Agriculture Reference
In-Depth Information
Table 7.13
Univariate probability that a genotype taken at random from a segregating
progeny with a common parent will have a Breeders' Preference greater than 4, on a 1
to 9 scale, relative raking of that probability, and proportion and ranking of genotypes
that survive the 4th selection stage at the Scottish Crop Research Institute.
Clone
Cross prediction
Rank
Percentage of
Rank
>
of preference
4
year 4 clones
selected in year 7
Maris Peer
69.17
1
17.69
1
3683.A.2
62.57
2
11.76
2
Pentland Ivory
60.40
3
7.11
4
G.6755.1
59.74
4
6.29
5
Cara
57.34
5
10.95
3
8204.A.4
54.42
6
5.13
6
Pentland Squire
49.25
7
3.18
7
8
=
Dr Macintosh
47.37
8
0.00
=
Self crosses
37.99
9
0.00
8
prediction trials. The numbers of genotypes from those
hybrid combinations with the highest probability of
producing a new cultivar were increased while the less
desirable cross combinations were discarded.
This scheme, in addition to providing information
of the commercial potential of each cross combination,
also was used to determine the suitability of individ-
ual parents. The progeny mean and genetic variance of
each parent was used in the prediction estimation. In
Table 7.13 are shown rankings of 9 parents based on this
system along with the number of desirable recombinant
lines which resulted from crosses involving the parents.
Despite one or two changes in rank order there was good
agreement in the predicted and observed indicators.
The differences which were observed could be explained
by morphological characters (i.e. Cara has a pink eye
and there was positive emphasis to select these types)
or pest preferences (i.e. Maris Piper has nematode resis-
tance and only clones which possessed the resistance
were continued irrespective of other characters).
In a straightforward commercial context, and for a
short-term objective, only a limited number of crosses
are to be considered, one simple and effective strategy
is to cross the best with the best. Therefore identify the
phenotypically and genetically best parents, intercross
these and select amongst their progeny.
Many breeders use the strategy of combining com-
plementary parents. For example, to inter-mate a high
yielding poor quality line with a low yielding but high
quality line. In theory this type of combination could
allow the selection of a high yield high quality recom-
binant. However, what is often achieved is an average
yield with average quality. It is usually necessary to use
some form of pre-breeding where the high yielding line
is first crossed (or backcrossed) to a high quality line,
parents are selected several times, and these are in turn
used in final cross combinations.
Similarly when a character is introduced from a wild
or unadapted genotype it may take many rounds of
backcrossing to get the desired character into a com-
mercial background before the trait is introduced into
a new cultivar. This is of course, where some of the
newer techniques of genetic transformation and marker
assisted selection offer other alternatives to the breeder.
Parental combinations
Having decided on a set of parental lines the next deci-
sion to be made is
how many crosses should be made
and
which combinations will yield best results?
If there is a means by which large numbers of crosses
can be evaluated then many crosses will yield better
results than if only a few are tried. However, it should
be noted that there is little to be gained by making more
crosses than can be screened in an effective manner.
Germplasm collections
Germplasm is the basic raw material of any plant breed-
ing programme. It is important that genetic diversity
is maintained if crop development is to continue and
