Download GENETICS DEFINITION

 The study of the means by which genetic
information is stored, replicated, translated,
mutated and transferred to future generations
such that anatomical and physiological systems
are integrated and parental characters appear in
subsequent generations.
 There are several structures and sub-cellular
organelles in the cell, but the main ones of
interest in genetics are the nucleus, ribosomes,
nucleolus and centrioles.
The nucleus contains the chromosomes, the
ribosomes control RNA transcription, the
nucleolus produces t-RNA and the centrioles serve
as focal points for cell division.
 DNA is an acronym for deoxyribonucleic acid.
It is composed of a 5 carbon sugar,
deoxyribose, phosphate bonds, and
nitrogenous bases. These nitrogenous bases
are adenine, thymine, cytosine and guanine.
Chemically adenine can only bond with
thymine and cytosine only with guanine. It is
in the form of a double helix.
 To visualize the structure of DNA, imagine a
rubber ladder in space. The uprights of the ladder
are composed of molecules of deoxyribose
connected in single strands with phosphate
bonds. The steps of the ladder are paired bases
that join to each other and also the uprights of
sugar and phosphate.
 The steps then would be a pair of bases
composed of either adenine bonded with
thymine or cytosine bonded with guanine.
 If you were to grasp the top and bottom of this
imaginary ladder and twist in opposite
directions, you would form a twisted structure
that would be in the shape of a double helix.
 Three of these bases in a row are called a codon or
triplet. The sequence will code for a specific
amino acid. Thus, as codons are put in sequence,
the structure of a protein is coded.
 If we put a codon that calls for the start of a gene
in front of the above sequence and another at the
end to stop it we have a gene.
 Locus-Point on a chromosome where a specific
gene is always found.
 Allele-Genes that are found at the same locus on
homologous chromosome that affect the same
trait in different ways.
 Homologous-Chromosomes of the same shape,
size and length that carry similar genes.
 Each normal animal has an even number of total
chromosomes. These are arranged in pairs of
homologous chromosomes one of each pair
coming from the sire and the dam.
 Cattle=60, sheep=54, swine=38, goats=60,
horses=64, donkey=62
 Same length, size and shape that carry similar or
the same genes at the same loci.
 Sex chromosomes are not homologous. In
mammals are X and Y; fowl are Z and W. Males XY,
Female fowl are ZW.
 Each animal or plant has a maximum of two alleles.
 Each gamete will receive one gene from a locus.
 Thus the probability of a gamete receiving a particular
gene depends upon the genotype of the animal or
plant for that locus.
 The probability of an event happening can never be
less than zero nor more than 1.0.
 Thus the sum of all the probabilities will add to 1.0.
 E.g. with a legal coin a toss can either result in a head
(1/2) or a tail (1/2). And (½ +½ = 1.0).
 Mendel’s first law indicates that genes are segregated
into gametes at random based on the genotype of the
individual.
 Thus, an animal with a genotype Aa will form gametes
with either A or a and it will do so at equal
probabilities.
Segregation of genes into gametes
Aa
A
a
 Recall that homologous chromosomes arrive at the
metaphase plate and line up randomly one on top of
the other with no regard to whether the chromosome
came from the sire or the dam.
 Thus, each pair of homologous chromosomes act
independent of every other pair.
 The probability of two or more independent events
happening at the same time is the product of their
independent probabilities.
 Genes carried on nonhomologous chromosomes are
independent events.
 In calculating, we can find each loci probability and
then multiply them to reach the final answer.
 If an animal were to have the genotype AaBb and the
two loci were on different pairs of homologous
chromosomes, then the A locus could produce an A or
an a gamete.
 Likewise, the B locus could produce either a B or a b
gamete.
 Remember that a gamete contains only ONE of each
gene from a locus.
 Thus, in constructing the possible gametes from the
previous example we could have an AB, an Ab, an aB or
an ab.
