somatic
cell) has 1 pair of sex chromosomes.
The other chromosomes within the
somatic cell are called autosomes.
Haploid and Diploid Numbers
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All somatic cells contain a diploid (2n)
number of chromosomes.
The germ cells, sperm and egg, contain
a haploid (1n) number.
Types of Cell Division
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Mitosis is the process of somatic cell
division.
It is replication of cells and responsible for
the maintenance of body cells, tissues and
organs. In addition, mitosis is responsible for
growth of young animals.
The cell contains a diploid number of
chromosomes and through the replication
process two cells emerge with diploid
numbers.
Stages of Mitosis
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Prophase
Metaphase
Anaphase
Telophase
YouTube video
Meiosis
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Meiosis is responsible for the process of
gametogenesis or the formation of
gametes.
Two processes of gametogenesis are:
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Spermatogenesis – sperm cells
Oogenesis – egg cells
YouTube video
Gene Expression
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Dominant genes – one member of each
gene pair is expressed to the exclusion
of the other.
Recessive genes – one member of each
gene pair is only expressed when the
dominant allele is absent from the
animal genome.
Genetic Expression Possibilities
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Homozygous dominant (RR)
Homozygous recessive (rr)
Heterozygous dominant (Rr)
Codominance – both alleles are expressed in the
phenotype when present in a heterozygous state. Roan
shorthorns
Incomplete dominance – no dominance and the
heterozygous phenotypic state is an intermediate. Gray
horse, palomino horse
Epistasis – when gene expression is influenced by the
presence of another. Bay horse (brown with black tail),
chestnut horse
Sex Linked or Related
Inheritance
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XX chromosomes are female
XY chromosomes are male
The passage of Y with appropriate gene
expression provides easy determination of the
phenotype of the male offspring.
The passage of X masks the genotype of the
offspring and expression phenotypically is
seen but not necessarily projectable for
future offspring.
Sex Influenced Inheritance
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Best example: Polled and Horned
Dorset sheep
HH or homozygous dominant – both
male and female have horns.
Hh or heterozygous individuals – male
has scurs while the female is polled.
hh or homozygous recessive individuals
– all are polled.
Population Genetics
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Is the study of how gene and genotypic frequencies
change within a given population.
Gene frequency is defined as the proportion of loci in
a population that contain a particular allele.
Phenotypic frequency refers to how often we see the
particular allele. It is expressed as a % or as a
decimal.
What we see is usually an indicator of the genetic
make up of the individual. Remember that what we
see may or may not be what we have present
genetically.
Making Genetic Change to the
Population
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Mutations and genetic drift can be bring about
change in the population.
Mutations are changes in the chemical composition of
gene that alters the DNA of the individual.
Genetic drift is a change in the gene frequency owing
to chance. An inverse relationship occurs relating to
the size of the population.
Neither one of the above can be used effectively to
make direct change to the gene frequency of a
population. Too much is left to chance.
Genetic Improvement by
Migration and Selection
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Migration is the process of bring new
breeding stock into a population.
Wholesale change is made in the influx
of new genes to the existing population.
Migration success in a breeding
program is accomplished due to the
new genes dramatically changing the
gene frequency within a herd or flock.
Selection
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Is the process of allowing certain
animals to be parents while other are
not bred or are used as slaughter
individuals.
Two types of selection:
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Natural
Artificial
Natural Selection
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Using multiple males in herd or flock results
in certain males becoming dominant in
passing along their genetic makeup along to
offspring as compared to other males in the
herd or flock.
Physical traits of the male: size, dominant
personalities, mobility and reproductive
efficiency can lead to changes in the genetic
frequency of the offspring.
Carload sales of bulls in the Western states
are an example.
Artificial Selection
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Management determines the use of
males or females in the breeding herd.
The managerial decision is based on the
desired outcome from the offspring of
particular matings, i.e. breeding for
carcass vs breeding for reproductive
efficiency by using selected males and
females.
Quantity vs Quality Traits
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Qualitative traits are defined as those phenotypes
that are classified into groups rather than numerically
measured. Examples given: color of hair coat, horns,
white faces, black hooves
Quantitative traits are numerically measured and
are controlled by many genes which individually have
small influence. Examples are: ADG, FE, pigs
weaned/litter, #of milk
Remember – environment will affect the quantitative
trait expression. Some environment may aid while
other may hinder the positive effect of selecting
quantitative traits for herd improvement.
Heritability
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Definition – it is a measure of the proportion
of phenotypic variation that can be passed on
to offspring.
It is used as an indicator of the amount of
genetic progress that can be achieved by
choosing superior sires and dams.
It is the proportion of the difference between
individuals that is due to additive gene
effects.
Additive gene action or effect is when the
total phenotypic effect is the sum of the
individual effects of the alleles.
Improvement from Heritability
Understanding
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Selection differential is the phenotypic
advantage of the parents chosen to provide
offspring. The difference in the mean of the
parents compared to the mean of the
population.
Example from text: Herd average is 7
pigs/litter born. Producer chooses sires and
dams who average 11 pig/litter. Difference is
4 pigs. Litter size is 10% heritable thus
4(10%) = .4 pigs. We can expect litters from
gilts produced from the crossing to produce
an average of 7.4 pigs/litter born.
Percents of Heritability
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Reproductive traits are low in
heritability. <20%
Growth related traits are moderate in
heritability. 20-40%
Carcass traits are considered high in
heritability. >40%
Systems of Mating
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Inbreeding
Linebreeding
Outbreeding
Crossbreeding
Inbreeding
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The mating of closely related individuals, i.e.
father to daughter, son to daughter.
Inbreeding decreases the variation in the
genes existing in a herd, thus increasing the
homozygosity of desired traits as well as
some undesired traits.
Undesired performance is called inbreeding
depression.
Linebreeding
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A form of inbreeding where the
concentration of genes centers around
a common ancestor, i.e. grandson to
granddaughter.
Also used to concentrate genes for
phenotypic expression.
Common among companion animals,
dogs in particular.
Outbreeding or out crossing
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Heterosis is defined as the improvement in
performance by individual offspring from the
crossing over the performance of the parent
population.
Increased heterozygosity or hybrid vigor
results.
Do not confuse heterosis with in purebreds
with the heterosis resulting from
crossbreeding.
Crossbreeding
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The mating of individuals from two
different breeds.
Maximization of heterosis occurs with
the two breed cross.
Breed rotations are common methods.