SAT Subject Biology Practice Question 228: Answer and Explanation

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Question: 228

18. 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 only
B. II only
C. I and II
D. II and IV
E. III and IV

Correct Answer: A

Explanation:

It is not known whether the F2 fly with red eyes is homozygous or heterozygous for this trait. Therefore, the offspring may exhibit either red or purple eyes. Because both parents have a dwarf body, a recessive trait, all offspring must also exhibit this trait.

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