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Gene Linkage
Mendel made 4 major conclusions based on his pea experiments that
have become the basis for modern genetics.
1. Traits are controlled by two “factors” (now called alleles).
2. Some alleles are dominant, others are recessive. Mendel did not
know about other modes of inheritance.
3. The alleles segregate (separate) from each other when gametes are
formed. New generations get only one allele or the other.
4. Alleles assort independently (inheriting one trait does not affect
whether another trait is inherited as well.)
How did Mendel come to all these conclusions and are they correct?
What would Mendel’s conclusion be if when he studied a two factor
crosses like seed color and shape, every time he crossed a yellow round
seed with anything, he only got yellow round seeds (never yellow and
wrinkled or green and wrinkled)?
Geneticists have concluded not all alleles segregate independently, but
chromosomes do. These genes that seem to “stick” together are now
called linked genes. They are linked because they are located close
together on the same chromosome. Genes on the same chromosome
only combine in new ways if the alleles are exchanged when crossing
over in a tetrad.
How did Mendel miss this? By luck, six of the seven traits he studied
were on separate chromosomes. The genes for the other trait are so far
apart on the same chromosome that because of crossing over, they seem
independent.