The salivary gland chromosomes in the dipteran larvae, are useful in gene mapping because:

Step-by-step solution

Polytene chromosomes are giant chromosomes found in the salivary gland cells of some dipterian larvae. They also occur in the epithelial cells of intestine, fat bodies, Malpighian tubules etc. They begin as normal chromosomes, but through repeated rounds of DNA replication without any cell division (called endoreplication/endomitosis), they become large, having a high content of DNA. Related Theory In some developmental stages, one or more bands bear conspicuous swellings called chromosome puffs or bulbs. The large