Agriculture Reference
In-Depth Information
Phenotypic parent evaluation trials can be carried out
at relatively low cost. When many new parents are to
be assessed then specific trials can be arranged. These
trials should be organized with the same criteria of good
experimental design that other evaluation requires. In
cases where only one or two new parents are to be
considered it is often useful to include these genotypes
as controls in one of the breeding trials.
General combining ability can also be estimated
using North Carolina crossing designs. These, as noted
earlier, are of two forms:
•
North Carolina I designs
, where a number of parent
genotypes are crossed to one or more tester lines. In
these designs it is not necessary to hybridize each par-
ent to a common set of tester lines. From the design
an analysis of variance can estimate general comb-
ing ability of parents, which is tested for significance
against the testers within parents' mean square.
Genotypic evaluation
North Carolina II designs
, where a number of parent
genotypes are crossed to one or more tester and each
parent is crossed to the same tester lines. In this case an
analysis of variance can partition the total variation
into differences between specific combining ability
of the parents, combing ability of the testers and an
interaction term (parents
•
However, although often only after a new parent has
proven to have some merit on its phenotypic perfor-
mance is a more detailed examination of genotypic
worth carried out, the possibility that a valuable geno-
type (in terms of becoming a parent) might hide within
a poor phenotype still exists. Nevertheless, because of
limited resources and a lesser probability of a poor phe-
notype proving to be a good genotype, most effort is
devoted to further evaluating proven material to deter-
mine the true value of the parent in cross combination.
The most common means to determine the genetic
potential of new parental lines is to examine a series of
progeny in which the new parent features as one of the
parents. From these studies it is possible to determine
the
general combining ability
of a series of different
genotypes and to use this information to select the most
desirable parental lines.
General combining ability is an indication of how
the progeny from a particular genotype crossed to a
range of other genotypes responds. The most effective
means of determining general combining ability is by
diallel crossing designs, where the variation observed
in the diallel table is divided into general combining
ability of the parents used and specific combing ability
(all variation which cannot be explained by an additive
model of parental values). But as noted before, this does
limit the number of lines that can be examined.
General combing ability can be estimated from other
crossing designs. The simplest of these involves evalu-
ating the progeny that are produced by crossing the
potential parent with one or more tester lines. Tester
lines are chosen because of past experience in produc-
ing worthwhile results. For example, a new parent may
be crossed to a genetically productive genotype and also
to one with little genetic worth. The contribution of the
parent can be observed by examination of the offspring
from the crosses.
×
testers) which indicates
specific combing ability.
In addition to the statistical analysis of diallels
and other crossing designs more information can be
obtained from genetic analysis. The most common
means to achieve this is from a Hayman and Jinks'
Analysis where within array variances (
V
r
) and between
array covariances (
W
r
) are used to estimate the pro-
portion of dominant to recessive alleles for a given
character. Hayman and Jinks' Analysis can be used
therefore to choose parents which have high phenotypic
performance and with a high degree of dominant alleles.
When a suitable cross prediction scheme is employed
in the early generation of a plant breeding scheme it
is possible to use the cross prediction data to indi-
cate which specific parents have the highest probability
of producing desirable recombinants. A potential new
parent is hybridized to a number of different genotypes
and the progeny are examined to estimate the mean of
all crosses in which the parent is used and the genetic
variance of all crosses in which the parent appears. These
data can be used in the same way as illustrated earlier in
cross prediction.
Similar probabilities based on several traits simul-
taneously can provide useful indicators of the exact
worth of a new parent without waiting several years
to determine this potential from survivors in a selection
scheme.
At the Scottish Crop Research Institute cross pre-
diction at the seedling stage of the potato breeding
programme became a standard practice. Each year
between 200 and 300 crosses were evaluated in cross
