Genetics 101

SUSAN SCHOENIANSheep & Goat Specialist

University of Maryland Extensionsschoen@umd.edu - www.sheepandgoat.com

2013 Winter Webinar Series: Breeding Better Sheep & Goats

DNA, chromosomes, genes, and alleles

DNA (deoxyribonucleic acid)• The genetic material that

controls how an animal looks and performs.

• Exists as two long strands spiraled into a double helix.

• Is organized into chromosomes.

Chromosomes

• Found in pairs in the nucleus of every cell of the body.

• There are two kinds of chromosomes.1. Autosomes2. Sex chromosomes

Species differ in the number of chromosomes they have.

n=5427 pairs

n=6030 pairs

AUTOSOMES

• Comprise all but one pair of chromosomes.

• (Mostly) control features the same in male and females.

SEX CHROMOSOMES

• Every animal has one pair of sex chromosomes.

• There are two types of sex chromosomes denoted by the letters X and Y.

• Females have two of the same kind of sex chromosomes (XX).

• Males have two distinct sex chromosomes (XY).

Sex determination• The sex of the

offspring is determined by the sex chromosomes.

• Males contribute either an X or Y chromosome, thus determine the sex of the offspring.

• Diet may also affect sex ratio.

Genes• A unit of inheritance

composed of a segment of DNA.

• Also in pairs.• Encodes the amino acid

sequence of a protein.• Physical location of

gene on DNA molecule is called a locus (loci).

Alleles (usually denoted by letters)

• An alternative form of a gene (one of the pair) that is located at a specific position of a specific chromosome.

• Organisms have two alleles for each trait or the expression of a trait may be affected by multiple alleles.

HOMOZYGOUS• When the two alleles are

the same.

HETEROZYGOUS• When the two alleles are

different.

DOMINANT (Capital letter)• One allele masks the

expression of another allele at the same loci.

• Overrides the traits of a recessive allele in a heterozygous pairing.

RECESSIVE (lower case letter)• Allele that causes a

phenotypic trait that is only seen in the homozygous genotype.

A

a

aA

aa

Aa

Aa

AA

Dominant traits in sheep and goats• Hairy fleece• Red, tan hair sheep• White wool

(most breeds)• White goats• Red Boers• Polled• Brown eyes• Wattles

Recessive traits in sheep and goats• Woolly fleece• Colored fleece

(most breeds)• Horns• Blue eyes• Myotonia• Colored goats• Black Boers• Genetic defects

– Spider lamb disease– Cryptorchidism– Entropion

Incomplete or partial dominance

• When the dominant allele is only partially dominant.

• Example: scurs

Sex-limited inheritance• Sex-linked - on X or Y

chromosomeExample: hemophilia

• Sex-limited - all or none expressed by sexExample: milk production

• Sex-influenced - genotype + sex determines phenotypeExamples: horns in most sheep and beards in goats.

Epistasis• When the action of one

gene depends upon another.

Example: fleece color, skin color, coat color

• Inheritance of fleece and coat color can be complicated and varies by breed.

Genetic linkages or co-inheritance• Traits that tend to be

inherited together as a consequence of an association between their genes.

Examples: polledness in goats and hermaphrodism (intersex in the homozygous female).

Genetic correlations and responses• Extent to which the

genotypic values for one trait predict the genotypic values for the second trait.

• Can be a positive or negative relationship. -1 ↔ +1

• Correlation can also be due to environmental influences.

Positive correlations (0-1)Birth weight

Weaning weightPost weaning weight

Rib eye areaOvulation rate

Fecal egg countFecal consistency score

Frame sizeScrotal circumference

Ultrasound rib eye area

Weaning weightPost weaning weightYearling weightPercent retail cutsLitter sizeFAMACHA© scoreDag scoreCarcass weightSemen productionActual rib eye area

(-1 – 0) Negative correlationsBirth weight

Fleece weightStaple length

Ovulation rateFecal egg count

Lambing ease Fiber diameterFiber diameterEmbryo survivalPacked cell volume

No correlation (zero)Scrapie genotype

Resistance to a diseaseReproductive rateReproductive rate

Horn conditionColor

Production traitsResistance to another diseaseGrowth and carcass traitsWool traitsProductivityProductivity

Genetic correlations tend to vary by breed and study.

Genetic defectsSimple recessive inheritance• Spider lamb syndrome• Cryptorchidism• Myotonia• Hairy lamb syndrome• Polledness in goats

More complicated• Jaw defects• Entropion• Hernias• Teat defects• Structural defects• Fleece defects• Rectal and vaginal prolapse• Birth (congenital) defects

Inheritance• Each sperm and egg

contains one chromosome from each pair of chromosomes of the parent.

• Which chromosome of a pair ends up in a particular sperm or egg is determined purely by chance.

Inheritance• There are 134,217,728

(227) possible combinations of the 54 or 60 chromosomes in a sperm or egg produced by a male and female.

• This results in considerable variation in the progeny from the same mating.

?

Genotype vs. Phenotype

PHENOTYPE (P)

• The observable or measurable traits of an organism: what we can see or measure.

• Determined by genetics, plus environmental influences (E).

P = G + E

GENOTYPE (G)

• The genetic make-up of a cell, organism, or animal.

• Determines hereditary potential and limitations of the individual.

Environmental influences• Diet• Health • Weather• Age• Type of birth and

rearing• Age of dam• Housing• Season of birth

Which buck is better?

Two kinds of traits

1. Qualitative

2. Quantitative (or polygenic)

QUALITATIVE

• Phenotype falls into specific category.

• Usually affect by one or few genes.

• Very little influence of the environment.

QUANTITATIVE

• Continuous in expression.

• Considerable variation in phenotype.

• Under the influence of many genes

• Much more environmental influence

Qualitative traits• Blood type• Eye color• Coat type• Fleece or coat color• Horns• Wattles• Beards• Inherited defects

– Entropion– Spider lamb disease– Cryptorchidism– Myotonia

Quantitative traits• Reproductive rate• Growth rate• Milk production• Fiber production• Carcass characteristics• Disease resistance• Conformation• Wool shedding• Feed efficiency

Quantitative traits

• Are usually the traits of greatest economic importance.

Methods of genetic improvement

1. Crossbreeding

2. Selection

Next webinar: Jan 29, 7 pm EST“Breeding systems” with Jeff Semler