Autosomal Linkage and Crossing Over
and Crossing Over
Since Mendel’s laws were rediscovered
in 1900, it became clear that, contrary
to Mendel’s second law, not all factors
segregate independently. Indeed,
many traits are inherited together.
Since the number of chromosomes in
any organism is relatively small compared
with the number of traits, each
chromosome must contain many
genes. All genes present on a chromosome
are said to be linked. Linkage
simply means that the genes are on the
same chromosome, and all genes present
on homologous chromosomes
belong to the same linkage groups.
Therefore there should be as many
linkage groups as there are chromosome
Geneticists commonly use the word “linkage”
in two somewhat different senses. Sex
linkage refers to inheritance of a trait on
the sex chromosomes, and thus its phenotypic
expression depends on the sex of the
organism and the factors already discussed.
Autosomal linkage, or simply, linkage, refers
to inheritance of the genes on a given autosomal
chromosome. Letters used to represent
such genes are normally written without
a slash mark between them, indicating
that they are on the same chromosome. For
example, AB/ab shows that genes A and B
are on the same chromosome. Interestingly,
Mendel studied seven characteristics of garden
peas, which assorted independently
because they were on seven different chromosomes.
If he had studied eight characteristics,
he would not have found independent
assortment in two of the traits
because garden peas have only seven pairs
of homologous chromosomes.
In Drosophila, in which this principle
has been studied most extensively,
there are four linkage groups that correspond
to the four pairs of chromosomes
found in these fruit flies. Usually,
small chromosomes have small
linkage groups, and large chromosomes
have large groups.
Linkage, however, is usually not complete.
If we perform an experiment in
which animals such as Drosophila are
crossed, we find that linked traits separate
in some percentage of the offspring.
Separation of alleles located on
the same chromosome occurs because
of crossing over.
As described earlier, during the
protracted prophase of the first meiotic
versa (Figure 5-10). Each chromosome
consists of two sister chromatids held
together by means of a proteinaceous
structure called a synaptonemal
complex. Breaks and exchanges occur
at corresponding points on nonsister
chromatids. (Breaks and exchanges also
occur between sister chromatids but
usually have no genetic significance
because sister chromatids are identical.)
Crossing over is a means for exchanging
genes between homologous chromosomes
and as such greatly increases
the amount of genetic recombination.
The frequency of crossing over varies
depending on the species, but usually
at least one and often several crossovers
occur each time chromosomes pair.
Because the frequency of recombination
is proportional to the distance
between loci, the comparative linear
position of each locus can be determined.
Genes located far apart on very
large chromosomes may assort independently
because the probability of a
crossover occurring between them in
each meiosis is close to 100%. Such
genes are found to be carried on the
same chromosome only because each
one is genetically linked to additional
genes located physically between them
on the chromosome. Laborious genetic
experiments over many years have
produced gene maps that indicate the
positions of more than 500 genes distributed
on the four chromosomes of Drosophila melanogaster.