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Genetics and Breeding
Interest Approach Ask students why they don’t look
like each other (if they had the same parents)
Terms
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.
Phenotype The physical appearance of the
animal because of its genetic makeup
Terms cont.
Genotype Genetic constitution (makeup)
of an individual
Terms cont. 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.
Dominant Refers to genes which hide or
mask the presence of other genes
Terms cont.
Recessive Gene whose phenotypic
expression is masked when in the presence of a dominant allele
Terms cont.
Heterozygous An animal that carries a gene
with two different alleles
Terms cont.
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 A dominant gene covers (masks)
the presence of other genes Capital letters are used to
designate dominant genes
Recessive Genes 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
Dominanttrait
Recessive trait
RrDominant
trait
Recessive trait
Incomplete Dominance 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 - whitewhite
Punnett Squares
Parents alleles are: AA and aa
A A
a
a
Aa Aa
Aa Aa
Phenotypic and Genotypic Ratios Phenotypic Ratios
Shows ratio of what is seen Example: previous Punnett square: the
phenotypic ratio is 2:2 or 1:1
Phenotypic and Genotypic Ratios Genotypic Ratios
Shows what is is seen and what is not seen
Example: previous Punnett square: the genotype ratio is 0:4:0
Example Angus cattle: coat
color
B is dominant
b is recessive
cross a BB cow with
a bb bull
B B
b
b
Bb Bb
Bb Bb
Example cont. All of the offspring have a black phenotype only
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
BPBull - BbPp
Bp bP bp
Cow - BBpp
Bp
Bp
BBPp BBpp
BBPp BBpp
BbPp
BbPp
Bbpp
Bbpp
Example cont.
BBPp – black polled
BBpp – black horned
BbPp – black polled
Bbpp – black horned
Phenotypic Ratio8:4:4
Or2:1:1
Genotypic Ratio4:4:4:4
Or1:1:1:1