Principles of Plant Genetics and Breeding

Chiasma
A A
B B
A
B
A A
Bivalents
b
Aa
a
B
a a
B b B b
b
a a
Recombinants
b b
Figure 3.10 Crossover is preceded by the formation of
bivalents, the pairing of homologous chromosomes.
Adjacent chromatids physically exchange parts during the
formation of a characteristic x-configuration, called the
chiasma.
the F 2 , in the ratio of 1(AABB) :2(AaBb) :1(aabb).
The meiotic products are either parental or noncrossover
gametes. In the example in Figure 3.10, the
phenomenon of crossing over that occurs in meiosis has
caused some alteration in linkage (called incomplete
linkage). In the absence of linkage, the testcross products
would segregate in the genotypic ratio of 1 :1:1:
1 for the four products, AaBb, Aabb, aaBb, and aabb.
However, in this example, the presence of linkage
allowed most gametes to inherit parental genotypes
(AaBb, aabb), as a result of normal gamete formation.
Crossing over created new genotypes (Aabb, aaBb; nonparental),
called recombinants (because they are products
of recombination). When the genes of interest are
arranged in a homolog such that one chromosome has
both dominant alleles (in this example) while the other
has both recessive alleles (AB/ab), the condition is
described as linkage in the coupling phase. However,
when the arrangement is Ab/aB, the linkage is in the
repulsion phase.
Again, in this example, the numbers (frequency) of
parental gametes were roughly equal, and so were the
numbers for the recombinants. The proportion of recombinant
gametes produced in meiosis in the multiple
PLANT CELLULAR ORGANIZATION AND GENETIC STRUCTURE 45
Homologous chromosomes
consisting of two chromatids
form bivalents
Single chiasma forms as
chromatids exchange
parts
At the end of meiosis,
chromatids separate into
individual haploid cells
hybrid is called the recombination frequency (RF). If
two genes are completely linked, RF = 0. Detection of
linkage is accomplished by using the chi-square test (see
Chapter 9):
Observed Expected
Genotype frequency (O) frequency (E) (O –E) 2 /E
A–B– 284 214.3 22.67
A–bb 21 71.4 35.58
aaB– 21 71.4 35.58
aabb 5 23.8 40.9
381 380.9 134.72
Degrees of freedom (df ) = 3; chi-square at α =0.05 is
7.82. Since the calculated χ 2 is greater than tabulated,
we reject the null hypothesis and declare the presence of
linkage.
When a cross involves three gene pairs (a trihybrid
cross), ABC, there may be recombination between A
and B, A and C, and B and C. This cross is called a threepoint
cross. The most common genetic types are the
parental types, and the least common, the double
crossovers. A testcross should reveal eight genotypes in
the progeny. The order of the genes can be deduced
from a three-point cross because one gene in the middle
will be the one that apparently changes places in going
from the parental to the double crossover type. For
example:
Recombinational events Gametes Testcross data
No crossover ABC 401
abc 409
Crossover in AB region Abc 32
aBC 28
Crossover in BC region ABc 61
abC 64
Crossover in both regions AbC 2
(double crossovers) aBc 3
1,000
Recombination between A and B is calculated for:
Parental types (ABC, ABc, abC, abc) = (401 + 61) +
(64 + 409) = 935 = 93.5%
Recombinant types (AbC, Abc, aBC, aBc) = (2 + 32) +
(28 + 3) = 65 = 6.5%
Recombination between B and C can be similarly
calculated.
Fortunately, for the plant breeder, genes in a chromosome
are not completely linked. If this were so, the