Animal Genetics Jr/Sr Vet Science

Introduction Genetics: study of inheritance (passage of traits from one generation to the next) Phenotype: physical appearance of animal –1. Genetics (Genotype) –2. Environment Genotype: genetic makeup of animal –Influenced by selection of breeding animals & transfer of genetic traits

Genetics Gene: Unit of inheritance for a trait Chromosomes: paired strands of genes within cell nucleus –Offspring inherit ONE chromosome from each parent –Gametes (ova and sperm) therefore have ONE chromosome for each trait –# depends on species

Inheritance DNA: Deoxyribonucleic Acid –Long strands of molecules contained by chromosomes –Segments are divided into genes –Helix: coiled, spiral, ladder shape –Only 4 proteins make up entire code! AdenineA ThymineT GuanineG CytosineC

Inheritance, continued RNA: Ribonucleic Acid –“Messenger” substance that “reads” DNA sequences & allows them to be copied Punnett Square: “checkerboard” system of determining probability of inheritance

Sex Determination Determined by “X” and “Y” chromosome Female: XX Male: XY Sex of Offspring: –50% male, 50% female Phenotypic Ratio: observable characteristics Genotypic Ratio: actual genetic makeup X Y X XX XY X XX XY

Expressing Traits Dominant Gene: Hides another gene’s characteristic Recessive Gene: characteristic is hidden by a dominant gene Heterozygous: carries two different genes for a trait Homozygous: carries the same genes for a trait

Homozygous Cross A: Polled Cattle –Dominant a: Horned Cattle –Recessive Phenotypic Ratio: –100% Polled Cattle Genotypic Ratio: –100% Heterozygous Polled Cattle A A a Aa a

Heterozygous Cross A: Polled Cattle –Dominant a: Horned Cattle –Recessive Phenotypic Ratio: –75% Polled Cattle –25% Horned Cattle Genotypic Ratio: –50% Heterozygous Polled Cattle –25% Homozygous Polled –25% Homozygous Horned A a A AA Aa a aa

Co-dominance Shared expression for a trait; no dominance Ex: Roan coat color –R: red color –W: white color –RW: roan color Genotypic & Phenotypic Ratio: –100% Roan R R W RW W

Roan Color in Cattle

Co-dominance, continued Roan Cross Genotypic & Phenotypic Ratio: –25% Red –50% Roan –25% White R W R RR RW W WW

Epistasis Masking Effect of a Gene Ex: All Labrador Retrievers are Chocolate or Black! Yellow color is caused by epistasis Basic Coat Colors: –BB: homozygous black –Bb: heterozygous black –bb: homozygous chocolate

Basic Crosses Homozygous Black x Homozygous Choc. –BB x bb Phenotypic Ratio: –100% Black Genotypic Ratio: –100% Bb (Heterozygous Black) B B b Bb b

Heterozygous Cross Heterozygous Black: –Bb x Bb Phenotypic Ratio: –75% Black –25% Chocolate Genotypic Ratio: –25% BB (Black) –50% Bb (Black) –25% bb (Chocolate) B b B BBBb b bb

Effect of Epistasis “Yellow” is caused by another gene, that is recessive and “masks” the black or chocolate color The letter “e” is used: –EE: not yellow –Ee: not yellow –ee: yellow

Color Genotypes in Labs BBEE: Black BBEe: Black BBee: Yellow BbEE: Black BbEe: Black Bbee: Yellow bbEE: Chocolate bbEe: Chocolate bbee: Yellow w/ a brown nose!

Black (BbEe) x Yellow (Bbee) BE Be bE be Be BBEe BBeeBbEeBbee BeBBEeBBeeBbEeBbee beBbEeBbeebbEebbee beBbEeBbeebbEebbee

Results! Genotypic Ratio: BBEe: 2/16 = 1/8 BbEe: 4/16 = 2/8 Bbee: 2/16 =1/8 BBee: 2/16 = 2/8 bbEe: 2/16 = 1/8 bbee: 2/16 = 1/8 Phenotypic Ratio: Black pups: BBEe: 1/8, BbEe: 2/8 3/8 Black Puppies Chocolate Pups: bbEe: 1/8 Choc. Pups Yellow Pups: Bbee: 1/8, BBee: 2/8, bbee: 1/8 4/8 (1/2!) Yellows