Topic 3033

Animal GeneticsGenotype and Phenotype

Anna Blight

Genetic Terminology 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 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 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 PoultryTurkeys

Chickens

Horses

Cattle

Goats

Sheep

Humans*(comparison)

Swine

41

39

32

30

30

27

23

19

Genotype and Phenotype Genotype Genetic constitution

of an individual For any pair of

alleles, three genotypes are possible

AA Aa aa

Phenotype Characteristics of an

animal that can be seen

Example: presence or absence of horns, color, or weight

Determined by genotype

Dominant Expression Dominant gene

overpowers expression of its recessive allele

Represented by a capital letter

Example: a black holstein is dominant over red coat color

Dominance 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 LivestockDominant Polled cattle Black Holstein or

Angus Belted Pattern in swine Trotting Gait Black horse

Recessive Horned cattle Red Holstein or

Angus Non-Belted Pattern-

swine Pacing Gait Chestnut horse

Genotypic & Phenotypic RatiosUsing the Punnett Square

Heterozygous mated with homozygous dominant

1:1 genotypic ratio 50% homozygous,

50% heterozygous. All offspring look the

same.

W W

W W W WW

w Ww Ww

Genotypic & Phenotypic RatiosUsing the Punnett Square

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

W w

W WW Ww

w Ww ww

Multiple Gene Pairs 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 PairsB=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.