This document discusses the key concepts of phenotypic variance, environmental variance, genetic variance, heritability, and the different components of genetic variance. It explains that phenotypic variance is the total observable variability and includes both genotypic and environmental influences. Genetic variance refers only to the inheritable portion that is important for crop improvement. Genetic variance can be further divided into additive, dominance, and epistatic components. The document also defines broad and narrow sense heritability and how they are estimated. It provides examples to illustrate concepts like additive and dominance effects.

1. SUBMITTED TO:
Dr. ARNA DAS
ASSITANT PROFESSOR
DEPARTMENT OF GENETICS
AND PLANT BREEDING
BACA, AAU ,ANAND.
SUBMITTED BY:
N.VISHNUREKHA
2010118086
M.Sc. AGRICULTURAL
STATISTICS
BACA,AAU, ANAND.
COURSE : GP 501 PRINCIPLES OF
GENETICS

2. Phenotypic variance:
 It is the total variability.
 It includes both genotypic and environmental variation.
Environmental variance:
 It refers to the non heritable variation.
 It is measured in terms of error mean square.
 The variation in true breeding line and in F1 is non heritable.

3.  Genetic Variance:
it is the heritable portion of total or phenotypic
variance.
 In crop improvements programme, only the genetic
components of variation are important because only
this component is transmitted to the next generation.
 According to Fisher in 1918, components of genetic
variance divided into three components viz.
1) Additive,
2) Dominance
3) Epistatic

4. 1) Additive Variance:
Genetic variance due to average effects of genes on all
segregating loci.
•Arises from differences between two homozygotes of a gene
i.e., AA and aa.
• Show lack of dominance and have intermediate expression
in Aa.
•Fixable and hence selection for traits governed by such
variance is effective.
• Only variance responds to selection.
• Breeding value of individual is measured directly by
Additive gene effects. General combining ability of a parent is a
measure of additive gene effects.

5.  It is the chief cause of transgressive segregation.
 it is required for the estimation of Narrow sense heritability.
 higher in magnitude in natural population.
Eg: In a CORN, genes A and B control the yield of the corn.
 A – 2 corns, a – 1 corn ; B – 6 corns , b – 3 corns
each allele will contribute to the yield of the corn.
 AaBb X AaBb
 AABB - 16 (2+2+6+6) ; AABb - 13
AABb - 13 ; AaBB - 15 ; Aabb - 9 ; AaBb - 12
aaBB - 14 ; aabb - 8 ; aaBb - 11

6. 2) Dominance Component:
 It is due to the deviation of heterozygote (Aa) phenotype from
the average of phenotypic value of the two homozygotes (AA
and aa).
 It is also referred as intra-allelic interaction.
 Associated with heterozygosity, maximum in cross pollinated
crops and minimum in self pollinated crops.
 It shows incomplete , complete , over dominance.
 Not fixable and hence selection is ineffective.
 Gets depleted through selfing or inbreeding.

7.  SCA is the measure of dominance variance in diallele, partial
diallele and line x tester cross analysis.
 It is the chief cause of heterosis.
 Moderate in magnitude in natural population.
Eg: A plant produces white flowers if its genotypes are A1A1
and red flowers if its genotypes are A2A2
 we would assume that A1 A2 leads to pink flower but in
dominance variance there is NO MIDPOINT
 If A2 is dominant, A1 A2 would produce red flowers.

8. 3) Epistatic or Interaction Components:
It results from an interaction between two or more genes.
Later Hayman and Mather classified the epistatic components
into three types interaction viz.
1) Additive X additive,
2) Additive X dominance,
3) Dominance X dominance.

9.  Eg: In the same flower, a B1 gene produces pigment whereas a
B2 gene does not produce any pigment (flower appears white)
 A2A2B1B1 – red flower
 A1A1B2B2 – white flower
 A2A2B2B2 – white flower

10.  The ratio of genetic variance to the total variance (i.e)
phenotypic variance is known as heritability.
 The extent of contribution of genotype to the phenotypic
variation for a trait in a population is ordinarily expressed as
the ratio of genetic variance to the total variance.
 Heritability ranges from 0 to 1.
Heritability H = VG/ VP Or = VG / VG + VE
Where VG - genotypic variance
 VP - phenotypic variance and
 VE - environmental variance .

