SAT Subject Biology Practice Question 229: Answer and Explanation

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Question: 229

19. In fruit flies, normal-sized, solid-colored body and red eyes are wild- type (normal) traits. Flies can be assumed to have these traits unless otherwise stated. A geneticist crosses fruit flies from a strain with purple eyes and with fruit flies from a strain with dwarf bodies. The results of the F1 cross are shown below. The scientist then crosses F1 flies together. Results from the F2 generation are included in the table.

GENERATIONNORMAL SIZE, PURPLE EYENORMAL SIZE, RED EYEDWARF, PURPLE EYEDWARF, RED EYETOTAL
F1074200742
F21023183395548/

The allele resulting in purple eyes is located near an allele resulting in streaks across the body, on the same chromosome. The streaked body phenotype is recessive. Crosses are performed with

A. I and II
B. I and III
C. I and IV
D. II and III
E. III and IV

Correct Answer: D

Explanation:

Because the allele resulting in purple eyes is located close to the allele resulting in streaks, these traits are expected to be genetically linked. The combination of alleles present in the parents will be inherited together more often than expected, because the genes do not assort, or move into gametes, independently. The combinations of streaked body with purple eyes and normal body with red eyes will occur less often than expected in the F2 flies.

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