heritability genetic advance

1. HERITABILITY , GENETIC
ADVANCE , GENOTYPE -
ENVIRONMENT INTERACTION

2. Components of variation
Heritability
Types of heritability
Genetic advance
Environment
Genotype x Environment interaction

3. COMPONENTS OF VARIATION
The quantitative variation in a population is of three types ,
Phenotypic variation
Genotypic variation
Environmental variation
FISHER 1918 , divided the genetic variance into three
components
Additive variance
Dominance variance
Epistasis variance

4. In crop improvement only the genetic component of variation is
important since only this component is transmitted to the next
generation
Heritability is the ratio of genotypic variance to the phenotypic
variance
Heritability denotes the proportion of phenotypic variance that is due
to genotype i.e., heritable .
It is generally expressed in percent (%)
It is a good index of transmission of characters from parents to their
offspring

5. TYPES OF HERITABILITY
Depending upon the components of variance used as numerator
in the calculation ,there are 2 definitions of Heritability
1.Broad sense heritability
2. Narrow sense heritability

6. Broad sense heritability
• According to Falconer, broad sense heritability is the ratio of
genotypic variance to total or phenotypic variance
• It is calculated with the help of following formula
where , Vg= genotypic variance
Vp = phenotypic variance
Ve = error variance
Heritability (h²) = Vg / Vp x 100 = Vg / Vg + Ve x 100

7. Broad sense heritability
broad heritability (h2) separates genotypic from environmentally
induced variance: h2 = Vg / Vp
It can be estimated from both parental as well as segregating
populations
It express the extent to which the phenotype is determined by the
genotype , so called degree of genetic determination
It is most useful in clonal or highly selfing species in which genotypes
are passed from parents to offspring more or less intact
It is useful in selection of superior lines from homozygous lines

8. Narrow sense heritability
In outbreeding species evolutionary rates are affected by narrow-
sense heritability
It is the ratio of additive or fixable genetic variance to the total
or phenotypic variance
Also known as degree of genetic resumblance
it is calculated with the help of following formula
where VA or D = additive genetic variance
VP or VP = phenotypic variance
Heritability (h²) = VA / VP x 100 or ½ D / VP

9. NARROW SENSE HERITABILITY
It plays an important role in the selection process in plant breeding
For estimation of narrow sense heritability , crosses have to be
made in a definite fashion
It is estimated from additive genetic variance
It is useful for plant breeding in selection of elite types from
segregating populations

10. 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

11. If heritability in narrow sense is high
characters are govern by additive gene action
Selection for improvement of such characters would be rewarding
If low heritability in narrow sense
Non additive gene action
Heterosis breeding will be beneficial

12. 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

13. Type of genetic material : the magnitude of heritability is
largely governed by the amount of genetic variance present in a
population for the character under study
Sample size : Large sample is necessary for accurate estimates
Sampling methods : 2 sampling methods , Random and
Biased . The random sampling methods provide true estimates of
genetic variance and hence of heritability

14. Layout or conduct of experiment : Increasing the plot size
and no. of replications we can reduce experimental error and get
reliable estimates
Method of calculation : heritability is estimated by several
methods
Effect of linkage : high frequency of coupling phase (AB/ab)
causes upward bias in estimates of additive and dominance variances
. Excess of repulsion phase linkage (Ab/aB ) leads to upward bias in
dominance variance and downward bias in additive variances

15. Improvement in the mean genotypic value of selected plants over the
parental population is known as genetic advance
It is the measure of genetic gain under selection
The success of genetic advance under selection depends upon three factors
(Allard , 1960)
 Genetic variability : greater the amount of genetic variability in base
populations higher the genetic advance
 Heritability : the G.A. is high with characters having high heritability
Selection intensity : the proportion of individuals selected for the study is
called selection intensity . high selection intensity gives better results

16. It is the difference between the mean phenotypic value of selected
population and mean phenotype of original population
This is the measure of the selection intensity and denoted by K
where , Xs = mean of phenotypic value of selected plants
Xo = mean of phenotypic value of parental population
Selection
intensity
1 % 2% 5% 10%
value of K 2.64 2.42 2.06 1.76
K = Xs – Xo

17. The difference between the mean phenotypic value of the progeny of
selected plants and the original parental population is known as
genetic gain
It is denoted by R
where , Xp = mean phenotypic value of progeny of selected plants
Xo = mean of phenotypic value of base population
R = Xp – Xo

18. The genetic advance is calculated by the following formula
where , K = standardize selection differential
h² = heritability of the character under selection
δp = phenotypic standard deviation
The estimates of GS have same unit as those of the mean
The genetic advance from mixture of purelines or clones should be
calculated using h² (bs)
From segregating populations using h² (ns)
GS = K x h² x δp

19. If the value of Genetic advance high
The character is governed by additive genes and selection will be
beneficial for such traits
If Genetic advance is low
The character is governed by non additive genes and heterosis
breeding may be useful

20. The external condition that affects the expression of genes
of genotype
Comstock and Moll, 1963 classified in two groups
Micro environment :
environment of single organism , as opposed to that of another growing
at the same time and place e.g. physical attributes of soil , temp ,
humidity , insect-pests and diseases
Macro environment :
associated with a general location and period of time . A collection of
micro environment

21. Allard and Bradshaw ,1964 classified Environmental
variables into two groups
Predictable or controllable environment :
 includes permanent features of environment ( climate , soil type, day
length) controllable variable : fertilizer level, sowing date & density,
methods of harvesting . High level of interaction is desirable
Unpredictable or uncontrollable environment :
difference between seasons, amount & distribution of rainfall,
prevailing temperature . Low level of interaction is desirable

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

23. A phenotype is the result of interplay of a genotype and
each environment .
A specific genotype does not exhibit the same phenotypic
characteristics under all environment, or different genotype
respond differently to a specified environment.
This variation arising from the lack of correspondence between
genetic and non genetic effects is known as Genotype X
Environment Interactions.
 Differences in performance of genotypes in different
environments is referred to as Genotype X Environment
Interactions.
The low magnitude of genotype x environment interaction
indicates consistence performance of the population .Or it
shows high buffering ability of the population

24. Quantitative G x E interaction or Non crossover interaction
When performance of the varieties does not change over the
environments ,the differential response of genotypes is only a
matter of scale , such G x E interaction is termed as quantitative GxE
interaction
Qualitative or Cross over G x E interaction
In case of qualitative or cross over G x E interaction the
performance of varieties changes with the environment and a given
environment favours some genotype or detrimental to some . As a
result the differential response of genotypes differ in type (not
scale) of response (promotion or inhibition)

25. No
G x E interaction
G x E interaction is
quantitative
G x E interaction is
qualitative

26. Quantitative interactions are less important to breeders
 while , Qualitative G x E interactions complicate
identification and selection of superior genotypes.
A common strategy to manage the G X E interaction is to
test the genotypes over a representative range of conditions
( both locations and years)

27. REFERENCE
B. D. SINGH , Plant Breeding : Principles and
Methods
N. K. S. Kute , A. R. Kumar : Principles of Plant
Breeding
J. Brown , P. Caligari : An introduction to Plant
Breeding