1. Quantitative Variation
Quantitative (metric or polygenic) characters of
Productive traits
Examples : milk production, egg production, meat
production, body weight etc

2. Quantitative characters typically have continuous,
approximately normal distributions and include
characters such as reproductive fitness, longevity,
height, weight, disease resistance, etc.

3. A major challenge in the study of quantitative
genetics is to determine how much of the observed
variation is due to genetics and how much is due
to environment.
One of the central concepts of quantitative genetics
is heritability.
Heritability is the proportion of the total
phenotypic variance in a population due to genetic
differences among individuals.

4. Algebraically, we can define the phenotypic value
Of an individual as the consequence of the alleles
It inherits together with environmental influences
As:
P = G + E
Where P = phenotype, G = Genotype, and
E = Environment.

5. The genetic component can be partitioned from
the environmental component as:
VP = VG + VE + 2CovGE
Where, CovGE is the covariance between genetic
and environmental effects.
The covariance for this component is expected
to be 0 if conditions for different genotypes
are equalized by randomly allocating individuals
across the range of environment, which is
difficult to achieve in wild populations.

6. For example, in territorial species of birds and
mammals, the genetically fittest parents may
obtain the best territories.
Offspring inheriting the best fitness genotypes
also inherit the best environments.
This results in a genotype X environment
correlation that increases phenotypic resemblance
among relatives.

7. Differences in performance of genotypes in
different environments is referred to as
Genotype X Environment Interactions.
These develop when populations adapt to particular
environmental conditions, and survive and
reproduce better in their native conditions than
in other environments.
Genotype X Environment Interactions are of major
significance to the genetic management of
endangered species as follows:

8. Quantitative genetic variation has contributions
from the average effects of loci VA, from their
dominance deviations VD, and from interactions
(epistatic) deviations among gene loci VI as:
VG = VA + VD + VI
These are referred to as additive genetic
variance (VA), dominance variance (VD), and
interaction variance (VI).
Each of these has major conservation implications
as follows:

9. VA and especially the ratio VA/VP (heritability)
reflect the adaptive evolutionary potential of the
population for the character under study.
VD reflects susceptibility to inbreeding depression.
VI influences the effects of outbreeding, whether
beneficial or deleterious.

10. Heritabilities range from 0 to 1.
Heritabilities of 0 are found in highly inbred
populations with no genetic variation.
Heritabilities of 1 are expected for characters with
no environmental variance in an outbred population
if all genetic variance is additive.
Heritabilities are specific to particular populations
living under specific environmental conditions.

11. Heritability and VA are fundamentally measures of
how well quantitative traits are transmitted from
one generation to the next.
Unfortunately, very few heritability estimates
exist for endangered species and there clearly is
need for many more estimates of heritability in
threatened and endangered species.

13. THE USE OF HERITABILITY :
SELECTION BASIS FOR ALL ORGANISM
THE PRINCIPLE TO DO ANIMA;\L BREEDING PROGRAM
THE BASIC EQUATION :
- BREEDING VALUE
_ _
BV = h2 ( Y – H)