Question

If the mating of two organisms produce a 1:1 phenotypic ratio in the progeny, then the correct parental cross-combination is

• A. Aa x Aa
• B. Aa x aa
• C. AA x AA
• D. AA x aa

Hint:

Monohybrid test cross ($Aa \times aa$) gives a $1:1$ ratio; Dihybrid test cross ($AaBb \times aabb$) gives a $1:1:1:1$ ratio.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
A 1:1 phenotypic and genotypic segregation ratio is the diagnostic hallmark of a monohybrid test cross.

Step 2: Key Formula or Approach:
Evaluating the progeny ratios for each cross:
1. $Aa \times Aa \implies 3:1$ phenotypic ratio ($1\,AA : 2\,Aa : 1\,aa$).
2. $Aa \times aa \implies$ Gametes from $Aa$ ($A, a$) fertilize gametes from $aa$ ($a$):
\[\text{Progeny} = 1\,Aa \text{ (Dominant)} : 1\,aa \text{ (Recessive)}\] This gives a $1:1$ phenotypic ratio.
3. $AA \times AA \implies 100\%\,AA$ (all dominant).
4. $AA \times aa \implies 100\%\,Aa$ (all dominant).

Step 3: Detailed Explanation:
Therefore, the parental cross-combination yielding a 1:1 ratio is $Aa \times aa$.

Step 4: Final Answer:
Hence, the correct option is (B).