Principles of Genetics in Animal Breeding

Genetics

The scientific study of genes, genetic variation, and heredity.

Trait

A distinguishing quality or characteristic.

Artificial selection

Human-guided mating decisions.

Blending theory

Parental essence blends together resulting in the appearance of their offspring being intermediate.

Qualitative traits

Traits that can be placed into categories (i.e. horned OR polled).

F₁ generation

The resulting offspring from the first cross of two parent plants.

F₂ generation

The resulting offspring from self-pollination of the F₁ generation.

Mendel's conclusions

1. Expressed traits: inherited unchanged in a hybridization (dominant phenotype). 2. Latent traits: disappear in the offspring of a hybridization (recessive phenotype). 3. Because the recessive trait reappears in progeny of F₁, the traits remained separate. 4. Plants must have 2 copies of an 'element' but only pass one 'element' on to offspring. 5. Dominant appearance could mean the plant had 2 dominant 'elements' or a dominant and recessive element. 6. Plants exhibiting a recessive appearance lacked a dominant 'element'.

Mendel's pea plants

Results of Mendel's breedings.

3/4

The ratio of plants produced with purple flowers in the F₂ generation.

1/4

The ratio of plants produced with white flowers in the F₂ generation.

Gregor Mendel

Father of modern genetics.

Observed qualitative traits

Traits of pea plants that can be categorized.

Dominant phenotype

The phenotype that is expressed in the presence of a dominant allele.

Recessive phenotype

The phenotype that is expressed only in the absence of a dominant allele.

Element

A term used by Mendel to describe the genetic factors passed from parents to offspring.

Hybridization

The process of crossing two different varieties or species.

Progeny

The offspring produced from a particular cross.

Intermediate appearance

The phenotype that results from blending theory, where offspring exhibit characteristics that are a blend of the parents.

Observed traits

The traits that are visible in the offspring.

Continuous improvement

An understanding of genetics is vital for enhancing livestock species.

Deoxyribonucleic acid

Self replicating material present in nearly all living organisms

Deoxyribonucleic acid

Carries genetic information

Deoxyribonucleic acid

Comprised of nucleotides

Chromosomes

Tightly coiled packages of SOME of an organism's DNA

Genome

The entire "set" of DNA

Genome

Hereditary instructions for building, running, and maintaining an organism

Regions of DNA

DNA has coding and non-coding regions

Non-coding regions

Known as spacer DNA

Coding regions of DNA

Known as genes

Genes

Functional unit of heredity

Genes

Small region of DNA that codes for a functional molecule (i.e. a protein)

Locus (loci)

Specific location on a specific chromosome where a gene is found

Allele

Alternative form of a gene

Allele

Chemical/functional difference between them

Alleles of Extension gene in dogs

More than two alleles can be present in a population

Allele pairings

If an individual possesses two of the same alleles they are said to be homozygous

Homozygous

If an individual possesses two of the same alleles

Heterozygous

If an individual possesses two different alleles

Phenotype

Physical expression of the genotype

Punnett Square

2-dimensional grid used to determine the possible genotypes (and resulting phenotypes) from a mating

Mendel's law of dominance

A certain allele will be dominant over another

Dominant allele

Will express its phenotype and mask the phenotype of the other

Recessive allele

Will be represented with a lowercase letter

Partial dominance

The heterozygous organisms possess a phenotype that is intermediate to the homozygous genotypes

No dominance

The heterozygous organisms possess a phenotype that is exactly intermediate to the phenotypes of the homozygous organisms

20 sec ¼ mile time

A specific time measurement for running a quarter mile.

Co-dominance

The heterozygous organisms fully express the phenotypes of both homozygous genotypes.

Dahlia 'Carnival'

Another example of co-dominance.

Epistasis

The expression of genes at one locus depends on the alleles present at one or more other loci.

Extension gene

Determines if a horse will be black or red; black is dominant (BB and Bb genotypes).

