# DBT JRF 2020 — Question 43

Exam: DBT JRF 2020 · Section: B · Topic: Genetics · Difficulty: Medium

## Question

A combination of gametes that can be formed by the genotype AaBbCcDdEeFfGg are

## Options

- **A.** 16
- **B.** 32
- **C.** 64
- **D.** 128

## Answer

Option D

## Explanation

Proline (Pro, P) is unique among the 20 standard amino acids: its side chain (a propyl group) forms a covalent bond with the backbone nitrogen, creating a rigid pyrrolidine ring. Consequences for $\alpha$ -helix: (1) The backbone N-H is absent (N is tertiary, no H available) — proline cannot donate an H-bond in the helix, breaking the H-bonding pattern that stabilises $\alpha$ -helices. (2) The pyrrolidine ring constrains the backbone $\phi$ angle to approximately $-60°$ (fixed), incompatible with ideal $\alpha$ -helical $\phi$ values ( $\approx -57°$ , but more importantly the rigidity means proline cannot adjust to the helix geometry). Result: proline introduces a rigid kink/bend, acting as a 'helix breaker.' It is commonly found at turns, hinges, and at the N-terminal first position of helices.

Source: https://mypyqbuddy.com/questions/a-combination-of-gametes-that-can-be-formed-6a05a8a02adafec470293243