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