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