Question

the table shows the crossing - over frequencies for three genes in the fruit fly drosophila. the greater the frequency, the more likely that crossing over separates the genes. the gene for eye color (a), wing shape (b), and body color (c) are found on the same chromosome.

what are the two most likely sequences of these genes on a gene map for this chromosome? select two options.

□ a. b, a, c
□ b. a, b, c
□ c. c, b, a
□ d. c, a, b

Explanation:

In gene mapping, the closer two genes are on a chromosome, the lower the crossing - over frequency between them. The crossing - over frequency between \(A\) and \(C\) is \(6\%\), which is the lowest, so \(A\) and \(C\) are the closest. The crossing - over frequency between \(A\) and \(B\) is \(12.5\%\), and between \(B\) and \(C\) is \(18.5\%\).
If the sequence is \(B - A - C\), the distance between \(B\) and \(A\) is \(12.5\) map units and between \(A\) and \(C\) is \(6\) map units, and between \(B\) and \(C\) is \(12.5 + 6=18.5\) map units (matches the given crossing - over frequency).
If the sequence is \(C - A - B\), the distance between \(C\) and \(A\) is \(6\) map units and between \(A\) and \(B\) is \(12.5\) map units, and between \(C\) and \(B\) is \(6 + 12.5 = 18.5\) map units (also matches the given crossing - over frequency).

Answer:

A. \(B,A,C\)
D. \(C,A,B\)