Download Consortium for Educational Communication Summary

Consortium for Educational Communication
Summary
The concept of factor was given by Gregor John Mendel in 1860’s
while performing his hybridization experiments in garden pea.
According to this concept, each character is controlled by a factor
(now called gene). For each character there is always a pair of
factors involved one each contributed by male and female parents
during reproduction. The law of independent assortment states
that the distribution of alleles to gametes during meiosis is random.
If one particular allele goes to one gamete, it has no influence on
the likelihood of any other allele going to the same gamete. The
law of independent assortment holds true for genes on separate
chromosomes. This law specifies a random distribution of genes
located on different chromosomes to the gametes. A dihybrid ratio
of 9:3:3:1 and a test cross ratio of 1:1:1:1 is always expected if
genes show independent assortment. However, there are many
cases where the law of independent assortment does not hold
true. When the alleles are present on the same linkage group
or chromosome, they are physically attached to each other and
cannot show independent assortment. The farther apart the
alleles are, however, the more likely that there will be a crossover
event between them that results in their subsequent ability to
show independent assortment. Sutton and Boveri very early
noted from cytology studies that there must be many more “unit
factors” than chromosomes. Therefore individual chromosomes
must encode multiple genes. Bateson and Punnet (1903) for
the first time observed deviations from normal dihybrid and test
cross ratios in sweet pea and gave the concept of “Coupling and
Repulsion’. T.H. Morgan in his experiments in Drosophila also
showed that not all genes were transmitted according to laws
of independent assortment and some genes behave as if they
were joined or “linked together”. Further studies showed that
such genes were indeed physically linked. These genes existed
on the same chromosome. We now know that chromosomes
encode many thousands of genes. Thus genes located on the
same chromosome are said to be linked and demonstrate linkage
in genetic crosses.