Another cross in Drosophila involved the recessive, X-linked genes yellow (y), w

Question

Another cross in Drosophila involved the recessive, X-linked genes yellow (y), white (w), and cut (at). A yellow-bodied, white- eyed female with normal wings was crossed to a male whose eyes and body were normal but whose wings were cut. The F females were wild type for all three traits, while the F, males expressed the yellow-body and white-eye traits. The cross was carried to an F2 progeny, and only male offspring were tallied. On the basis of the data shown here, a genetic map was con- structed. Phenotype Male Offspring ct ct 90 95 424 376 0 0 ct (a) Diagram the genotypes of the Fi parents. (b) Construct a map, assuming that white is at locus 1.5 on the X chromosome. (c) Were any double-crossover offspring expected? (d) Could the F2 female offspring be used to construct the map? Why or why not?

Explanation / Answer

a)y+ w+ ct/ y w ct^+ female X y w ct^+ /Y male

b) The least frequent classes (in this case zero!) are the double crossovers, which are compared to the parental (most frequent to get the order): y-w-ct.

Distances: y-w: 9+6/1000*100 = 1.5 cM

w-ct: 90+95/1000*100 = 18.5 cM

y-ct: 90+95+9+6/1000*100 = 20 cM

Now draw a map, with w at 1.5:

_____________________________

y w ct

0 1.5 20

c)Yes. (0.015*.185) = 0.0278 * 1000= 2.78. There must be interference, but we can’t calculate it because we didn’t observe any double crossovers (0.0278/0).

d)

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