Heritability in Plant Breeding

Heritability

Ø  The ratio of genotypic variance to the phenotypic or total variance is called as heritability.

Ø  It is generally expressed in percentage.

Ø  It is the good index of the transmission of characters from parents to their off spring.

Ø  The estimates of heritability helps the plant breeder in selection of elite genotypes from diverse genetic populations.

Types of Heritability

Ø  Depending upon the components of variance used as numerator in the calculation, heritability is of two types, viz; broad sense and narrow sense.

Broad Sense Heritability

Ø  It is the ratio of genotypic variance to the phenotypic variance.

Ø  It is calculated from total genetic variance which consists of additive, dominance and epistatic variances.

Main Features

Ø  It can be estimated from both parental as well as segregating populations.

Ø  It is estimated from total genetic variance.

Ø  It is useful in selecton of elite types from homozygous material.

Ø  It is calculated from following formula:

Heritability (H) = Vg/Vp = Vg/(Vg + Ve)

Where, Vg, Vp, Ve are genotypic, phenotypic and environmental components of variances, respectively.

Narrow Sense Heritability

Ø  It is the ratio of additive or flexible genetic variance to the total or penotypic variance.

Ø  It plays an important role in selection process in plant breeding.

Main Features

For estimation of narrow sense heritability, crosses have to be made in definite fashion.

It is estimated from additive genetic variance.

It is useful in selecton of elite types from segregating material.

It is calculated from following formula:

Heritability = ½ D / VP

Where, D is the additive geneti variance and VP is the phenotypic variance.

            Since, narrow sense heritability is estimated from additive genetic variance it plays important role in selection of elite genotypes from the segregating populations, whereas broad sense heritability estimates are more useful in selecting superior lines from the homozygous materials.

Estimation of Heritability

In Broad Sense

Ø  The calculation of heritability in broad sense requires an estimate of genotypic variance in a population.

Ø  The genotypic variance consists of additive, dominance and epistatic variances.

Ø  The broad sense heritability from simple trials and generation mean analysis is estimated as follows:

From Simple Trials

            From the data of simple replicated experiment of several genotypes first the genotypic, phenotypic and environmental variances are calculated and hen heritability is estimated as given below:

Heritability (bs) = VG / VP x 100

Where, VG and VP are geypic, phenotypic variances, respectively.

From Generation Mean Analysis

The heritability is worked out as per the given formula:

Heritability (bs) = (VF₂-VF₁) / VF₂ x 100

Where, VF₁ and VF₂ are the variance of F₁ and F₂ respectively.

In Narrow Sense

Ø  The calculation of heritability in narrow sense requires an estimate of additive genetic variance in a population.

Ø  In narrow sense, the heritability is calculated from diallel analysis and generation mean analysis as follows:

Diallel Analysis

From the diallel analysis the heritability in narrow sense can be calculated from the formula:

Heritability (ns) = (1/2 D + ½ H1 – ½ H2 – ½ F) / ½ D + ½ H1 – ¼ H2 – ½ F + E x 100

Verhalen and Murray (1969) proposed the following formula for calculation of heritability in narrow sense from F2 generation of diallel cross.

Heritability (ns) = ¼ D / (1/4 D + 1/16 H1 – 1/8 F + E) x 100

Generation Mean Analysis

From the estimates of generation mean analysis heritability in narrow sense can be worked out as:

Heritability as per Warner (1952) = [(1/2 D) / VF₂] x 100

where, D and VF₂ are additive and phenotypic variances, respectively.

As per Mather (1949) = [D / D + H + E] x 100

where, D, H and E are additive, non-additive and error variances, respectively.

            Only ½ D is used to calculate the heritability in narrow sense, because the phenotypic variance of F₂ has only ½ D as its additive component.

Factors Influencing Heritability

1.       Type of Genetic Material

Ø  The magnitude of heritability is largely governed by the amount of genetic variance present in a population for the character under study.

Ø  Greater the genotypic variance higher the heritability. The derivatives of several divergent parents are expected to express more genetic variance than derivatives of closely related parents.

Ø  The degree of inbreeding also influenes the genetic variances.

Ø  The geneticvariances increases with increase in rate of inbreeding. Thus heriability estimates are higher in F₄ or F₅ than in F₂ after inbreeding.

