Question

Why does more crossing over occur between two distantly linked genes than between two genes that are very close together on the same chromosome?

Short answer

Answer: More crossing over occurs between two distantly linked genes because the physical distance between the genes on the chromosome plays a significant role in the frequency of crossing over events. The farther apart two genes are, the more likely it is that a crossing over event will occur between them. This is due to the higher probability of a crossover event happening somewhere in the intervening DNA sequence when the genes are spaced further apart. In contrast, when genes are located close together on a chromosome, the chances of a crossover event happening between them are relatively low, resulting in less recombination and lower genetic diversity.

Step-by-step solution

1. Define Crossing Over

Crossing over is a process that occurs during the production of sex cells (meiosis), in which homologous chromosomes exchange equivalent genetic material. This exchange generates genetic diversity, which is vital for the survival and evolution of a species.

2. Mechanism of Crossing Over

Crossing over occurs during a stage in meiosis called prophase I. Homologous chromosomes – one from each parent – pair up and form structures called tetrads or bivalents. The chromosomes then break at specific points and recombine with their homologous partners, forming new combinations of genetic information.

3. Factors Affecting Crossing Over Frequency

The frequency of crossing over events between two genes depends on their physical distance on the chromosome. The farther apart two genes are, the more likely it is that a crossing over event will occur between them. This is because there is a higher probability of a crossover event happening somewhere in the intervening DNA sequence when the genes are spaced further apart.

4. Explaining the Exercise

The question asks why more crossing over occurs between two distantly linked genes than between two genes that are very close together on the same chromosome. Based on the information presented above, we can conclude that the primary reason for this difference in crossing over frequency is the physical distance between the genes on the chromosome. When genes are far apart, there is a higher probability of a crossover event occurring between them, leading to more gene recombination and higher genetic diversity. In contrast, when genes are located close together on a chromosome, the chances of a crossover event happening between them are relatively low, resulting in less recombination and lower genetic diversity.