Two double heterozygous plants (PpQq), derived from two different pairs of true-breeding parents of unknown genotype, produce gametes in the proportions as given below.
Which one of the following options correctly represents the genotype of the parents?
Show Hint
The gametes with the highest percentages are always the parental types.
For Cross 1, they are $pQ$ and $Pq$, meaning parents must have those exact alleles homozygous (ppQQ and PPqq).
This tip lets you solve linkage problems in seconds.
Step 1 : Understanding the Question:
The question asks to determine the genotypes of the homozygous, true-breeding parental lines (R1, R2, R3, R4) that produced the $F_1$ dihybrids based on the ratios of gametes produced by those $F_1$ plants. Step 2 : Key Formulas and Approach:
The approach relies on the principles of genetic linkage and recombination.
In a linked dihybrid system, parental gametes are always produced in higher frequencies ($> 25\%$ each) due to linkage, while recombinant gametes are produced in lower frequencies ($< 25\%$ each) due to crossing over. Step 3 : Detailed Explanation:
• Analyzing Cross 1 (R1 $\times$ R2):
The $F_1$ dihybrid produces gametes in the proportions: $35\%\ pQ$, $35\%\ Pq$, $15\%\ PQ$, and $15\%\ pq$.
The gametes with the highest frequencies ($35\%$ each) represent the non-recombinant (parental) configurations: $pQ$ and $Pq$.
This indicates the $F_1$ alleles are in the repulsion (trans) phase, represented as $pQ/Pq$.
Since the parents R1 and R2 are true-breeding (homozygous), one must have contributed the $pQ$ chromosome and the other the $Pq$ chromosome.
Thus, the parental genotypes must be $ppQQ$ and $PPqq$.
• Analyzing Cross 2 (R3 $\times$ R4):
The $F_1$ dihybrid produces gametes in the proportions: $35\%\ PQ$, $35\%\ pq$, $15\%\ pQ$, and $15\%\ Pq$.
Here, the high-frequency parental gametes are $PQ$ and $pq$ ($35\%$ each).
This indicates the $F_1$ alleles are in the coupling (cis) phase, represented as $PQ/pq$.
The homozygous parents R3 and R4 must have contributed the $PQ$ and $pq$ chromosomes.
Thus, their genotypes are $PPQQ$ and $ppqq$. Step 4 : Final Answer:
Matching these findings, we get: R1 = ppQQ, R2 = PPqq, R3 = PPQQ, R4 = ppqq.
This is represented by Option (A).
