Agriculture Reference
In-Depth Information
storage, short-term storage, and long-term
storage.
When a new breeding program is initiated the
breeder seeks genetic resources to be evaluated
relative to the breeding objective or ideotype
(specifi c trait). Informal networks are formed
among breeders that are not necessarily confi ned
locally, resulting in exchanges of materials of
mutual interest among breeders in new or estab-
lished breeding programs who understand the
importance genetic variation.
With the advent of intellectual property pro-
tection it became possible to derive commercial
benefi t from unique genes or combinations of
genes that control specifi c traits. These property
rights have led both private companies and public
institutions to seek commercial advantage of these
gene combinations (discussed further later). The
exchange of germplasm tends to be covered by
material transfer and other legal agreements. The
insertion of a patented gene into a previously
unprotected cultivar can render the unprotected
cultivar with the inserted gene unavailable for
subsequent breeding, unless an agreement is
made with the owner of the patented gene. Simi-
larly, investors in cultivar development are seeking
a return on their investment by not sharing
new genetic materials for a specifi ed period
after its release. Though these concerns certainly
affect plant breeding and plant breeders, most
plant breeding companies and plant breeders
recognize the critical need for germplasm
exchange, and hence create mechanisms for germ-
plasm exchange. They may not be as simple as
before, but germplasm exchanges maintain an
essential presence in contemporary wheat breed-
ing programs.
also described as crossing), modifi cation in chro-
mosome composition or number, and mutations.
With advances in biological knowledge and tech-
nology (e.g., recombinant DNA, gene transfer,
and somaclonal variation), new mechanisms have
been developed to augment and generate new
genetic variability, that is, variation that does not
exist within the species and its crossable relatives.
However, it should be remembered that common
wheat (
T. aestivum
L., 2
n
= 6
x
= 42; genomes
BBAADD, cytoplasm donor listed fi rst) and
durum wheat (
T. turgidum
ssp.
durum
Desf., 2
n
=
4
x
= 28; genomes BBAA) are both polyploids with
extensive interspecifi c and intergeneric germ-
plasm resources (see Chapter 1).
A primary step in wheat cultivar improvement
is to generate heritable genetic variation. By far,
the most common way of generating genetic vari-
ation is to mate (cross) two or more wheat parents
that have contrasting genotypes. Once the cross
is made, it is critical to estimate the population
size required for the chance occurrence of the
desired recombinant (a line that has all of the
desired genes). The number of different homozy-
gous individuals that is possible from a single
hybridization is 2
n
, where
n
is the number of gene-
pair differences between the two parents or the
number of loci expected to segregate in the F
2
generation. The F
2
generation is the fi rst genera-
tion of segregation and the preferred alleles can
be in either a homozygous or heterozygous condi-
tion. Each gene-pair difference will be reduced in
heterozygosity by 50% for each generation of
inbreeding or selfi ng.
The consequences of number of gene-pair dif-
ferences and inbreeding have a geometric impact
on the frequency of the desirable alleles. Thomas
and DePauw (2003) elaborated on a calculation
put forward by Shebeski (1967) to estimate the
proportion of plants within a population having
all the desirable alleles. They defi ned the desir-
able gene quotient (DGQ) of a parental mating as
that proportion of plants (
P
) in which a segregat-
ing locus is either heterozygous or homozygous
for the preferred allele. For segregating alleles at
locus 1 through locus
j
, the desirable gene quo-
tient would be DGQ = (
P
1
×
P
2
×
P
3
×
METHODS TO GENERATE
GENETIC VARIATION
Hybridization
We will consider mechanisms to generate genetic
variation and subsequently examine methods for
its assessment. Genetic variability originates via
genetic recombination (through hybridization,
. . .
×
P
j
).
In the absence of selection at these loci, DGQ
