What ratios of these three tetrads would you expect to see if two genes are linked? Why is each type of tetrad more or less common? What ratios would you expect to see if the two genes are unlinked? Why?
What will be an ideal response?
If two genes are linked, you will see that PD > TT > NPD. PD tetrads are most common; they are produced when no crossover occurs between genes and when two-strand double crossover takes place. TT tetrads are less frequent than PD; they occur when single crossovers or three-strand double crossovers take place. NPD tetrads are least frequent; they form only when four-strand double crossover occurs.
If PD = NPD, then the two genes are unlinked. This ratio indicates that either the genes are located on different chromosomes or they are syntenic but very far apart.
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