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at the same locus (intra-allelic), are possible in
tetraploids than in diploids.
Cultivated potato also has diploid deriva-
tives and wild relatives that are useful in genetic
and molecular analyses. For example, sequen-
cing of the potato genome utilized two diploid
clones that were identified as having a relatively
high frequency of presence/absence variants,
and other deleterious gene mutations, that likely
contributed to observations of inbreeding de-
pression in potato (Potato Genome Sequencing
Consortium, 2011).
The maintenance of desirable gene inter-
actions in highly heterozygous, outcrossing potato,
while minimizing homozygosity which contrib-
utes to the expression of deleterious genes,
requires breeders to hybridize parents that are
not closely related. The backcross method, as
practiced in self-pollinated crops, is not normally
utilized in potato for the incorporation of simply
inherited traits, due to the inherent inbreeding
depression associated with the use of a recur-
rent parent. Instead, a modified backcross tech-
nique whereby a different parent is used as the
recurrent parent in each backcross generation
is commonly employed in potato (Hoopes and
Plaisted, 1987).
Relative to diploids, the segregation of traits
becomes much more complex. For example, in a
simple gene model with two alleles, “A” (domin-
ant) and “a” (recessive) at a single locus, diploid
genotypes would consist of two homozygote
classes (AA and aa) and one heterozygote class
(Aa). In tetraploid potato, two homozygote classes
are also represented: AAAA and aaaa, defined as
quadruplex and nulliplex genotypes, respectively.
However, rather than there being just one hete-
rozyote genotype, triplex (A 3 a), duplex (A 2 a 2 ), and
simplex (Aa 3 ) heterozygote genotypes are now
represented. The ratio of phenotypes obtained from
the crossing of the three heterozygote classes to
a nulliplex where the presence of the A allele
confers one phenotypic class (dominant gene ef-
fect) with the other phenotypic class being nul-
liplex (recessive) can vary considerably ( Table
16.1 ). Under a chromosome segregation model
whereby no crossing over is expected between the
locus and the centromere, only dominant pheno-
types would be expected with triplex heterozygotes,
whereas recessive phenotypes are represented in
the progeny of duplex and simplex heterozygotes
( Table 16.1 ). A greater frequency of recessive
phenotypes is obtained if the locus is at a suffi-
cient distance from the centromere for crossing
over to occur on a regular basis—termed chro-
matid segregation, with recessive phenotypes
now also being represented in the progeny of trip-
lex heterozygotes where none were found previ-
ously with chromosome segregation ( Table 16.1 ).
The complex genetics associated with the
higher ploidy of potato has limited the under-
standing of the inheritance of genetic traits in
potato relative to diploid crops. Even the advent
of molecular biology and its associated molecu-
lar markers has not alleviated the difficulty of
working with a tetraploid crop. As discussed by
van Eck (2007), most molecular marker techniques
currently available do not allow full classification
of multiple alleles at a locus in tetraploids—
thereby limiting their usefulness in associating
traits of interest with molecular markers. Cur-
rently in potato, the development and use of
marker-assisted selection in potato breeding has
been primarily for traits conferred by a single
major gene, such as resistance to viruses or
nematodes (Simko et al ., 2007), with less pro-
gress having been made for polygenic or quanti-
tative traits.
The recent sequencing of the 844 Mb po-
tato genome will aid in identifying allelic vari-
ants of genes contributing to important quanti-
tative traits in potato (Potato Genome Sequencing
Table 16.1. Heterozygote genotypes in potato and their expected chromosome and chromatid phenotypic
segregation in progeny following crosses to a nulliplex (a 4 ). The “A” allele conferring complete dominance
over the recessive “a” allele, with two phenotypic classes therefore being possible.
Expected phenotypic classes
Heterozygote cross
Chromosome segregation
Chromatid segregation
A 3 a × a 4
All A phenotype
27A:1a
A 2 a 2 × a 4
5A:1a
3.7A:1a
Aa 3 × a 4
1A:1a
0.87A:1a
 
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