Principles of Plant Genetics and Breeding

5 Every plant originates from a different F 2 plant, resulting
in greater genetic diversity in each generation.
6 It is suited to environments that do not represent
those in which the ultimate cultivar will be commercially
produced (no natural selection imposed).
Disadvantages
1 Natural selection has no effect (hence no benefit from
its possible positive impact).
2 Plants are selected based on individual phenotype not
progeny performance.
3 An inability of seed to germinate or a plant to set seed
may prohibit every F 2 plant from being represented in
the subsequent population.
4 The number of plants in the F 2 is equal to the number
of plants in the F 4 . Selecting a single seed per plant
runs the risks of losing desirable genes. The assumption
is that the single seed represents the genetic base
of each F 2 . This may not be true.
W. T. B. Thomas
Table 1 Characters listed in the current UK recommended lists of
barley (www.hgca.com).
Character Spring barley Winter barley
Yield (overall and regional with fungicide) Yes Yes
Yield without fungicide Yes Yes
Height Yes Yes
Lodging resistance Yes Yes
Brackling resistance Yes
Maturity Yes Yes
Winter hardiness Yes
Powdery mildew resistance Yes Yes
Rhynchosporium resistance Yes Yes
Yellow rust resistance Yes Yes
Brown rust resistance Yes Yes
Net blotch resistance Yes
BaYMV complex resistance Yes
BYDV resistance Yes
Grain nitrogen content Yes Yes
Hot water extract Yes Yes
Screenings (2.25 and 2.5 mm) Yes Yes
Specific weight Yes Yes
BREEDING SELF-POLLINATED SPECIES 299
Modifications
The procedure described so far is the classic singleseed
descent breeding method. There are two main
modifications of this basic procedure. The multiple seed
procedure (or modified single-seed descent) entails
selecting 2–4 seeds per plant, bulking and splitting the
bulk into two, one for planting the next generation, and
the other half held as a reserve. Because some soybean
breeders simply harvest one multiseeded pod per plant,
the procedure is also referred to by some as the bulk
pod method.
Another modification is the single hill method in
which progeny from individual plants are maintained as
separate lines during the early generations by planting
a few seeds in a hill. Seeds are harvested from the hill
and planted in another hill the next generation. A plant
is harvested from each line when homozygosity is
attained.
Industry highlights
Barley breeding in the United Kingdom
Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UK
Targets
Barley breeding in the UK aims to produce
new cultivars that offer an improvement in one
or more of the key characters for the region
(Table 1). New cultivars must have a good
yield, preferably in excess of the current established
cultivars, if targeted solely at the feed
market. To be accepted for malting use, a new
cultivar must offer improvement in one or
more key facets of malting quality, primarily
hot water extract, with no major defects in, for
instance, processability characters. Additionally,
new cultivars must have minimum levels of
disease resistance, which equates to being no
worse than moderately susceptible, to the key
diseases listed in Table 1.
Crossing to commercialization
Barley breeders therefore design crosses in
which the parents complement each other for
these target characters and attempt to select
out recombinants that offer a better balanced
overall phenotype. Whilst a wide cross may
offer a better chance of producing superior