Genes on Chromosomes
Country: United States
Date: May 2005
How do you know if two genes (say, for wing color and eye
color in fruit flies) reside on the same chromosome or different chromosome?
If the two pairs of genes are on different chromosomes and you set up a
di-hybrid cross involving these two pair of genes, the F2 generation will
show a ratio of 9 A_B_:3 A_bb:3aaB_:1 aabb (Mendel's 2nd Law of Independent
Assortment). If the two pairs of genes are on the same chromosome (linked),
the F2 generation will show a ratio which is determined by the distance
between the two loci. A word of caution: if the two genes are separated by
50 map units or more, they will appear to assort independently due to
frequent crossing over between the two sites during meiosis.
Ron Baker, Ph.D.
This is done by linkage analysis. Remember that during meiosis crossing
over occurs. Part of the chromosome on one partner breaks off and the same
part of the other of the pair also breaks off and they switch places. If you
follow two genes you can determine how far apart they are on the chromosome
by how often they are separated by cross over. So, let's say that you have
two genes that are right next to each other on the chromosome. During
meiosis the break in the chromosome would have to be right between them.
That might happen, but very rarely. So those two traits would travel
together into the same cell most of the time and the gamete would have both
traits. If the genes are very far apart on the chromosome they will be
separated into different gametes almost all the time.
Red hair and freckles is a good example of two genes that are tightly
linked. These two genes are very close to each other on the same chromosome.
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Update: June 2012