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To answer the last part you need to know the expected number of double crossovers which you normally calculate using observed data e.g. 498 flies had ruby eyes

1b) ......ct.....

rb<-----20...>t

1c) cut wing 500, ruby eyed and tan body 500, normal 1000


There are lots of things to consider when answering this question.

Firstly all of the traits are recessive.

Males = XY Females =XX

Homozyous means both chromosomes have the same alleles (form of a gene)

I would start by mapping out the traits like this:

rb<---12.5----->ct<----7.5----->t

We start by crossing a ct Male (who therefore has the chromosomes XY) with the female (XX) who is homogenous.

This means the male only has the ct trait on his X chromosome and the female has the recessive versions of rb and t on each X chromosome.

To cross them make a grid like on the whiteboard.  

The F1 female will inherit one X chromosome from the male and one from the female. So on one X chromosome will have the ct and the other will have rb and t. However because all the traits are recessive and feature on only one of the two chromosomes none of these traits will actually display themselves and so the F1 female will normal. Or wont have cut wings, ruby eyes or tan body.

i need help finding the answer for this question 

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