3. In genetic map, distance between two genes is measured in units of
(1) cM        (2) A⁰
(3) bp        (4) nm

Title: Understanding Genetic Map Distance: Units and Their Significance

Introduction:
Genetic mapping is crucial in genetics to determine the relative position and distance between genes on a chromosome. The distance between genes on a genetic map is not measured in physical units but based on recombination frequency, which reflects the likelihood of crossover events during meiosis. This article explains the unit of measurement used in genetic mapping and clarifies common terms and options related to it.

Explanation of Options:

1. cM (centiMorgan): This is the correct unit for measuring genetic map distance. One centiMorgan (cM) represents a 1% chance of crossing over between two genes during meiosis. It is a measure of genetic linkage and recombination frequency rather than a physical distance. The concept was introduced by geneticist Thomas Hunt Morgan and his student Alfred Sturtevant. If two genes are 1 cM apart, there is a 1% chance that a crossover will occur between them, indicating they are relatively close on the chromosome.

2. Å (Angstrom, A0): An angstrom is a unit of length equal to 10^-10 meters typically used to measure atomic and molecular dimensions. It is a physical unit of distance, not used for genetic mapping.

3. bp (base pairs): Base pairs represent the actual physical length of DNA sequences, counting the number of nucleotide pairs between genes. While important for physical genome mapping, bp is not the unit used on genetic maps based on recombination frequency.

4. nm (nanometers): A nanometer equals one-billionth of a meter and is used to describe physical distances at the molecular level, such as the length of DNA segments under a microscope. Like angstroms and base pairs, it is a physical measurement, not a unit for genetic map distances.

This detailed explanation clarifies that genetic map distances are measured in centiMorgans (cM), representing recombination frequency and genetic linkage, distinguishing it from physical length units like angstroms, base pairs, and nanometers.