AaBb
AB
Ab
aB
ab
 The probability of each of these gametes being formed
is ¼. Pr (A) = ½ and Pr (B) = ½. ½ x ½ = ¼
 There are four possible gametes each with a probability
of ¼, so they add to 4/4 or 1.0 and all the probabilities
are accounted for.
 “What is the probability of producing a gamete
(abcDEf) from AaBbCcDdEEFf?
 1/32
 a=1/2, b=1/2, c=1/2, D=1/2, E=1.0, f=1/2.
 ½ x ½ x ½ x ½ x 1.0 x ½ = 1/32
 Each is an independent event and so each probability
is multiplied by all the others.
 For practice, make up genotypes and calculate the
probability of several gametic genotypes.
 Gametic production in one sex has no influence over
gametic production in the other. Thus, these too are
independent events.
 If we want to know what the possibilities of certain
genotypes resulting from known matings, we need
only to calculate the probabilities of gametes and
multiply.
 The probability of the first genotype producing an A or
an a gamete is ½ each.
 Likewise for the second genotype.
 If an A sperm fertilized an A ovum, then the result
would be an AA zygote. The probability of this
occurring is ½ x ½ = ¼.
 The same would be true for an a sperm fertilizing an a
ovum.
 If an A sperm fertilized an a ovum the result would be
an Aa zygote (also ¼).
 But, if an a sperm fertilized an A ovum the result
would be aA. (also ¼)
 The two genotypes are equivalent, it making no
difference which sex contributed what gene to the
heterozygote.
 Thus, we add the two probabilities to = ½.
 From any heterozygous mating, we will get 1
homozygote, 2 heterozygotes and 1 homozygote
(opposite).
 This is the classic 1:2:1 ratio.
 We can construct a chart known as a Punnet Square to
illustrate this.
Genotype of
Sire and dam
gamete.
A
a
A
AA
Aa
a
aA
aa
 If we add another independent heterozygous locus, we
can still treat each as an independent event.
 Thus, gametes at each locus are created in equal
numbers.
 AaBb would yield the four gametes referred to earlier.
If we construct a chart for their recombination we
would get:
Sire/dam
gametes
AB
Ab
aB
ab
AB
Ab
aB
ab
AABB AABb AaBB AaBb
AABb AAbb AaBb Aabb
aABB aABb aaBB aaBb
aAbB aAbb aabB aabb
Sire/dam
gametes
AB
Ab
aB
ab
AB
AABB AABb AaBB AaBb
Ab
AABb AAbb AaBb Aabb
aB
aABB aABb aaBB aaBb
ab
aABb aAbb aabB aabb
Sire/dam
gametes
AB
Ab
aB
ab
AB
AABB AABb AaBB AaBb
Ab
AABb AAbb AaBb Aabb
aB
aABB aABb aaBB aaBb
ab
aABb aAbb aabB aabb
Sire/dam
gametes
AB
Ab
aB
ab
AB
AABB AABb AaBB AaBb
Ab
AABb AAbb AaBb Aabb
aB
aABB aABb aaBB aaBb
ab
aABb aAbb aabB aabb
Sire/dam
gametes
AB
Ab
aB
ab
AB
AABB AABb AaBB AaBb
Ab
AABb AAbb AaBb Aabb
aB
aABB aABb aaBB aaBb
ab
aABb aAbb aabB aabb
Sire/dam
gametes
AB
Ab
aB
ab
AB
AABB AABb AaBB AaBb
Ab
AABb AAbb AaBb Aabb
aB
aABB aABb aaBB aaBb
ab
aABb aAbb aabB aabb
Sire/dam
gametes
AB
Ab
aB
ab
AB
AABB AABb AaBB AaBb
Ab
AABb AAbb AaBb Aabb
aB
aABB aABb aaBB aaBb
ab
aABb aAbb aabB aabb
 Non-additive gene action-shows some type of
dominance
 Complete or total dominance
 Incomplete or partial dominance
 Co-dominance
 Over-dominance
 Epistasis
 One allele when in the homozygous or heterozygous
form will always be expressed in the phenotype.
