Need help with genetic problems. Please explain so I can learn! The R/r and S/s

Question

Need help with genetic problems. Please explain so I can learn!

The R/r and S/s genes are linked and 20 map units apart. In the cross RS/rs * RS/rs what fraction of the progeny will be R_ss?

Also, Suppose the A and B genes are on the same chromosome but separated by 20 map units. What fraction of the progeny from the cross AB/ab * ab/ab would be phenotypically AB?

I know how to do the crosses but can figure out how the 20 map units fits into it. Please include the crosses so I can double check mine.

Explanation / Answer

R/r and S/s genes are linked and 20 map units apart.

Cross is RrSs * RrSs

Parental types will be "RS" and "rs". Recombinants will be "Rs" and "rS". The two genes are 20mu apart. So, percentage of recombination is 20%. So, there should be 10% Rs and 10% rS.

So, the fraction of progeny which will be "RS" = 40; the fraction of progeny which will be "rs" = 40

Fraction of the progeny will be R_ss? = 3/4*1/4 = 3/16 (if genes were independently assorting)

R_ss means 'Rs/Rs' and/or rs/rs. So, result should be 10% or 40%.

Answer 2.

AB/ab * ab/ab

Phenotypically 'AB' means A_B_

Now, the genes are linked. 'AB' and 'ab' are parental types. Ab and aB are recombinants = 20%

Parental types will be 80%; so, 40 'AB' and 40 'ab' will be there.

So, phenotypically 'AB' will be 40%.

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