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 than between two closely linked genes on the same chromosome because the formation of chiasmata is more likely between distant genes. The increased likelihood of chiasmata formation between distant genes results in a higher frequency of crossing-over events and genetic recombination, ultimately contributing to greater genetic diversity in the offspring.

Step-by-step solution

Understanding Crossing Over

Crossing over is a process that occurs during meiosis, where homologous chromosomes exchange genetic material. This process increases genetic diversity within the offspring, as it creates new combinations of alleles in the resulting gametes. Crossing over occurs during the prophase I stage of meiosis, where homologous chromosomes pair up and form structures known as chiasmata.

Proximity and Crossing Over

The probability of crossing over between two genes is influenced by their physical distance on a chromosome. If two genes are very close or nearby, it is less likely that crossing over will occur between them. This is because the chiasmata, which are the points of exchange, are less likely to form between two closely linked genes. As a result, their alleles will tend to be inherited together, also referred to as genetic linkage.

Greater Distance and Increased Frequency of Crossing Over

Conversely, when two genes are further apart on the chromosome, there is a greater likelihood that one or more chiasmata will form between them during prophase I of meiosis. This increases the chance of genetic recombination and crossing over between the two genes, resulting in a greater diversity of alleles in the offspring. The frequency of crossing-over events between two genes is proportional to the distance separating them.

Conclusion

In summary, more crossing over occurs between two distantly linked genes than between two closely linked genes on the same chromosome because of the increased likelihood of chiasmata formation between distant genes. This increased probability of chiasmata formation results in a higher frequency of crossing-over events and genetic recombination, ultimately contributing to greater genetic diversity in the offspring.