2.       Sample Size

Ø  Heritability estimates are influenced by the sample size on which the estimates are based.

Ø  If the estimates are based on the entire population it will give the true genetic variance of a population, but evaluation of entire population is not practically possible.

Ø  Large sample will give estimates near to the population mean, while small sample may give misleading information.

3.   Sampling Method

Ø  There are two main sampling methods, viz., random and biased sampling. The random sampling method generally provides true estimates of genetic variance and hence of heritability.

Ø  The biased sampling on the other hand will not give true representative estimates of genetic variance and thereby heritability.

Ø  When a non-random sample of genotypes is evaluated to compute the ratio of genotypic variance to the phenotypic variance, the ratio is not called as heritability. This is termed as repeatability.

4.   Conduct of Experiment

Ø  The heritability estimates are influenced by expenimental error or error variance, because error variance is a part of denominator used in the estimation of heritability.

Ø  The degree of uniformity in the test environment will influence the experimental error. Any precaution that the breeder can take to reduce experimental error (VE) will improve the heritability of a character.

Ø  The error variance can be reduced by either increasing the plot size or number of replications.

Ø  Moreover, multilocation testing will provide more reliable estimates of heritability than testing at one location only.

5.       Method of Calculation

Ø Heritability is estimated by several methods. The estimates of heritability obtained by various methods will vary to some extent.

Ø The estimates of broad sense heritability arc always higher than narrow sense heritability.

6.   Effect of Linkage

Ø Heritability estimates are considerably influenced by the presence of linkage. Linkage influences the heritability estimates by causing an upward or downward bias in the estimates of additive and dominance genetic variance.

Ø High frequency of coupling phase linkage (AB/ab) causes an upward bias in the estimates of additive and dominance variances (Hallauer and Miranda, 1981).

Ø An excess of repulsion phase linkage (Abl aB) leads to upward bias in the dominance variance and downward bias in the additive variance.

Ø Linkage disequilibrium can be reduced by random mating of a population.

Ø The number of generations of intermating required for breaking the linkage depends on the closeness of the linkage (Hanson, 1959).

Advantages

1. Estimates of heritability are useful in predicting the transmission of characters from the parents to their offspring.

2. Estimates of heritability are free from genetical assumptions.

3. Heritability estimates are based on empirical results.

4. The broad sense heritability can be estimated from parental as well as hybrid populations.

5. Heritability helps in the selection of elite types from the mixed parental populations or segregating populations.

6. Narrow sense heritability gives an idea about the additive genetic variance.

7. Heritability estimates can be worked out from both inbreeders as well as outbreeders.

Demerits

1.   Estimates of heritability are based on variances and its components and, therefore, are not statistically very robust and reliable.

2.   The variance and its components, as compared to means, are also inaccurately estimated.

3.   The partition of variance makes the things worse. Components of variance sometimes tuna out to be negative, resulting in negative estimates of heritability.

4.   Heritability is the property of a specific population in a specific experiment. In other words, heritability is specific to material under study and structure of the experiment.

Co-Heritability

Ø Coheritability refers to the ratio of genetic covariance to the phenotypic covariance.

Ø Thus, it deals with the simultaneous inheritance of two characters.

Ø The main features of coheritability are given below:

Ø It measures inheritance of two characters simultaneously.

Ø Estimation of coheritability is based on genotypic and phenotypic covariances of the characters under consideration.

Ø Estimates of coheritability are useful in the simultaneous improvement of two characters.

Ø Coheritability is estimated with the help of following formula.

Co-heritability (x₁.x₂) = GCOV x₁.x₂ / (PCOV x₁.x₂) x 100

where. GCOV and PCOV are the genotypic and phenotypic covariances of characters x₁ and x₂, respectively. Coheritability is generally expressed in percentage.

            Coheritability takes both genotypic as well as phenotypic covariances into account and helps in understanding changes taking place in pairs of polygenic characters. High values of coheritability estimate suggest that increase in one polygenic trait will lead to simultaneous increase in another col-writable enaracter. Thus coheritability may form a more meaningful index for achieving the breeding objectives (Riswas and Sasmal, 1989).