Question

There are three genes a, b and c. The percentage of crossing over between a and b is 20%, b and c is 28% and a and c is 8%. What is the sequence of genes on chromosome

• A. b, a, c
• B. a, b, c
• C. a, c, b
• D. None

Answer 1

The Correct Option is A

To determine the sequence of genes on the chromosome based on the given crossover percentages, we need to understand the concept of genetic linkage and crossing over:

1. Understanding Crossing Over: Crossing over refers to the exchange of genetic material between homologous chromosomes during meiosis. The likelihood of crossing over between two genes is proportional to the distance between them on a chromosome. The percentage of recombination (or crossover frequency) can be used to estimate this distance.
2. Given Data:
   • Percentage of crossing over between genes a and b = 20%
   • Percentage of crossing over between genes b and c = 28%
   • Percentage of crossing over between genes a and c = 8%
3. Analysis of Data:
   • The lower the crossover percentage, the closer the genes are. Here, genes a and c have the smallest percentage (8%), indicating they are closest to each other.
   • Gene b, having a higher crossover percentage with both a (20%) and c (28%), is located furthest from a and c but between them.
4. Arranging the Sequence:
   • Since a and c are closer (8%) and b has intermediary distances with both (20% with a and 28% with c), it suggests that b is positioned between a and c on the chromosome.
5. Conclusion: Based on the crossover percentages, the correct sequence of the genes is b, a, c.

Thus, the correct option is b, a, c.