35. In Drosophila order of genes under investigation is as shown in figure.

b⁺          cm⁺               w⁺
5cM           10cM

if drosophila with genotype    b⁺    cm⁺    w⁺

b       cm     w

is test crossed what would be observed frequency of progenies with genotype

b⁺   cm      w⁺

b      cm      w

assuming zero interference ?
(1) 0.005       (2) 0.02
(3) 0.0025     (4) 1.5

Calculation of Required Progeny Frequency in Drosophila Test Cross

The required frequency of the progeny class b+ cm w+ / b cm w under zero interference is 0.005. Therefore, the correct option is (1) 0.005.

Given Data

The gene order and map distances on the X chromosome of Drosophila are:

b+ − 5 cM − cm+ − 10 cM − w+

• Distance between b and cm = 5 cM → Crossover frequency in region b − cm = 0.05
• Distance between cm and w = 10 cM → Crossover frequency in region cm − w = 0.10

Test Cross Setup

The heterozygous female used for the test cross is:

b+ cm+ w+ / b cm w

Since she is crossed with a triple recessive male (b cm w), each gamete from the female appears directly as a progeny genotype.

Step-by-Step Genetic Calculation

1. Identify the Target Gamete

The required progeny genotype is b+ cm w+ / b cm w. Thus, the maternal gamete must be b+ cm w+.

Compare with parentals:

• Parental 1: b+ cm+ w+
• Parental 2: b cm w

Observation:

• At b: same as Parental 1 (no change).
• At w: same as Parental 1 (no change).
• At cm: allele changed from cm+ to cm, i.e., from Parental 1 to Parental 2.

This pattern results from a double crossover (one between b − cm and another between cm − w).

2. Use Zero-Interference Assumption

Under zero interference, crossovers occur independently.

Thus, the expected frequency of progeny with genotype b+ cm w+ / b cm w is 0.005 (0.5%).

Evaluation of Options

Conclusion

Only Option (1) — 0.005 — correctly represents the double crossover frequency under zero interference.

Short SEO‑oriented introduction

In classical genetics, three‑point test crosses in Drosophila are a powerful way to map gene order and calculate recombination frequencies between linked genes. Understanding how to convert map distances (in centiMorgans) into single‑ and double‑crossover frequencies is essential for accurately predicting progeny classes under zero interference. This worked example with genes $b$, $cm$, and $w$ shows how a 5 cM and 10 cM interval leads to a 0.005 frequency for a specific double‑crossover genotype in a test cross.​