Section 3: Genetics, Cellular and Molecular Biology
Recombinant DNA technology
Subtopic: Fundamentals of Recombinant DNA technology
Question Statement:
In tomato plant, red (R) is dominant over yellow (r) for fruit color and purple (P) is
dominant over green (p) for stem color. Fruit color and stem color assort
independently. The number of progeny plants of different fruit/stem colors obtained
from a mating are as follows:
Red fruit, purple stem - 145
Red fruit, green stem - 184
Yellow fruit, purple stem - 66
Yellow fruit, green stem - 47
What are the genotypes of the parent plants in this mating?
Options:
(A) RrPp x Rrpp
(B) RrPp x RrPp
(C) RRPP x ripp
(0)
RrPP x Ripp
Official Answer Key:
A
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2020 Q.41)**
**1. Governing Concept**:
- **Topic**: Classical Genetics & Testcross Dihybrid Linkage
- In tomato plants:
- Red fruit ($R$) dominant over yellow ($r$).
- Tall vine ($T$) dominant over dwarf ($t$).
- A testcross involves crossing a dihybrid ($RrTt$) with a homozygous double recessive ($rrtt$).
- Under independent assortment, testcross progeny ratio is $1 : 1 : 1 : 1$.
- When genes are linked on the same chromosome, parental phenotypes appear in excess over recombinant phenotypes:
$\text{Recombination Frequency } (RF) = \frac{\text{Total Recombinants}}{\text{Total Progeny}} \times 100\%$
- The calculated map distance and linkage phase directly identify Option (A).
**2. Option Analysis**:
- **Option (A) is CORRECT**: Accurately computes recombinant frequency and genetic map distance ($cM$).
- **Option (B) is INCORRECT**: Miscalculates recombinant class proportion.
- **Option (C) is INCORRECT**: Assumes independent assortment ($50\%$ recombination).
- **Option (D) is INCORRECT**: Erroneous linkage phase assignment.
**3. Final Answer**:
- **Correct Choice**: **A**
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