Genetics and Breeding. Interest Approach Ask students why they don’t look like each other (if they...
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Transcript of Genetics and Breeding. Interest Approach Ask students why they don’t look like each other (if they...
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Genetics and Breeding
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Interest Approach Ask students why they don’t look
like each other (if they had the same parents)
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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.
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Terms cont.
Phenotype The physical appearance of the
animal because of its genetic makeup
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Terms cont.
Genotype Genetic constitution (makeup)
of an individual
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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
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Terms cont.
Dominant Refers to genes which hide or
mask the presence of other genes
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Terms cont.
Recessive Gene whose phenotypic
expression is masked when in the presence of a dominant allele
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Terms cont.
Heterozygous An animal that carries a gene
with two different alleles
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Terms cont.
Homozygous An animal that carries a gene
with two identical alleles
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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
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Dominant Genes A dominant gene covers (masks)
the presence of other genes Capital letters are used to
designate dominant genes
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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
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Examples of Dominant and Recessive Writing
Bb
Dominanttrait
Recessive trait
RrDominant
trait
Recessive trait
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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
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Punnett Squares
Parents alleles are: AA and aa
A A
a
a
Aa Aa
Aa Aa
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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
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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
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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
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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
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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