Heritability , genetic advance

Heritability , genetic advance

1. HERITABILITY , GENETIC ADVANCE , GENOTYPE -ENVIRONMENT INTERACTION SUBMITTED BY Pawan Nagar M Sc. Horti ROLL NO . O4-2690S-2015

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

3. 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. 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 , V g = genotypic variance V p = phenotypic variance V e = error variance Heritability ( h² ) = V g / V p x 100 = V g / V g + V e x 100

6. broad heritability ( h 2 ) separates genotypic from environmentally induced variance : h 2 = V g / V p 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

7. 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 V A or D = additive genetic variance V P or VP = phenotypic variance Heritability ( h² ) = V A / V P x 100 or ½ D / VP

8. 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

9. 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

10. 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

11. H 2 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 (V A ) are fundamentally measures of how well quantitative traits are transmitted from one generation to the next

12. 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

13. 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

14. 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

15. 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

16. 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

17. The genetic advance is calculated by the following formula where , K = standardize selection differential h² = heritability of the character under selection δ p = 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 δ p

18. 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

19. 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

20. 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

21. 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 + G x E

22. 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

23. 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 C ross 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 . A s a result the differential response of genotypes differ in type (not scale) of response (promotion or inhibition)

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

25. 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)

26. 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