1. Frequency of crossing over is directly proportional to-
(1) Length of chromosomes
(2) Number of genes
(3) Distance between genes
(4) Number of chromosomes

Frequency of crossing over is directly proportional to the distance between genes on a chromosome.

Explanation of Options:

1. Length of chromosomes: While the length of a chromosome provides a physical space, crossing over frequency is not directly proportional to the total chromosome length but rather the specific distance between the genes involved.

2. Number of genes: The number of genes on a chromosome does not directly affect crossing over frequency; instead, it depends on how far apart the genes are.

3. Distance between genes: This is the correct answer. The farther apart two genes are on a chromosome, the higher the chance that a crossover will occur between them during meiosis.

4. Number of chromosomes: This factor does not influence the frequency of crossing over for genes on a single chromosome.

Introduction:
Crossing over is a crucial genetic process during meiosis where homologous chromosomes exchange genetic material. A key factor influencing the frequency of crossing over between genes is the physical distance between them on the chromosome. This article explains why crossing over frequency is directly proportional to the distance between genes and discusses other related factors.

Detailed Explanation:

• Crossing over occurs during meiosis, facilitating genetic recombination and diversity by exchanging chromosomal segments between homologous chromosomes.

• The likelihood of a crossover event between two genes depends primarily on the distance separating them on the chromosome; greater distances increase the chance of crossover.

• The length of the chromosome and the number of genes are less directly influential, as crossing over is localized between specific gene loci rather than the entire chromosome.

• The number of chromosomes in a cell does not affect the crossing over frequency between genes located on a particular chromosome.

This understanding forms the basis for genetic mapping, where recombination frequency helps determine the relative positions of genes on chromosomes.

The correct answer is (3) Distance between genes.