This document discusses quantitative genetics and the concepts of phenotypic value, genotypic value, environmental deviation, and population mean. It explains that quantitative traits are controlled by multiple genes and their inheritance is called quantitative or polygenic inheritance. The phenotypic value of an individual is determined by the sum of their genotypic value and environmental deviation. The population mean genotypic value, which is equal to the population mean phenotypic value if environmental deviations average to zero, can be calculated based on allele frequencies at loci and their additive and dominance effects.

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. a. Sexual selection
b. Heritability
c. Linkage Equilibrium
d. Fossil Record
e. 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.
 Genotypic 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)
Environmental 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, A1 and A2.
The genotypic value of
A1 A1 homozygote = +a
A2 A2 homozygote = –a and
A1 A2 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 A1 and A2 be p and q

10. POPULATION MEAN
Genotype Frequency Genotypic value Frequency-weighted
genotypic value
A1 A1
p2 +a p2a
A1 A2
2pq d 2pqd
A2 A2
q2 -a - q2a
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