A testcross of trihybrid Drosophila produced the following phenotypes and number of offspring. A table showing phenotype and the number of offspring with each phenotype is below. A plus sign is used for wild-type phenotype; a letter indicates the mutant phenotype for that gene.$\begin{array} { l r }
+ + + & 669 \
a b c & 653 \
+ + c & 121 \
a b + & 139 \
+ b + & 2280 \
a + c & 2215 \
+ b c & 3 \
a + + & 2
\end{array}$
-Without performing any calculations,which of the following is the gene order?
A)abc
B)acb
C)cab
D)+++
E)None of these

Question

A testcross of trihybrid Drosophila produced the following phenotypes and number of offspring. A table showing phenotype and the number of offspring with each phenotype is below. A plus sign is used for wild-type phenotype; a letter indicates the mutant phenotype for that gene.$\begin{array} { l r } 
+ + + & 669 \
a b c & 653 \
+ + c & 121 \
a b + & 139 \
+ b + & 2280 \
a + c & 2215 \
+ b c & 3 \
a + + & 2
\end{array}$
-Without performing any calculations,which of the following is the gene order?
A)abc
B)acb
C)cab
D)+++
E)None of these

Quizplus · Accepted Answer

The gene order can be determined by identifying the parental and double crossover phenotypes. The most frequent phenotypes (+ + + and a b c) are the parental types, and the least frequent phenotypes (+ b c and a + +) are the double crossovers. Comparing these, the gene that differs between the parental and double crossover phenotypes is in the middle, indicating the gene order is acb.

A Testcross of Trihybrid Drosophila Produced the Following Phenotypes and Number