Question

A represents the dominant allele and a represents the recessive allele of a gene. If, in 2000 offspring, 1000 are aa and 1000 are of some other genotype, which of the following are most probably the genotypes of the parents?

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

Hint:

A \(1:1\) phenotypic ratio among offspring when crossing a dominant phenotype with a recessive one indicates that the dominant parent is heterozygous (Aa), confirming a testcross.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Mendelian genetics describes how alleles segregate and combine during sexual reproduction.
Analyzing the phenotypic or genotypic ratios among offspring can help determine the genotypes of the parents.

Step 2: Detailed Explanation:
We are given that out of 2,000 offspring:
- \(1,000\) are homozygous recessive (\(aa\)).
- \(1,000\) are of some other genotype (which must be \(Aa\) or \(AA\)).
This represents a \(1:1\) (or \(50\%:50\%\)) ratio of recessive to dominant-expressing offspring.
Let us analyze the crosses to identify which parent combination yields this ratio:
Aa \(\times\) Aa (Heterozygous cross): Yields a \(1:2:1\) genotypic ratio (\(25\%\) \(AA\), \(50\%\) \(Aa\), \(25\%\) \(aa\)). This does not produce a \(1:1\) ratio.
Aa \(\times\) aa (Testcross): The heterozygous parent (\(Aa\)) produces two types of gametes (\(A\) and \(a\)), while the homozygous recessive parent (\(aa\)) produces only one type (\(a\)).
The Punnett square yields: \[ \text{Offspring} = \frac{1}{2} Aa \text{ and } \frac{1}{2} aa \]
This matches the observed \(1:1\) ratio exactly, with \(1,000\) \(aa\) and \(1,000\) \(Aa\) offspring.
AA \(\times\) Aa: Yields \(50\%\) \(AA\) and \(50\%\) \(Aa\). No \(aa\) offspring are produced.
AA \(\times\) aa: Yields \(100\%\) \(Aa\) offspring. No \(aa\) offspring are produced.

Step 3: Final Answer:
The genotypes of the parents are Aa and aa.