Genetics 24231 Faculty of Agriculture

1. Genetics 24231Faculty of Agriculture Instructor: Dr. Jihad Abdallah Topic 12: Introduction to Quantitative Genetics

2. Simply Inherited and Polygenic Traits • Simply inherited traits: traits affected by one or few genes (coat color, presence of horns, shape of seed in garden pea, …..). • Phenotypes of simply inherited traits are placed into categories (qualitative or categorical or discontinuous traits) • Not affected or very little affected by the environment.

3. Polygenic traits: are traits affected by many genes (no single gene having an overriding effect) like growth rate, milk production, birth weight, etc. • Generally described in numbers. • Typically quantitative or continuous in their expression (quantitative or continuous traits) • Polygenic traits are affected by the environment. • They generally have a normal distribution

4. The locations on the chromosome that contain the genes which affect quantitative traits are called Quantitative Trait Loci (QTLs). • QTLs are are identified by genetic mapping using DNA markers. • An QTL region may contain a single gene or two or more closely linked genes.

5. Quantitative genetics: is the science which deals with the inheritance of quantitative traits.

6. The basic model for quantitative traits • P = G + E • P = phenotypic value for the trait of one individual (plant or animal). • G = the effect of the genes carried by the individual (genotypic value). • E = the effect of the environmental factors on the phenotype of the animal.

7. Genotypic Value • Genotypic value is the overall effect of all the genes carried by the individual on its phenotype. It includes: • Additive effects of genes (A): the sum of individual effects (average effects) of alleles. • Dominance effects of genes (D): interaction between alleles at the same gene • Epistatic effects (I): interaction between alleles on different genes • G = A + D + I P = A+ D + I + E

8. The phenotypic variance is decomposed into genetic and environmental variances • VP = VA + VD + VI + VE • VP = phenotypic variance (total variance) • VA = the variance due to additive effects of genes • VD = the variance due to dominance effects of genes • VI = the variance due to epistatic effects of genes • VE = the variance due to environmental effects.

9. Heritability • Heritability in the broad sense (H2):is the proportion of the phenotypic variance that is due to all genetic effects ( additive, dominance and epistasis): It measures the strength of the relationship between the phenotypic values of the individuals and their genotypic values.

10. Heritability in the narrow sense (h2):is the proportion of the phenotypic variance that is due to additive genetic effects only. • It measures two things: • The degree to which the offspring resemble their parents in the phenotype for a trait. • The strength of the relationship between the phenotypic values and the additive genetic effects (the relationship between P and A).

11. Notes: • Heritability is a measure on a population of individuals in a given environment for a given character. It is NOT measured on one individual. • Heritability can be estimated for each quantitative trait. • It varies from population to another and from environment to another for the same traits.

12. Importance of heritability • Heritability is very important in selection (in genetic improvement) • It determines if phenotypic selection would be efficient or not: • Small heritability: phenotypic selection is not efficient (low accuracy of selection). • High heritability: phenotypic selection is efficient (high accuracy of selection)

13. Example • Suppose that for lamb weight at six months for the sheep population in Palestine: VA = 40, VD=8, VI =2, VE=50. Calculate the heritability in the broad sense and heritability in the narrow sense for this trait. Heritability in the broad sense: Heritability in the narrow sense: