REVIEW Chapter 6 - McGraw-Hill Ryerson

Figure 6.11 In most of the
gametes formed, there is
no crossing over—they
maintain the linkage of the
alleles. In a small minority
of gametes, crossing
over occurs and alleles of
previously linked genes
become unlinked.
Describe why alleles
of genes that are closer
together on a chromosome
are more likely to remain
linked during meiosis.
252 MHR • Unit 2 Genetic Processes
Crossing Over and the Inheritance of Linked Genes
A chromosome may contain up to a few thousand genes. All of the genes on any one
chromosome are called a linkage group because they tend to be inherited together.
However, linked genes do not always stay linked—researchers have found that they
segregate on a regular basis. Th is is due to the process of crossing over, which you
learned about in Chapter 5. Recall that crossing over occurs in prophase I of meiosis,
when non-sister chromatids exchange pieces of chromosomes.
Suppose you are studying two genes that are on the same chromosome and,
therefore, linked. Crossing over between homologous chromosomes can occur. As
shown in Figure 6.11, this will result in the alleles of the linked genes no longer being
on the same chromosome. Th e alleles of the previously linked genes are now unlinked.
Th is means that they will migrate into diff erent gametes. Th e result is that instead of
two types of gametes being produced, four diff erent types of gametes will be produced
in diff ering proportions. Th ere are fewer gametes with the recombined alleles because
crossing over is a random event and it occurs infrequently.
A
B
no crossing over
during meiosis
97%
a
b
A
B
four types of gametes in unequal proportions
a
b
crossing over
during meiosis
a
B
3%
recombinant gametes
Using Gene Linkage for Chromosome Mapping
Scientists have discovered that alleles for a given pair of linked genes will separate
with a predictable frequency and that this frequency is diff erent for diff erent pairs of
linked genes. Th e frequency depends on how close the alleles of the linked genes are
positioned on a chromosome. Crossing over occurs more frequently between alleles that
are far apart on a chromosome than between alleles that are close together. Th erefore,
a given pair of linked genes will separate more frequently than the alleles for another
pair of linked genes if their alleles are farther apart on the chromosome. Th is process
of determining the relative locations of genes on chromosomes is called chromosome
mapping. Th ese types of linkage studies are useful for mapping chromosomes in species
that reproduce rapidly and produce many off spring, such as plants and insects. But
chromosome mapping is not useful in mapping human chromosomes. Chromosome
mapping of humans only became possible when modern techniques that allow
scientists to directly see the chromosomes became available.
A
b