Principles of Plant Genetics and Breeding

Table 13.4 Genetics of autoploids.
Diploid Polyploidy Name
Cross
Aa × Aa AAaa × AAaa
Products
1/4 AA 1/36 AAAA Quadruplex
2/4 Aa 8/36 AAAa Triplex
1/4 aa 18/36 AAaa Duplex
8/36 Aaaa Simplex
1/36 aaaa Nulliplex
Table 13.5 Genetic frequencies following chromosome
segregation of an autotetraploid.
Gametic frequency
Genotype AA Aa aa
AAAA 1 0 0
AAAa 1/2 1/2 0
AAaa 1/6 4/6 1/6
Aaaa 0 1/2 1/2
aaaa 0 0 1
Note: chromatid segregation occurs less frequently than
chromosomes segregation and produces alternative types of
segregation. For example the simplex (Aaaa) can produce
gametes that are homozygous (AA) by the process called
double reduction.
Assuming complete dominance and chromosome segregation
the following phenotypic ratios are observed. Certain
segregation ratios are sometimes indicative of the nature of
autotetraploid inheritance.
Cross Progeny (dominant : recessive)
AAAA × AAAA 1 : 0
AAAa × AAAa 1 : 0
AAaa × AAaa 35 : 1
AAaa × Aaaa 11 : 1
AAaa × aaaa 5 : 1
Aaaa × Aaaa 3 : 1
Aaaa × aaaa 1 : 1
aaaa × aaaa 0 : 1
to allele a, there would be only two phenotypes. If
dominance is incomplete or the effect of allele A is
cumulative, there could be up to five phenotypes.
Upon selfing, a dominant phenotype in a diploid (AA,
Aa) would produce a progeny that is all dominant, or
POLYPLOIDY IN PLANT BREEDING 219
Table 13.6 Multiple allelelism in autotetraploids.
Tetrasomic condition
a1a1a1a1 a1a1a1a2 a1a1a2a2 a1a1a2a3 a1a2a3a4 All alleles are identical; monoallelic; balanced
Two different alleles; diallelic; unbalanced
Two different alleles; diallelic; balanced
Three different alleles; triallelic
Four different alleles; tetra-allelic
Number of possible interactions are: (i) first order (e.g., a1a2 ,
a1a3 ); (ii) second order (e.g., a1a2a3 , a1a3a4 ); and (iii) third
order interaction (a1a2a3a4 ). This depends on the tetrasomic
condition.
Tetrasomic condition 1st 2nd 3rd Total
a 1 a 2 a 3 a 4 6 4 1 11
a 1 a 1 a 2 a 3 3 1 0 4
a 1 a 1 a 2 a 2 1 0 0 1
a 1 a 1 a 1 a 2 1 0 0 1
a 1 a 1 a 1 a 1 0 0 0 0
segregate in the 3 : 1 ratio. Selfing each of the five
categories would produce many different outcomes in
autotetraploids, assuming random chromosome segregation
(Table 13.5).
An autoploid individual can have up to four alleles
(abcd) per locus. Five different genotype categories are
similarly possible except that there may be only four
nulliplex genotypes (aaaa, bbbb, cccc, dddd) and only
one tetragenic genotype (abcd), but numerous combinations
for the intermediates (Table 13.6). The possible
gametic array is shown for each genotype. Interallelic
and intra-allelic interactions may occur for as many as
four alleles per locus in an autotetraploid. The degree
to which intra-allelic interaction occurs determines the
expression of heterosis and inbreeding depression in
an autotetraploid. Because four identical alleles are
required to achieve homozygosity in an autotetraploid
compared with only two in a diploid, homozygosity
is achieved at a less rapid rate in autotetraploids
(Figure 13.4).
Another aspect of autoploid genetics with a plant
breeding implication, is the difficulty of distinguishing
between a triplex and a quadruplex on the basis of a
progeny test (assuming random chromosome segregation).
Both genotypes (AAAA and AAAa) will breed
true for the dominant allele. To identify a triplex plant,
the breeder would have to advance the progeny one
more generation to identify the duplex plants of the S 1 .
Achieving genetic purity in autotetraploid stocks is