Question

In a dihybrid cross, if an individual heterozygous for both the characters is crossed with double recessive, which phenotype ratio would be expected? Choose the correct answer from the options given below:

• A. 1 : 2 : 2 : 1
• B. 1 : 2 : 1 : 1
• C. 1 : 1 : 1 : 1
• D. 9 : 3 : 3 : 1

Hint:

A test cross always reveals the hidden genotypes. Since the double recessive parent contributes only recessive alleles, the offspring phenotypes directly reflect the gametes of the heterozygous parent.

Answer 1

The Correct Option is C

This is a test cross, where an organism heterozygous for two traits (AaBb) is crossed with one that is homozygous recessive for both traits (aabb).
Parent 1 genotype: AaBb
Parent 2 genotype: aabb Gametes from AaBb: Since AaBb is heterozygous for both traits, it can produce 4 types of gametes: \[ \text{AB},\ \text{Ab},\ \text{aB},\ \text{ab} \] Gametes from aabb: Since aabb is homozygous recessive, it can produce only 1 type of gamete: \[ \text{ab} \] F\textsubscript{1 Generation Offspring:} Crossing the above gametes gives 4 possible combinations: \begin{tabular}{ll} AB × ab & $\rightarrow$ AaBb
Ab × ab & $\rightarrow$ Aabb
aB × ab & $\rightarrow$ aaBb
ab × ab & $\rightarrow$ aabb
\end{tabular} Each genotype corresponds to a unique phenotype:
AaBb: dominant for both traits
Aabb: dominant for first, recessive for second
aaBb: recessive for first, dominant for second
aabb: recessive for both traits Thus, the phenotypic ratio = \[ \boxed{1 : 1 : 1 : 1} \]