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A chromosome is the discrete unit of the genome that carries genes.These are self-replicating molecules involved in heredity, mutation, evolution, variation, etc. Autosomes and sex chromosomes are the two types of chromosomes.

Genes that determine sex-linked traits are present on sex chromosomes, while genes that determine all other traits of an individual are present on autosomes.

Drosophila melanogasteris commonly known as the fruit fly. It is a small insect and feeds on fungi. This insect has four pairs of chromosomes; one pair of sex chromosomes and three pairs of autosomes.

Male Drosophila possesses one X and one Y chromosome, while females possess two X chromosomes. This fly produces many offspring in a single mating and has a short generation time. Thus, these insects are easy to grow in the lab.

The first mutant Drosophila was a male with white eyes.However, the wild type has red eyes.The gene for the eye color of Drosophila is located on X-chromosome.

Thus, a male with a defective X chromosome could exhibit the mutant phenotype as males possess one X and one Y chromosome.

However, if the gene linked to the mutant phenotype was present on autosomes, then both the alleles of the gene had to be mutant to exhibit the phenotype.This is quite a rare situation.

Thus, mutant phenotype linked to sex chromosome in male fruit fly was important to identify the mutant organism.