Ask A Scientist Molecular Biology Archive

name Momo 
status student 
grade 9-12 
location MA 

Question - 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? 
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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.
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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.

vanhoeck
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