Genetics and Breeding
Interest Approach
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Ask students why they don’t look like
each other (if they had the same
parents)
Terms
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Allele
A pair of genes which are located at the
same place on homologous
chromosomes (the matched pair).
Often applied to the traits associated
with the genes.
Terms cont.
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Phenotype
The physical appearance of the
animal because of its genetic
makeup
Terms cont.
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Genotype
Genetic constitution (makeup) of
an individual
Terms cont.
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Gene
A specific region of a chromosome
which is capable of determining the
development of a specific trait that is
composed partially or wholly of DNA
Terms cont.
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Dominant
Refers to genes which hide or mask
the presence of other genes
Terms cont.
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Recessive
Gene whose phenotypic expression
is masked when in the presence of
a dominant allele
Terms cont.
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Heterozygous
An animal that carries a gene with
two different alleles
Terms cont.
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Homozygous
An animal that carries a gene with
two identical alleles
Characteristics of Genes
Traits are controlled by genes
 Genes have different forms which are known
as alleles
 In diploid organisms, like man and the higher
animals, there are TWO alleles – one on each
chromosome of the homologous pair
One of each of the above example alleles could be found
on a pair of homologous chromosomes
Dominant Genes

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A dominant gene covers (masks) the
presence of other genes
Capital letters are used to designate
dominant genes
Recessive Genes
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A recessive gene is the one over-ridden by
the dominant gene
For a recessive trait to appear
(phenotypically) the animal must be
homozygous (have two of the same gene –
one on each homologous chromosome) for
that characteristic to appear
Recessive genes are designated by small
letters
Examples of Dominant and
Recessive Writing
Bb
Dominant
trait
Rr
Recessive
trait
Dominant
trait
Recessive
trait
Incomplete Dominance
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Sometime traits do not have one clear
dominant gene or one clear recessive gene
In incomplete dominance, traits appear to
blend together
When crossing a red snapdragon with a white
snapdragon, the offspring is pink if
incomplete dominance occurs
The heterozygous genotype in incomplete
dominance appears different than either of
the homozygous alternatives

Example: red – pink - white
Punnett Squares
A
A
Parents alleles are:
AA and aa
a
Aa
a
Aa
Aa
Aa
Phenotypic and Genotypic
Ratios
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Phenotypic Ratios
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Shows ratio of what is seen

Example: previous Punnett square: the
phenotypic ratio is 2:2 or 1:1
Phenotypic and Genotypic
Ratios
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Genotypic Ratios
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Shows what is is seen and what is not seen

Example: previous Punnett square: the
genotype ratio is 0:4:0
Example
B
B
b
Bb
Bb
b
Bb
Bb

Angus cattle: coat color
B is dominant
b is recessive
cross a
a
BB cow with
bb bull
Example cont.
All of the offspring have a
black phenotype only
Bull - BbPp
BP
Bp BBPp
Bp BBPp
Bp
BBpp
bP
bp
BbPp
Bbpp
BBpp BbPp
Cow - BBpp
Bbpp
Cross a BB cow to a Bb bull
But also, another allele is
considered.
Polled:
P is dominant
p is recessive
Cross the Bb heterozygous
polled bull with the BB
homozygous recessive polled
cow
Example cont.
BBPp – black polled
BBpp – black horned
Phenotypic Ratio
8:4:4
Or
2:1:1
BbPp – black polled
Bbpp – black horned
Genotypic Ratio
4:4:4:4
Or
1:1:1:1