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: The distance between genes on a chromosome affects the frequency of crossing over during meiosis because the likelihood of chiasmata formation increases with the distance between the genes. The farther apart two genes are, the more likely a chiasma will form between them, leading to a higher frequency of crossing over and greater genetic diversity in offspring. Conversely, when genes are positioned close together, they are less likely to undergo crossing over and may be inherited together as a group, known as genetic linkage.

Step-by-step solution

1. Understanding Crossing Over and Meiosis

Crossing over is an important process that occurs during meiosis, a type of cell division responsible for the production of gametes (sperm and egg cells). During meiosis, homologous chromosomes exchange genetic material through a process called recombination, which can result in new combinations of genes in the offspring. This is a significant source of genetic diversity.

2. The role of Chiasmata in Crossing Over

During the first stage of meiosis, known as prophase I, homologous chromosomes pair up and form structures called chiasmata, where genetic material is exchanged between the chromosomes. The frequency of chiasmata formation can vary depending on the total length of the chromosome and the distance between genes of interest.

3. Recombination Frequency and Gene Distance

The recombination frequency between two genes on a chromosome is proportional to the distance between those genes. The farther apart two genes are, the more likely a chiasma will form between them, leading to a higher frequency of crossing over. This relationship is often expressed using a unit called a centimorgan (cM), which represents the relative distance between genes.

4. The Impact of Gene Distance on Crossing Over

Since crossing over happens more frequently between distantly linked genes, we observe more recombination events for genes located further apart on the chromosome. This increased recombination can result in a greater variety of genetic combinations in offspring, giving them a greater chance of survival and adaptation to changing environments. Conversely, when genes are positioned close together on a chromosome, they are less likely to undergo crossing over and may be inherited together as a group, known as genetic linkage.

5. Conclusion

In summary, more crossing over occurs between distantly linked genes because the likelihood of chiasmata formation increases with the distance between the genes. This increased recombination frequency ensures genetic diversity and adaptation in offspring.