What are polytene chromosomes and how are they formed?

Polytene chromosomes:

1. Polytene chromosomes are large chromosomes found in the salivary glands of insects. Balbiani discovered them in 1881.
2. They're also known as salivary gland chromosomes because they were discovered in the salivary glands. Because they have a lot of chromonemata, they're called polytene.
3. They develop when chromosomes are regularly duplicated without being divided. The medical word for this ailment is endomitosis.
4. They have darkly stained bands, while the interband is the area between two bands that is lightly coloured.
5. The polytene chromosome is around 0.5 mm in length and 20 m in diameter.
6. Dipteran flies, unicellular protozoans, mammals (trophoblast), plants (antipodal cells), and other organisms are known to have polytene chromosomes.