Question

in fruit flies, + (wild-type) is dominant to se (sepia). a cross yields 1 wild-type : 1 sepia offspring. which parent genotypes best explain this ratio?
+se × +se
+se × sese
++ × ++
sese × sese

Explanation:

Step1: Analyze each cross

• For \(\boldsymbol{+Se \times +Se}\): Using Punnett square, offspring genotypes are \(++\), \(+Se\), \(+Se\), \(SeSe\). Phenotypic ratio is 3 wild - type : 1 sepia.
• For \(\boldsymbol{+Se \times SeSe}\): Offspring genotypes are \(+Se\) and \(SeSe\) (each with 50% chance). Phenotypic ratio is 1 wild - type (\(+Se\)) : 1 sepia (\(SeSe\)).
• For \(\boldsymbol{++ \times ++}\): All offspring are \(++\), so all wild - type.
• For \(\boldsymbol{SeSe \times SeSe}\): All offspring are \(SeSe\), so all sepia.

Step2: Match the ratio

The given ratio is 1 wild - type : 1 sepia, which matches the cross \(+Se \times SeSe\).

Answer:

\(+Se \times SeSe\)