Animal Breeding: Genetic Improvement Principles

Study Notes

Animal breeding applies principles of population genetics to optimize livestock production efficiency.
Genetics is the study of heredity and variation.
Heredity is the transmission of traits from parents to offspring through genetic material.
Variation refers to genetic differences among individuals within a species.
Genetics focuses on the genetic makeup of individuals and how hereditary information is passed down.

Transmission genetics focuses on genetic processes within individuals and gene transmission.
Molecular genetics investigates the molecular basis of heredity, including DNA, transcription, and translation.
Population genetics studies the heredity of traits in groups of individuals influenced by one or a few genes.
Quantitative genetics studies the heredity of traits influenced by many genes in groups of individuals.

Animal breeders aim to improve entire animal populations, not just individual animals.
The goal is to enhance future generations of livestock for improved traits.

A population is a group of individuals of the same species in a specific location.
Genetically, a population is a breeding group.
Populations are dynamic and experience changes in size and structure.

Traits are observable or measurable characteristics of individuals.
Observable traits describe an animal's appearance, such as coat color, size, muscling, leg set, and head shape.
Measurable traits relate to an animal's performance, such as weaning weight, lactation yield, or time to run a mile.
Phenotype refers to the observed or measured value of a trait.

Genotype represents the genes or set of genes responsible for a particular trait, reflecting the genetic makeup of an individual.
Phenotype is the observable or measurable expression of a trait, influenced by both genotype and environment.

Qualitative traits are determined by one or a few genes and remain unchanged throughout life (e.g. hair color).
Quantitative traits are influenced by many genes and change continually throughout life (e.g. milk yield).
Qualitative traits are not affected by the environment, while quantitative traits are influenced by environmental factors.

Phenotype (P) is a combination of genotype (G) and environment (E): P = G + E.
This equation highlights the interplay between genetic factors and environmental influences on an individual's observable characteristics.

This law establishes ideal conditions for predicting allele and genotype frequencies in populations.
It assumes random mating, absence of selection, mutation, and migration, and equal viability and fertility.
Ideal conditions are rarely met in natural populations, as they are constantly changing.

Large population size: Minimizes sampling errors and random effects.
Random mating: Individuals within the population mate without preference for specific genotypes.
No selection: All genotypes have equal chances of survival and reproduction.
Closed population: No immigration or emigration, preventing gene flow from external populations.
No mutation: Allele frequencies remain constant without changes due to mutations.

Genetic equilibrium: If all conditions are met, the population remains in balance with stable allele and genotype frequencies across generations.
Allele frequencies predict genotype frequencies: The specific proportions of alleles in the population determine the expected proportions of different genotypes.

The equation predicts genotype frequencies from allele frequencies: p^2 + 2pq + q^2 = 1
- p^2: frequency of homozygous dominant genotype (AA)
- 2pq: frequency of heterozygous genotype (Aa)
- q^2: frequency of homozygous recessive genotype (aa)
This relationship helps understand how allele frequencies influence the distribution of genotypes in a population.

The maximum frequency of heterozygotes (2pq) occurs when allele frequencies are equal (p=q=0.5).
As allele frequencies deviate from this point, heterozygosity decreases.
When one allele is rare, the homozygous genotype for that allele becomes the rarest.

Estimating allele frequencies: Determining frequencies of alleles in a given population.
Predicting allele transmission: Tracking how allele frequencies change from one generation to the next under specific conditions.
Monitoring genetic change: Comparing real populations to the ideal equilibrium to understand population dynamics.
Estimating recessive disorder frequency: Calculating the frequency of recessive alleles in a population based on observable phenotypes.
Identifying carriers: Estimating the frequency of heterozygotes (carriers) for a recessive trait.

Many genetic disorders are recessive, only expressed in homozygous individuals.
Phenotypically, the heterozygote is indistinguishable from the dominant homozygote.

The frequency of a recessive phenotype (q^2) can be determined from a population sample.
Under Hardy-Weinberg conditions, the frequency of heterozygotes (2pq) can be calculated.

The Hardy-Weinberg law provides a model for understanding allele and genotype frequencies in populations.
It serves as a benchmark for evaluating and understanding deviations from equilibrium in real populations.
By understanding the principles of population genetics, animal breeders can develop strategies to optimize livestock populations for desirable traits.