Assume 7 different strains of mice have been isolated; each shows a recessive wi

Question

Assume 7 different strains of mice have been isolated; each shows a recessive withered fur trait.Crosses are performed as follows; (w) indicates withered fur, (R) indicates wild-type refined fur.

            A            B            C        D          E           F              G

A          w          w             R        R           w          R              R

B                       w            R         R          w          R               R

C                                      w        w          R          R              R

D                                                w           R          R              R

E                                                              w         R              R

F                                                                           w            w

G                                                                                         w

Based on these crosses, how many different genes are present? Explain your answer.

Explanation / Answer

This is a problem in which you're trying to find out how many different genes in the mouse affect the fur type. The basic idea here is that while one recessive gene may promote withered fur, a different gene somewhere else in the genome may "override" it, allowing the mouse to have refined fur in the end. How this occurs isn't really given in the problem (it could be that these strains represents different positions in the genome - loci; so strain B contains a recessive gene at position B, the mouse has a dominant gene at position G so that when strain B and strain G are crossed, the dominant gene ensures that the overall fur type will be refined.) From here on in, pretend that strain A refers to gene A, strain B refers to gene B, and so on. The task is to determine which of the genes are the same and which are different so that you can know the number of DIFFERENT genes affecting the fur. Look at genes A and B. Gene A has withered fur, so does Gene B. If these two genes are really different, then when you cross strains A and B, you should get a refined type (see my explanation above about how each of these overrides the other if they are different). In this case, you see that the cross is still withered, so A and B must be the same. When you cross B and C, you get refined fur, so B and C must be different. Also, when you cross A and C, you get refined fur, so A and C must be different (which makes sense, since in the previous paragraph, we figured out that A = B). At this point, if you're lost, remember that every strain by itself starts out as withered fur, so if you cross it with a strain that is DIFFERENT, the crossed offspring will have refined fur. I guess I should also clarify that the "7 strains" given at the beginning of the problem doesn't mean 7 DIFFERENT strains - that's for you to figure out which are the same and which are different. You can see that C and D are the same, because when you cross C with D, the offspring is still withered fur. So, at this point, A=B, C=D, and A/B are different from C/D. You can see that E is different from C/D (because the offspring from the cross have refined fur). But notice that when you cross E with A/B, the offspring are still withered. Thus, A=B=E. F is a new one. When crossed with any of the previous ones (A through E), you get refined. Same thing with G. However, when you cross F with G, the offspring are still withered. So F=G, and F/G is not identical to any of the other strains. Summary: A = B = E ; C = D ; F = G. So you can see that there are a total of 3 different genes affecting the fur (how ironic that the other person who answered this question got the same answer but was thinking in terms of alleles).

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