Agouti gene

Restricts black pigment to the points; restriction to the points is dominant (AA and Aa genotypes).

Simply-inherited traits

Traits that are controlled by one - a handful of genes, tend to be categorical (either/or) and are not really affected by the environment.

Polygenic traits

Controlled by hundreds-thousands of different genes, tend to be quantitative (continuous), and are affected by the environment.

Phenotype

The observable characteristics of an organism, which is a result of the genotype and environment.

Genotype

The genetic constitution of an organism.

Carcass weight

Influenced by 100 genes, resulting in 200 alleles where desirable alleles each add 1 pound to carcass weight.

Birth weight

The weight of an animal at birth, which can be quantified (e.g., 75# or 95#).

Weaning weight

The weight of an animal when it is weaned, which can be quantified (e.g., 785# or 885#).

Yearling weight

The weight of an animal at one year of age, which can be quantified (e.g., 995# or 1050#).

Desirable alleles

Alleles that contribute positively to a trait, such as adding weight to carcass.

Undesirable alleles

Alleles that do not contribute positively to a trait, adding nothing (e.g., +0 pounds).

Greater number of desirable alleles

Animals that perform the best in a trait of interest must have a greater number of desirable alleles for that trait.

Pattern of inheritance

The specific way traits are passed from parents to offspring, including dominance and epistatic interactions.

Evaluate performance

Assessing animals based on their traits of interest to determine genetic potential.

Genotype Yearling weight

1065#

Environmental effect

-70#

Phenotype Yearling weight

995#

Genotype Yearling weight

1045#

Environmental effect

+5#

Phenotype Yearling weight

1050#

Performance

Phenotype is affected by genotype and environment.

Genetic potential

An animal will not reach its genetic potential if ideal conditions aren't met.

Heritability

The extent of the effect of environment on performance depends on the heritability of the trait.

Chromosomes

Segments of densely compacted DNA.

DNA

Self-replicating material present in nearly all living organisms.

Genes

Functional unit of heredity.

Locus

Specific location on a specific chromosome where genes are found.

Alleles

Multiple forms of a gene.

Polygenic inheritance

Affected by hundreds to thousands of genes.

Economically important traits

We have to rely on how animals perform in that trait to assess their value as a genetic parent.

Heritability values range

0 - 1 (proportion).

Lowly heritable traits

0 - 0.2 (reproductive traits: fertility, litter size).

Moderately heritable traits

0.2 - 0.4 (growth traits: birth, weaning, yearling weights).

Highly heritable traits

0.4 - 0.6 (carcass traits: HCW, REA, WBSF, marbling).

Phenotype formula

Phenotype = Genotype + Environment.

Estimated breeding values (EBVs)

Values that indicate genetic potential based on performance data.

Expected progeny differences (EPDs)

Values that indicate the expected performance of offspring.

Estimated transmitting abilities (ETAs)

Values that indicate the genetic potential passed to offspring.

Performance records

Extensive record-keeping typically done in seedstock operations.

Expected progeny differences example

Calves sired by Bull A will be 12.5 lbs. heavier at birth than calves sired by Bull D.

Mating plans

Can be based on randomness, performance/visual appearance (phenotype), or genetic relationship (pedigree).

Random mating plan

Selecting which animals are going to be bred without selecting the breeding pairs.

Gate-cutting

The first 25 females to enter a pasture.

Chute-cutting

The first 25 females that go through the chute.

Performance-based mating plans

Involves positive and negative assortative mating.

Positive assortative mating

Breeding animals of the same type of quality to create exceptional offspring.

Negative assortative mating

Breeding animals of opposite (dissimilar quality) to correct poor performance in a trait.

Corrective mating

Also called negative assortative mating, it involves breeding my worst females to my best male.

Inbreeding

Mating closely related animals, increasing homozygosity and the likelihood of expressing detrimental homozygous recessive traits.