Chromosome picture, how to interpret?
You need to look up polytene (giant) chromosomes.


  Polytene chromosomes are over-sized chromosomes which have developed from standard chromosomes and are commonly found in the salivary glands of Drosophila melanogaster. Specialized cells undergo repeated rounds of DNA replication without cell division (endomitosis), to increase cell volume, forming a giant polytene chromosome. Polytene chromosomes form when multiple rounds of replication produce many sister chromatids that remain synapsed together.


You can read more about it at wikipedia (http://en.wikipedia.org/wiki/Polytene_chromosome)