Question

The salivary gland chromosomes in the dipterian larvae are useful in gene mapping because

• A. These are much longer in size.
• B. These are easy to stain.
• C. These are fused.
• D. They have endo-reduplicated chromosomes.

Answer 1

Answer: (D)

They have endo-reduplicated chromosomes.

Absence of cell division after DNA replication which in turn causes multiplication of chromosome number of cells is called endo-reduplication. Polytene chromosomes in salivary glands of Drosophila are formed as a result of the endo-reduplication and the synapsis of homologues without cell division. Thus they are much larger in size, easy to stain and are somatically paired but these features are not useful in gene mapping. Endo-reduplication produces transverse banding pattern of alternating chromatic and achromatic regions which in turn assist in gene mapping.