Agriculture Reference
In-Depth Information
•
At least some of this phenotypic variation must have
a genetic basis
F
1
family, and will be reduced, by half, in each subse-
quent selfed progeny. Similarly, the dominance genetic
variance will be dependent on the degree of heterozy-
gosity in the population and will differ between filial
generations. A more useful form of heritability for plant
breeders, therefore, is
narrow-sense heritability
(
h
n
)
This relates to the concept of heritability, the propor-
tion of the phenotypic variance that is genetic in origin.
This proportion is called the heritability and this section
is concerned with the ways of estimating heritability.
Values of heritability (
h
2
,
which
is the ratio of
additive genetic variance
(
V
A
)
to
)
can range from zero to one.
If
h
2
is relatively high (e.g. close to 1) there is potential
for a breeding programme to alter the mean expression
of the character in future generations. On the other
hand, if
h
2
is close to zero, there will be little scope for
advancement and there would probably be little point
in trying to improve this character in a plant breeding
programme.
There are three main ways of estimating heritability.
total phenotypic variance.
Why should lack of resemblance between parents
and their offspring be attributable to dominance, but
not additive, components? Well, dominance effects are
a feature of particular genotypes; but, genotypes are
'made' and 'unmade' between generations as a result of
genetic segregation during the production of gametes.
Thus, the mean dominance effect in the offspring of a
particular cross can be different from that of the par-
ents, even when there is no selection. On the other
hand, when selection is applied, there may be no change
or even change in the 'wrong' direction. This is not
true of additive genetic effects. The additive genetic
component must remain more or less constant from
one generation to the next in the absence of selection.
While, if differential selection is applied, the change
between generations must be in the direction corre-
sponding to the favoured alleles. In addition, additive
genetic variance is constant between filial generations
and so narrow-sense heritability of recombinant inbred
lines can be estimated from early-generation segregating
families.
In the first filial generation, after hybridization
between two homozygous parents (F
1
)
Carrying out particular genetic crosses so that the
resulting data can be partitioned into their genetic
and environmental components
•
•
Based on the direct measurement of the degree of
resemblance between offspring and one, or both, of
their parents. This is achieved by regression of the
former onto the latter in the absence of selection
•
Measuring the response of a population to given levels
of selection (this will not be discussed until we cover
selection later)
The essential background theory of heritability was
presented in the previous quantitative genetics section.
Heritability is a ratio of genetic variance divided
by total phenotypic variance. In a simple additive-
dominance model of quantitative inheritance the total
genetic variance will contain
dominance genetic vari-
ance
(denoted by
V
D
)
, there is no
genetic variance between progeny and all variation
observed between F
1
plants will be entirely environ-
mental. The first generation for which there are both
genetic and environmental components of phenotypic
variance is the F
2
. Partitioning of phenotypic variance
and the calculation of the broad-sense (and ultimately
narrow-sense) heritabilities will be confined to this
generation.
and
additive genetic variance
(denoted by
V
A
)
. Dominance genetic variance is vari-
ation caused by heterozygotes in the population, while
additive genetic variance is variation between homozy-
gotes in the segregating population.
Broad-sense heritability
(
h
b
)
is the total genetic vari-
ance divided by the total phenotypic variance. The
total genetic variance in an additive-dominance model
is simply
V
A
+
Broad-sense heritability
V
D
. The total phenotypic variance is
obtained by summing the genetic variance plus the
environmental variance.
The degree of heterozygosity within segregating pop-
ulations will be related to the number of selfing gener-
ations. Maximum heterozygosity will be found in the
The first step is to derive an equation for the genetic
variance of the F
2
generation. The genetic variance of
the F
2
generation (without proof ) is:
1
2
V
A
+
1
4
V
D
2
σ
F
2
=
