Topic 3033
Animal Genetics
Genotype and Phenotype
Anna Blight
Genetic Terminology
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DNA to RNA to protein
Protein codes for genes
Genes code for traits
Each chromosome pair contains many gene
pairs
Alleles
genes
that occupy specific loci on
chromosomes and affect the same trait
There
are TWO alleles, one on each
chromosome of the homologous pair
If
loci correspond the animal is homozygous
(AA, aa), if the alleles contrast the animal is
heterozygous (Aa)
Coat
color is determined by dominance of one
of the alleles (Aa) or homozygosity of recessive
alleles (aa)
Genetic Terminology
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For example, during gametogenesis a
heterozygous animal passes a white allele to one
gamete and the black allele to the other gamete
Resulting germ cells pass on either a white allele
or a black allele, not both because of segregation
of the homologous chromosomes
Homozygous animals pass on two of the same
alleles to gametes
X and Y Chromosomes
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Determine sex of the animal
Females = XX (except bird eggs = XY)
Males = XY (bird species sperm = XX)
Females only contribute X to gametes
Males contribute both X and Y thus
determining sex of animal
Reversed in bird species
Pairs of Chromosomes in
Livestock and Poultry
Turkeys
Chickens
Horses
Cattle
Goats
Sheep
Humans*(comparison)
Swine
41
39
32
30
30
27
23
19
Genotype and Phenotype

Genotype

Phenotype

Genetic constitution
of an individual
For any pair of
alleles, three
genotypes are
possible
AA Aa aa

Characteristics of an
animal that can be
seen
Example: presence or
absence of horns,
color, or weight
Determined by
genotype
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Dominant Expression
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Dominant gene
overpowers
expression of its
recessive allele
Represented by a
capital letter
Example: a black
holstein is dominant
over red coat color
Dominance
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Homozygous
dominant, WW
W = White
Heterozygous sheep
Ww, w = black
Sheep will show
dominant trait and be
white
Recessive Expressions

Black wool
 Recessive trait for
sheep
 Homozygous
recessive
 ww
 Both parents must
carry recessive
allele
Common Known Dominant and
Recessive Traits in Livestock
Dominant
 Polled cattle
 Black Holstein or
Angus
 Belted Pattern in
swine
 Trotting Gait
 Black horse
Recessive
 Horned cattle
 Red Holstein or
Angus
 Non-Belted Patternswine
 Pacing Gait
 Chestnut horse
Genotypic & Phenotypic Ratios
Using the Punnett Square
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W
w
W
WW
Ww
W
WW
Ww
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Heterozygous mated
with homozygous
dominant
1:1 genotypic ratio
50% homozygous,
50% heterozygous.
All offspring look the
same.
Genotypic & Phenotypic Ratios
Using the Punnett Square
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W
w
W
WW
Ww
w
Ww
ww
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Mating two
heterozygous sheep
Genotypic ratio of
1:2:1
Phenotypic ratio is
3:1; three white
sheep, one black
sheep
Punnett square is an
easy way to
determine ratios
Multiple Gene Pairs
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Two traits, Red or black
coat color and polled or
horned cattle
Gives nine genotypes
Four phenotypes, 9:3:3:1
ratio
 9 Black polled,
3 black horned,
3 red polled, and 1
red horned
Multiple Gene Pairs
B=black coat b=red coat
P=polled p=horned cattle
BP
Bp
bP
bp
BP
BBPP BBPp
BbPP
BbPp
Bp
BBPp
BBpp
BbPp
Bbpp
bP
BbPP
BbPp
bbPP
bbPp
bp
BbPp
Bbpp
bbPp
bbpp
How would understanding
genotype and phenotype, and
dominant and recessive traits
contribute to a breeding program?
Bibliography
Taylor, Robert E., Field, Thomas G. 1998.
Scientific Farm Animal Production: An
Introduction to Animal Science. Upper
Saddle River: Prentice Hall.