Agriculture Reference
In-Depth Information
the use of backcrossing. The genetics of
backcrossing
will be covered later in the qualitative genetics section.
However a brief description will be given here.
on how closely the breeder wants to resemble the recur-
rent parent or how well the backcross genotypes are
performing. The proportion of the recurrent parent
genotype in each backcross family will increase with
increased backcrossing, and can be calculated by the
formula:
Backcrossing
g
Backcrossing is a commonly used technique in devel-
oping pure-line cultivars. This technique has been used
in plant breeding (not only in inbred species) to trans-
fer a small, valuable, portion (usually a single gene) of
the genome from a wild or unadapted genotype into
the background genotype of an adapted and already
improved cultivar.
Backcrossing is an operation that involves a
recurrent
parent
and a
non-recurrent parent
. In many cases the
non-recurrent parent is an unadapted line or geno-
type, and hence is not expected to contribute characters
other than the specific one that it is desired to trans-
fer to resulting selections. It is therefore usual to
choose a recurrent parent which is already suited to
the environment (i.e. the most adapted genotype avail-
able). The process of isogenic line production will
be identical and repeated for each line that is to be
produced.
The process will be described as where the homozy-
gous allele (
RR
) of interest from the non-recurrent
parent is completely dominant showing resistance to
a disease and the recurrent parent has the recessive
completely susceptible alleles (
rr
). First the recurrent
parent is crossed to the non-recurrent parent produc-
ing F
1
seeds which are therefore heterozygous (
Rr
) for
this locus and where each of the two parents contribute
equally to the genotype. The F
1
s(
Rr
) are crossed back
to the recurrent parent
rr
−
(
/
)
1
1
2
=
where
g
the number of backcrossing generations,
including the original cross (P
1
×
P
2
) to produce the
F
1
. The following are proportions of the genes that
are recovered from the recurrent parent according to
number of backcrosses:
F
1
=
50.0%
BC
1
=
75.0%
BC
2
=
87.5%
BC
3
=
93.8%
BC
4
=
96.9%
The above percentages of the recurrent parent geno-
type in the resulting progeny hold reasonably well
in the early backcross generations. However, with
increased backcrossing, the percentage genotype of the
non-recurrent parent (the '
wild
' type) will be more
influenced by linkage. The resulting backcross inbred
can often contain a higher proportion of wild-type
genotype than desired, or more backcross operations
will be required to obtain the desired proportion of the
cultivated/adapted (recurrent) parent.
The backcrossing method where the gene of interest
is recessive is slightly more complex (and often a more
lengthy) process. The general theory is the same but in
this case it is necessary to progeny test the backcrossed
generations in order to separate the homozygous and
heterozygous plants that need to be selected. Progeny
testing can be avoided (or reduced) when tightly linked
co-dominant molecular markers are available. Molecu-
lar markers also can be used to increase the frequency
of the adapted (recombinant) parent genome in the
backcross family. This is discussed in more detail in
Chapter 8.
Rr
, to produce backcross 1
(BC
1
). The seeds from this '
backcross
' are of the geno-
types
Rr
or
rr
, which can be screened to identify the
disease susceptible lines
rr
), as opposed to the dis-
ease resistant
Rr
. The
Rr
s can then be used to cross
back again to the recurrent parent
rr
, to produce BC
2
.
This process of screening for the presence of the het-
erozygous resistant lines and backcrossing them to the
recurrent parent is repeated a number of times with
the aim of developing a line which is comprised of all
the genes from the recurrent parent except at the '
resis-
tance locus
' which will have the resistance allele (
R
). In
other words we effectively '
add
' this to the genotype.
The number of backcrossing generations will depend
×
