genetics question please

[Rootz90]

The gel diagram below shows the DNA banding patterns observed among the
progeny of a cross between double heterozygous genotypes (MmNn x MmNn) shown on the left
(P1 and P2). On the right are shown the 9 patterns of bands observed in the progeny. The DNA
bands are numbered 1-4.

picture:

Indicate which DNA band correspond to which allele.

Allele M...
Allele m...
Allele N...
Allele n...

answers explanation
The critical part of the answer is realizing that you cannot have alleles of only one
gene (Mm or Nn) and not have the other gene altogether. So, when you have only 2
DNA bands (Lanes C, E, G and I), they must be the 4 homozygotes (MMNN,
MMnn, mmNN and mmnn). The parents are both doubly heterozygous (MmNn) ad
produce the 9 genotypes possible from such a dihybrid cross. As a result, there are
multiple possible allele assignments which are all correct, as long as the 4 alleles are
kept straight (see above).
Each pair of allele must be assigned correctly (M= 1, m = 3; or M = 2 m= 4
or reverse since for DNA bands there is no dominance)
Same for N and n alleles.
M cannot be 1 and m = 2 at the same time. N cannot be 1 and n = 2.
M cannot be 3 and m = 4; N cannot be 3 and n = 4


Can someone explain why M cannot be 1 and m=2 at the same time?? I'm so confused as to how to read these bands.

thanks

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[SN2ed]

Thread closed. These types of questions belong in the Study Q&A forum.