Heritability , genetic advance
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Transcript of Heritability , genetic advance
HERITABILITY , GENETIC ADVANCE , GENOTYPE -ENVIRONMENT INTERACTION
SUBMITTED BYPawan Nagar
M Sc. HortiROLL NO . O4-2690S-
2015
CONTENTS Components of variation
HeritabilityTypes of heritability
Genetic advance Environment
Genotype x Environment interaction
COMPONENTS OF VARIATION
The quantitative variation in a population is of three types ,
Phenotypic variation Genotypic variation
Environmental variationFISHER 1918 , divided the genetic variance into three
components Additive variance
Dominance variance Epistasis variance
HERITABILITY
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
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
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
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
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
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
INTERPRETATION OF RESULTS
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
INTERPRETATION OF RESULTS
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
HERITABILITY 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
FACTORS AFFECTING HERITABILITY
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
FACTORS AFFECTING HERITABILITY
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
GENETIC ADVANCE 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
Selection differential
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
Genetic gain 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
COMPUTATION OF GENETIC ADVANCE
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
INTERPRETATION OF GENETIC ADVANCE
If the value of Genetic advance highThe character is governed by additive genes and selection
will be beneficial for such traits
If Genetic advance is lowThe character is governed by non additive genes and
heterosis breeding may be useful
ENVIRONMENTThe external condition that affects the expression of genes
of genotypeComstock 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
EnvironmentAllard and Bradshaw ,1964 classified Environmental
variables into two groupsPredictable 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
PHENOTYPE IS THE FUNCTION OF GENOTYPE AND ENVIRONMENT
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
GENOTYPE x ENVIRONMENT INTERACTION
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
Genotype x Environment interaction
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 interactionQualitative 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)
No G x E interaction
G x E interaction is quantitative
G x E interaction is qualitative
Genotype x Environment Interactions
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)
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