Values and Means - Part 1

Basic concepts in quantitative genetics

1. TOPIC: Quantitative Genetics Values and Means Dr S.Shanaz Assoc. Prof Division Of AGB

2. Continuously varying characters are called quantitative characters or metric characters (Example: economically important traits such as height, weight, milk yield, wool yield, egg production etc.) and variation in them is called quantitative variation or continuous variations. QUANTITATIVE INHERITANCE   Quantitative genetics is the study of continuous traits and their underlying mechanisms . Quantitative traits are controlled by multiple genes, each segregating according to Mendel's laws The inheritance of quantitative traits or poly genes is called Quantitative inheritance, Multiple factor inheritance, Multiple gene inheritance or Polygenic inheritance.

3. Sexual selection Heritability Linkage Equilibrium Fossil Record Hardy Weinberg exceptions

4. VALUES AND MEANS Genetic properties of a population are expressed in terms of gene frequencies and genotype frequencies. Concept of value : To understand the connection between gene frequencies and the quantitative differences exhibited in a quantitative/metric character A given quantitative trait is characterized by a mean value and a standard deviation expressed in metric units by which the character is measured.

5. Phenotypic value ( P ) The phenotypic value of a given quantitative trait is the yield of the individual with respect to the trait. Phenotypic value symbol is  P The phenotypic value can be measured and is evaluated in relation to the population mean value The phenotypic value ( P ) of an individual is determined by the combined effect of the genotypic value ( G ) and the environmental deviation ( E ) P = G + E

6. Genotypic value ( G ) Genotype is the sum total of genes possessed by an individual in pairs Environment is all the non-genetic circumstances that influence the phenotypic value. G enotypic value (G) is determined by the combined effect of all genes in all loci which influence the trait. Genotypic value : Symbol G . Environmental deviation (E) E nvironmental deviation represents the combined effect of all non-genetic factors that have influenced the phenotypic values (Symbol : E ) Genotype confers certain value on the individual Environment causes a deviation from this in one direction or the other For a single locus, the mean environmental deviation in the whole population is zero . Thus, mean phenotypic value is equal to the mean genotypic value . P =G

7. GENOTYPIC VALUE Genotypic value may be calculated by taking a mean of a large population with same genotype raised under similar conditions. Here mean environmental deviation in the whole population is taken as zero. The genotypic value is partitioned into additive gene action, dominance and epistatis . G = A + D + I where G -  Genotypic value A -  Additive value D -  Dominance deviation and I -  Interaction or epistatic value Considering a single locus with two alleles,  A 1   and  A 2. The genotypic value of A 1  A 1   homozygote =  +a A 2   A 2   homozygote =  –a  and A 1  A 2   heterozygote =  d

8. GENOTYPIC VALUE The value of  d  of the heterozygote depends on the degree of dominance. The degree of dominance may be expressed as  d/a  .

9. POPULATION MEAN Consider the following assumption: Diploid organism Diallelic autosomal locus Random mating population The mean phenotype is obtained by summing the frequency weighted genotypic values (assuming that environmental deviation is zero for each genotype ) Let the gene frequency of A 1  and A 2  be p and q

10. POPULATION MEAN Genotype Frequency Genotypic value Frequency-weighted genotypic value A 1   A 1 p 2 +a p 2 a A 1  A 2 2pq d 2pqd A 2   A 2 q 2 -a - q 2 a Total a(p-q)+2pqd The contribution of any locus to the population mean has two terms:  a (p-q )   attributable to the homozygote and   2pqd   attributable to heterozygote With additive combinations the population mean value resulting from the joint effects of several loci is the sum of the contributions of each locus. Population mean (M) = Σ a (p - q) + 2 Σ  pqd

11. Population Mean Genotypic Value is a function allele frequency This Population Mean Genotypic Value is identical to the population mean phenotypic value if the mean environmental deviation is zero. Population mean (M) = Σ a (p - q) + 2 Σ  pqd

12. Thanks