Question

A pure-breeding Rat strain displays two distinct rare traits. When a male from this pure breeding mutant strain is
crossed to a wild type female all the female F1 display both traits and the male F1 look wild type.
A] What is the mode of inheritance for these traits?
B] The F1 mice described above are inter-crossed to produce an F2 generation. 40% of the male F2 generation show
both traits, 40% are wild type and the remaining 20% are split between displaying one trait or the other. What fraction
of the female F2 progeny are expected to show both of the rare traits?
C] What is the map distance (in m.u.) between the two genes specifying these rare traits?

Answer 1

Answer 1.a) mode of inheritance of these traits are X- linked dominant disorder. Because here male parent is true breeding whereas the female parent is wild type. And there cross results in all the female F1 displaying both the traits and the male F1 looked wild type.

Here we can see that distinct rare traits are dominant over the wild type because they are expressed in the F1 generation. And it is clear that all females progeny in F1 have theses two distinct rare traits while the males progeny have only wild type phenotype this can be because male progeny have recieved only the dominant gene present on the X chromosome of the mother I.e wild type. This means that single copy of the mutant gene is enough to cause the disease in both male and female. These are all the characteristics of X linked dominant disorder.

Answer B.) When the F1 are intercrossed to produce F2 generation. Then there will be similar fractions o to males ,in the F2 female progeny that are expected to show both of the rare traits.

As male have only one X chromosome that they would be giving to their daughter. Therefore there the loci are linked and no recombination would be possible. Therefore female progeny will same as that of male progeny.

Answer c. In the part B of the question it is written that the remaining 20% are split between displaying one trait or the other. From here we can say that the map distance (in m.u.) between the two genes specifying these rare traits are 20 m.u.

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