Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 170

Heritability is a property of the trait, the population, and the environment. Changing any of these factors will result in a different estimate of heritability. There are two different estimates of heritability.

1 Broad sense heritabilityHeritability estimated using the total genetic variance (VG) is called broad sense heritability. It is expressed mathematically as:It tends to yield a high value (Table 4.2). Some use the symbol H2 instead of H.

2 Narrow sense heritabilityBecause the additive component of genetic variance determines the response to selection, the narrow sense heritability estimate is more useful to plant breeders than the broad sense estimate. It is estimated as follows:

Table 4.2 Heritability (broad sense) estimates of some plant architectural traits.

Trait	Heritability
Plant height	45
Hypocotyl diameter	38
Number of branches/plant	56
Nodes in lower third	36
Nodes in mid section	45
Nodes in upper third	46
Pods in lower third	62
Pods in mid section	85
Pods in upper third	80
Pod width	81
Pod length	67
Seed number per pod	30
100 seed weight	77

However, when breeding clonally propagated species (e.g. sugarcane, banana), in which both additive and nonadditive gene action are fixed and transferred from parent to offspring, broad sense heritability is also useful. The magnitude of narrow sense heritability cannot exceed and is usually less than the corresponding broad sense heritability estimate.

Heritabilities are seldom precise estimates because of large standard errors. Characters that are closely related to reproductive fitness tend to have low heritability estimates. The estimates are expressed as a fraction, but may also be reported as a percentage by multiplying by 100. A heritability estimate may be unity (1) or less.