Basic Genetics and Selection

Basic Genetics and Selection Basic Genetics • The basic unit of inheritance is the gene, located on chromosomes which are made up of strands of complex molecules called DNA. • In mammals, these chromosomes contain about 60,000 genes, the entire set of which is called a genome. • Different forms of the same gene are called alleles. If an individual’s two alleles (one from each parent) are the same they are homozygous. If different they are heterozygous. This genetic makeup is called genotype. What can be seen or measured is called phenotype. • Progeny are created by the union of gametes (sperm from the male and egg from the female). Each gamete produced by an individual is unique. Patterns of inheritance depend on: – The gametes produced by the parents. – The possible combination of gametes. – The action of different alleles. Types of Inheritance • Qualitative - One pair or relatively few alleles generally involved. Traits have distinct differences in phenotype, such as color, horns, etc. Usually not influenced by the environment. • Quantitative - Many pairs of genes involved, acting additively. There is continuous variation in phenotypes. Virtually all of the economically important traits result from this type of inheritance. Generally influenced by the environment. Causes of Phenotypic Variation • Heredity: Animals genetic background for phenotype • Environment: Conditions under which the animals are born and raised; climate, nutrition, disease, general management, etc. Heritability • Degree of relationship between genotype and phenotype. Generally reported as a percentage. • Production traits vary in heritability. Most reproductive traits (conception rate, etc.) are low (< 20%), production traits (weight, etc.) are medium (20% to 40%), and product traits (lean tenderness, etc.) are high (> 40%). SELECTION • Process that determines which individuals become parents, how many offspring they produce, and how long they remain in the population. • Man practices “artificial” selection. Nature (the environment) practices “natural” selection, which is always present and may be the major influence on phenotype in some instances. • Single trait selection – Most rapid genetic progress – May be least adapted to the environment • Multiple trait selection – The more traits the less progress – Animals raised outside of confinement will need more multiple trait selection Factors Affecting Rate of Improvement Through Selection • Heritability level • Selection differential – difference between performance of individuals selected as parents and the average of the group they were selected from. Relatively few sires are needed, so they have more effect than dams • Generation interval – average age of parents when progeny are born – cattle > sheep and goats > rabbits • Rate of improvement per year from selection = (heritability x selection differential) / generation intervals Selection Methods • Selection on appearance is strictly subjective. • Selection on pedigree is done strictly on the basis of an individual’s ancestors. • Selection on individual performance relies on measuring production traits, and selecting on that basis. • Selection on progeny performance can be more accurate but also is the slowest. • Combining pedigree, individual performance, and progeny performance, using Expected Progeny Difference (EPD, discussed later), is most effective. Matching Genetic Programs to Existing Conditions THOSE CONDITIONS ARE: Climate Topography Pests Forage Labor Management skill Markets Genetics and Forage • Forage availability is the main determinant of optimum mature body size. Larger animals need more forage. • Forage quality is the main determinant of optimum milking ability. Higher-milking females need higher-quality forage.