Principles of Plant Genetics and Breeding

46 CHAPTER 3
lifeblood of plant breeding, genetic variation, would be
very limited. However, during meiosis, as was previously
indicated, the phenomenon of crossing over causes
recombination or shuffling of linked genes to occur,
thereby producing gametes that are unlike the mother
cell. Genetic recombination is the most common source
of variation in flowering species. Along with independent
assortment of genes, these two phenomena ensure
that all offspring will contain a diverse mixture of both
maternal and paternal alleles.
Whereas breaking linkages is desirable for the creation
of the much-needed variation, plant breeders would
sometimes rather have certain linkages left intact. This is
the case when several desirable genes are tightly linked.
On the other hand, there are some occasions when a
desirable gene is linked to an undesirable gene, in which
case breeders would like to break the association. The
probability of breaking a linkage depends on how close
the genes are in the group or block. A tight linkage
(close association) is more difficult to break than a loose
linkage. An opportunity for crossover occurs whenever
meiosis occurs.
Chromosome mapping
Plant breeders develop and use “biological maps” to
guide them in their work. The two basic types of maps
are the physical and genetic maps. Genetic maps are
constructed based on the linkage relationship between
genes. The degree of crossing over between any two
genes or loci on a single chromosome is proportional to
the distance between them. This correlation information
is used to construct chromosome maps.
Chromosome maps provide information about gene
locations, gene order, and the relative position of various
genes, according to genetic distances. Linkage maps
may be used by plant breeders to aid the selection process.
If a desired gene is closely linked with a genetic
marker, the breeder may use the marker to indirectly
select for the desired gene. In the example of a dihybrid
cross, it is possible to calculate the genetic distance
between the two genes (or markers), but one cannot tell
the order of the genes (i.e., whether A comes before B
or B before A). A trihybrid cross is needed for this determination,
as previously stated.
The distance between two genes is defined as the
recombination frequency between them. The unit of
measure is the map unit or centimorgan (cM), which
is defined as 1% of crossover. In a dihybrid cross, the
percent crossover (e.g., between genes A and B) is
calculated as the percentage of recombinant offspring
produced in a cross. For example, for 50 recombinants
out of 400 offspring, it is calculated as (50/400) × 100
= 12.5% = 12.5 map units. If the crossover between A
and C is calculated as 7.5% and between B and C as
19.8%, then the gene order is BAC.
B....................................A...............C
bffff 12.5ffffgbf 7.5fg
b———————19.8——————g
A low frequency of double crossover between B and C
will give the parental genotype, so that the crossover
units will be less than the sum of those between B and A,
and A and C combined. Further, genes that are separated
by 50 or more crossover units are essentially nonlinked
and will assort independently.
Physical maps are constructed based on nucleotides,
the building blocks of DNA. Genetic distance on a
linkage map expressed in centimorgans is not directly
correlated with the physical distance expressed in
nucleotides.
Penetrance and expressivity
It has previously been said that the environment in
which a gene occurs influences how it is expressed. The
source of this environmental effect could be as close
and intimate as the immediate cellular environments,
or as remote as the general plant external environment.
In plants, breeders may transfer genes from one genetic
background into another through hybridization.
Sometimes, they encounter a situation in which the
gene may be successfully transferred, but the desired
effect is not observed. A researcher can quantitatively
study the degree of expression of a trait. For example, in
one case, a disease-resistance gene may offer resistance
in one plant but fail to do the same in another plant from
the same population. This phenomenon is described as
variable gene penetrance and measures the percentage
of individuals that show some degree of expression of
the genotype of interest. If 20% of plants show the
desired resistance trait, the resistance gene is said to have
80% penetrance (Figure 3.11a). Sometimes, changes in
the plant environment may cause the same plant to
produce different phenotypes or degrees of expression
of a trait under these different conditions. For example,
the hibiscus plant normally produces single flowers
(a flower with one set of petals). A double-flowered
mutant (additional petals added to the primary set) has