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.$\begin{array} { l r }
+ + + & 669 \
a b c & 653 \
+ + c & 121 \
a b + & 139 \
+ b + & 2280 \
a + c & 2215 \
+ b c & 3 \
a + + & 2
\end{array}$
-Which gene pair is closer together; i.e.there are fewer map units between them?
A)a ? b
B)b ? c
C)a ? c
D)a ? b and b ? c are the same length
E)a ? c and b ? c are the same length

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.$\begin{array} { l r } 
+ + + & 669 \
a b c & 653 \
+ + c & 121 \
a b + & 139 \
+ b + & 2280 \
a + c & 2215 \
+ b c & 3 \
a + + & 2
\end{array}$
-Which gene pair is closer together; i.e.there are fewer map units between them?
A)a ? b
B)b ? c
C)a ? c
D)a ? b and b ? c are the same length
E)a ? c and b ? c are the same length

Quizplus · Accepted Answer

The gene pair with the fewest map units between them is the one with the lowest recombination frequency. The recombination frequency is calculated as the number of recombinant offspring divided by the total number of offspring. The recombinant offspring are those that have a different phenotype from both parents. In this case, the recombinant offspring are those that have the phenotype a b c, + + c, a b +, + b c, a + c, and + b c. The total number of offspring is 669 + 653 + 121 + 139 + 2280 + 2215 + 3 + 2 = 6082. The recombination frequency for the gene pair a ? b is (139 + 121) / 6082 = 0.043. The recombination frequency for the gene pair b ? c is (3 + 2215) / 6082 = 0.365. The recombination frequency for the gene pair a ? c is (2 + 653) / 6082 = 0.107. Therefore, the gene pair with the fewest map units between them is a ? b.

A Testcross of Trihybrid Drosophila Produced the Following Phenotypes and Number