Gregor Mendel discovered the law of independent assortment that is that
different genes are inherited independently of each other. If 2 genes are
located on different chromosomes the they will experience independent assortment
but if they are not located on the same chromosome then they will either experience
full or partial genetic linkage. Partial linkage occurs when 2 genes sit far
apart on the same chromosome, these genes are only partially linked as they can
easily be separated buy recombination. Complete genetic linkage occurs when 2
genes sit close together on the same autosome, genes can be genetically linked
or sex linked. Sex linkage occurs when 2 genes sit close together on the same
sex chromosome. As male and female expression and inheritance patterns differ
they are considered sex linked.
When genes are linked there is a bigger
chance for the 2 genes to be inherited together. The closer together 2 genes
are the less likely the chance for them to cross over and conversely the
further away 2 genes are the less likely they are to cross over as there is a
larger region where recombination can occur. Crossing over occurs when there is
a swapping of genetic material between non-sister chromatids of homologous chromosomes, this results in
increased variability and a mixture of parental characteristics in offspring.
Crossing over occurs in prophase 1 of meiosis when chromosomes pair up
to form a tetrad and undergo synapsis. At a certain spot 2 chromosomes may
overlap, this point is known as the chiasma. In some cases a protein complex
called the synaptonemal complex forms which causes the 2 chromosomes to
permanently swap genetic material.
Recombination increases variability as 4 different gametes are formed, whereas
if recombination didn’t occur only 2 different gametes would be formed. These 4
gametes that are produced are called recombinants.
Alfred Sturtevant created the 1st genetic map of a chromosome
from the fruit fly. He proposed that the frequency of recombination between 2
genes could help determine the location on the chromosome. The recombination
frequency is the prevalence that a single chromosomal cross over will take place
during prophase 1 of meiosis. The distance between 2 genes on a chromosome is
measured in centimorgans or map units. 1 map unit has a recombination frequency
of 1%, thus the further away 2 genes are the higher the recombination frequency.