Totally covers the expression of its recessive allele.
 Common type first explored by Mendel.
 Aa x Aa yields: AA, 2 Aa, aa
 Note: heterozygotes never breed true.
 One allele only partially covers the phenotypic
expression of its recessive allele.
 Results in a blending type inheritance. Recessive allele
‘bleeds’ into the phenotype.
 Example: red flower crossed with white flower yields a
pink flower.
 Yields three distinct phenotypes rather than only two
such as complete dominance results in.
 Neither allele will cover or dominate. Both will be
expressed completely in the phenotype.
 Red flower crossed with white flower would yield a red
and white flower. Also the type of gene action in roan
shorthorns and AB blood groups in humans.
 Three phenotypes will be expressed.
 Interaction between genes that are alleles such that
the heterozygote is superior in performance to either
homozygote.
 Is a major factor in the phenomenon known as
heterosis or hybrid vigor.
 Graph heterosis on blackboard.
 An increase in performance in crossbred or out bred
animals over their parents.
 Average of the F1 – the average of the P1 divided by the
average of the P1 times 100 gives the percent heterosis.
Note: the average of the progeny only has to exceed the
average of the parents, it does not have to be superior
to the best parent.
 Interaction between non-allelic genes such that there
is created a new phenotype.
 Very common gene action in coat color inheritance in
animals. The dilution gene in horses affects the b
locus to dilute to palomino or cremelo.
 The e locus in labs will result in a yellow lab if the e
locus is ee and the b locus is either BB, Bb or bb.
Difference is in the nose color.
 Shows no dominance. Instead has residual genes and
contributing genes.
 A residual gene is expressed in the phenotype as a
particular level of performance. A1, B1 are examples.
 A contributing gene only adds on to the performance
of the residual, doesn’t cover. A2, B2 are examples.
 A1A1 X A2A2 yields all A1A2 which would be
intermediate in phenotype to the homozygotes.
 Another attribute of additive genes are that they are
affected by environmental influences whereas nonadditive genes are affected very little by environment.
 This makes it difficult to tell the genotypes from the
phenotypes because a contributing gene homozygote
in a poor environment might be confused with a
heterozygote in a good environ.
 This type of gene action shows itself most often in
what are termed ‘economically important traits’. These
include average daily gain, feed efficiency, carcass
traits, etc.
 Many genes and environment affect them, males are
heavier and more efficient than females and the
measures are continuous.
 Other characteristics of additive genes:
 Involve many gene pairs-polygenic
 Shows no heterosis
 Shows sex effects
 Transgressive variation
 Medium to high heritability estimates
 Phenotypes are not distinct but instead follow a
continuous variable format.
 Little environmental influence.
 Few pairs of genes involved.
 Sex effects are few.
 Heterosis and inbreeding depression expressed.
 No transgressive variation.
 Zero to low heritability estimate.
 Distinct phenotypes.
 Some traits affected by only non-additive or additive




gene action.
Some traits are affected by both types of gene action.
Select differently depending on type.
Additive: identify the best and breed best to best.
Non-additive: easiest to outbreed or crossbreed.
 Most traits of economic importance are affected by
polygenes. This means that several loci of genes all
affect the same trait in some fashion. Our example of
coat color inheritance in horses is an example of
polygenes.
 Additive traits are affected by many pairs of polygenes
each affecting the trait in a small way.
 In a population of animals or plants there many times
exist at a loci more than 2 alleles.
 This fact may explain differences in outcomes when
two particular parents are mated.
 The A, B, o alleles in human blood type is an example.
An Aa mated with a Ba could give you an AB, an Aa, a
Ba or an aa
 AB- Co dominance
 Aa & Ba- Heterozygous
 A & B blood types
 aa- O blood type