Question

There are three genes a, b, c. Percentage of crossing over between a and b is 20%, b and c is 28% an a and c is 8%. What is the sequence of genes on chromosomes?
A. b,a,c
B. a,b,c
C. a,c,b
D. None of these

Answer 1

Hint: Certain genes located linearly on a chromosome are expressed together. These genes are also called linked genes. This disproves Mendel’s independent law of assortment.

Complete Step by Step Answer:
Linkage theory was given by T.H. Morgan and Castle after working on Drosophila flies. They figured out that the strength of linkage is inversely proportional to the distance of position of genes. Therefore, genes far away from each other are more prone to crossing over. Another way to put it would be crossing over probability is proportional to the distance between genes.
Bearing this theory in mind and the percentage of crossing over between various genes, we see that genes
b and c genes are the farthest, with 28% crossing over;
a and c genes are the closest with 8% crossing over;
a and b genes are moderately far with 20% crossing over
Thus, we can draw an inference that the sequence of genes is : b,a,c
Hence, the correct option is A: b,a,c

Note: Crossing over is a process that involves exchange of genetic material between two homologous non-sister chromosomes, each received by each parent. This is a crucial step that occurs in meiosis. It increases genetic variability., thus, contributing to increased genetic diversity. Law of Independent Assortment given by Gregor Mendel states that the alleles of two (or more) different genes get sorted into gametes independently of one another. This was opposed by Linkage theory.