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, c
- b, a, c
- a, c, b
- None
Answer
(2) b, a, c
Solution
Frequency of crossing over is proportional to the distance between two genes (1% crossing over = 1 map unit).
The largest value, b–c = 28, gives the two end genes: b and c.
⇒ a lies between b and c.
Check: b–a + a–c = 20 + 8 = 28 = b–c ✓
∴ Sequence of genes: b, a, c (b — 20 — a — 8 — c)
Answer: (2)