11. There are two types of heritability viz
1) Broad sense heritability and
2) Narrow sense heritability.

12.  It is the ratio of genotypic variance to the total phenotypic
variance.
h
2 (bs) = VG/VP or VG/VG+VE
 Estimates are valid for homozygous lines, or populations.
 However, when we are dealing with segregating generation.
The genetic variance consists of additive and dominance
component.
 Since in self pollinated crop we develop homozygous lines,
the dominance component will not contribute to the phenotype
of homozygous lines derived from a population.
 In such cases only the additive component of variation is
important.
 Therefore, for segregating generation broad sense heritability
is less important because it cannot realize fully in the

13.  Heritability values are characterized as fallow.
 Low - less than 30%
 Moderate - 30-60%
 High - More than 60% (Johnson et al. 1955).
 Useful in selection of elite types from homozygous
population.
 Can be estimated from both parental and segregating material.
 Estimated from total genetic variance.
 More useful in animal breeding.

14.  It is the ratio of additive genetic variance VA to the
total phenotypic variance VP (smith, 1952)
 h
2 (ns) = VA/VP = VA/VG + VE
 Narrow sense heritability is reliable measures, as it is
based on breeding value.
 The magnitude of narrow sense heritability is always
less than or equal to broad sense heritability.
 Requires crossing in a definite fashion.

15.  Narrow sense heritability is classified as follow.
i. Low -5 to 10 % .
ii. Medium -10-30%
iii. High - more than 30% Robinson et.al(1966).
 Estimated from additive genetic variance.
 Useful in both plant and animal breeding.
 Useful in selection of elite types from segregating material.

16.  Heritability can be estimated by three different methods.
a) From analysis of variance table of a trial consisting of a
large number of genotypes.
b) Estimation of VG and VE from the variance of P1, P2, P3,
P4 generation of a cross.
c) Parent – offspring regression upon doubling provides
estimates of heritability.
 Thus, H = 2b, where b is the regression of progeny means on
parent value.
 When heritability is estimated from the above three methods is
known as broad sense heritability

17. Broad sense heritability
 It require estimation of genetic variance in a population . It is
estimated as two way
1) Simple trials:
h
2 (bs) = VG / VP X 100
where VP = VG + VE
2) From Generation mean analysis
( VF2 - VF1) / VF2 X 100
 Where VF2, VF1 are variance of F1 and F2 respectively

18. Narrow sense heritability
 For the estimation of narrow sense heritability require
estimation of additive genetic variance
1)Diallel Analysis
h
2 (ns) = (1/2 D + ½ H1- ½ H2 – ½ F) X 100 (1/2 D +
½ H1- ¼ H2 – ½ F+ E)
2) Generation mean analysis.
 Heritability= ½ D / VF2 X 100 Warner (1952)….
Heritability = D/ (D+H+E) X 100 Mather (1949)…

19.  Because it allows us to predict a trait’s response to selection
R = h
2 S.
Where;
 R = response to selection
 h
2 = heritability
 S = selection differential

20.  Heritability does not indicate the degree to which a
characteristic is genetically determined.
 An individual does not have heritability.
 Narrow-sense heritability of 0.6 in population does not indicate
that an individual’s characteristic is 60% additive.
 There is no universal heritability for a characteristic.
 Two populations will have different heritability due to
environment.
 Even when heritability is high, environmental factors may
influence a characteristic.
 Heritability indicates nothing about the nature of population
differences in a characteristic.

21.  useful in predicting the effectiveness of selection.
 helpful for deciding breeding methods to be followed for
effective selection.
 It gives us an idea about the response of various characters to
selection pressure.
 useful in predicting the performance under different degree of
intensity of selection.
 helps for construction of selection index.
 Estimates of heritability serve as a useful guide to the breeder,
to appreciate the proportion of variation that is due to
genotypic or additive effects.

22.  If heritability in broad sense is high
 It indicates character are least influenced by environment
 selection for improvement of such characters may be useful.
 If heritability in broad sense is low
 The character is highly influenced by environmental effects
 Genetic improvement through selection will be difficult.
 If heritability in narrow sense is high
 characters are govern by additive gene action,
 Selection for improvement of such characters would be
rewarding(selection method)
 If low heritability in narrow sense
 governed by non-additive, non-fixable genes.
 Heterosis breeding will be beneficial

23.  H2 varies from 0 (all environment) to 1 (all genetic)
 Heritability of 0 are found in highly inbred populations with
no genetic variation.
 Heritability of 1 are expected for characters with no
environmental variance in an outbred population if all genetic
variance is additive.
 Heritability are specific to particular populations living under
specific environmental conditions
 Heritability (h²) and Additive Variance (VA ) are
fundamentally measures of how well quantitative traits are
transmitted from one generation to the next.

24.  Falconer D.S and Mackay J. 1998.Introduction to Quantitative
Genetics. Longman.
 Naryanan S.S and Singh P. 2007. Biometrical Techniques in
Plant Breeding. Kalyani.
 George Acquaah.2012. Principles of Plant Genetics and
Breeding. Wiley blackwell.