Agriculture Reference
In-Depth Information
Qualitative inheritance in
tetraploid species
A character can be controlled by genes that are inher-
ited independently but that interact to form the final
phenotype. The interaction of genes at different loci
that affect the same character is called either non-allelic
interaction or
epistasis
. Epistasis was originally used to
describe two different genes that affect the same char-
acter, one that masks the expression of the other. The
gene that masks the other is said to be epistatic to it.
The gene that is masked was termed hypostatic. Epis-
tasis causes deviations from the common phenotypic
ratios in F
2
such as 9:3:3:1that indicates segregation
of two independent genes, each with complete domi-
nance. Phenotypic ratios in F
2
for two unlinked genes
as influenced by degree of dominance at each locus and
epistasis between loci are shown in Table 5.4.
In some ways the interaction between two different
loci, in which the allele at one locus affects the expres-
sion of the alleles at another, will remind you of the
phenomenon of dominance. The two phenomena are
essentially different, however.
Dominance
always refers
to the expression of one member of a pair of alleles
relative to the other at the same locus, as opposed
to another locus;
epistasis
is the term generally used
to describe effects of non-allelic genes on each other's
expression, in other words their interaction.
Compared to crop species that are cultivated as pure
inbred lines, there has been comparatively little research
carried out on the inheritance in auto-tetraploid species.
This has been primarily due to the fact that the major
autotetraploid crop species (e.g. potato) are clonally
reproduced or are outbreeding species that suffer severe
inbreeding depression (i.e. alfalfa). Many (or all) of
these cultivars are highly heterozygous and hence it is
not as easy to carry out simple genetic experiments.
Major gene inheritance in auto-tetraploids has been
the topic of many research/breeding groups with the
aim of parental development. Consider for example the
potato crop, where there are several single gene traits
that control resistance to Potato Virus X, Potato Virus Y,
Potato Cyst Nematode (
G. rostochiensis
) and late blight
(
Phytophthora infestans
). All these qualitative traits show
complete dominance. The technique used to develop
parents in a breeding programme is aimed at increasing
the proportion of desirable offspring in sexual crosses
and in the extreme to avoid the need to test breed-
ing lines for the presence of the allele of interest. The
technique is called
multiplex breeding
.
In tetraploid crops any genotype may be
nulliplex
(
aaaa
), having no copies of the desired allele at the spe-
cific (
A
) locus;
simplex
(
Aaaa)
, having only one copy
of the desirable resistance allele (
A
);
duplex
(
AAaa
),
having two copies of the allele;
triplex
(
AAAa
), having
three copies of the allele or
quadruplex
(
AAAA
) having
four copies of the allele (i.e. homozygous at that locus).
If the alleles show dominance then genotypes which
have at least one copy of the gene will, phenotypically,
appear identical in terms of their resistance. But they
will differ in their effectiveness as parents in a breeding
programme. To determine the genotype of a clonal line,
test crossing is necessary.
To illustrate the usefulness of multiplex breeding in
potato, consider the problem of developing a parental
line which, when crossed to any other line (irrespec-
tive of the genotype of the second parent) will give
progeny all of which will be resistant to potato cyst
nematode by having at least one copy of the H
1
gene
(a qualitative resistance gene conferring resistance to all
UK populations of the damaging nematode
Globodera
rostochiensis
and which has been shown to give relatively
durable resistance).
Table 5.4
Phenotypic ratios of progeny in the F
2
genera-
tion for two unlinked genes (where
A
is dominant to
a
, and
B
is dominant to
b
), and epistasis between loci.
F
2
phenotype
Genetic explanation
A_B_
A_bb
aaB_
aabb
9
3
3
1
No epistasis
9
3
4
0
Recessive epistasis:
aa
epistatic to
B
and
b
12
0
3
1
Dominant epistasis:
A
epistatic to
B
,or
b
13
0
3
0
Dominant and recessive
epistasis:
A
epistatic to
B
and
b
;
bb
epistatic to
A
and
a
.
A_
and
bb
produce identical
phenotypes
9
0
7
0
Duplicate recessive epistasis:
aa
epistatic to
B
, and
b
; and
bb
epistatic to
A
and
aa
15
0
0
1
Duplicate dominant
epistasis:
A
epistatic to
B
and
b
;
B
epistatic to
A
and
aa
