Q15.1-3CC - Campbell Biology

Question

Propose a possible reason that the first naturally occurring mutant fruit fly Morgan saw involved a gene on a sex chromosome and was found a male.

Step-by-Step Solution

Answer

When the gene is located on the sex chromosome, male Drosophila needs one mutant allele to exhibit a mutant phenotype. However, if the genes are present on autosomes, two mutant alleles are required to produce a mutant phenotype.

Step 1: Autosomes and sex chromosomes

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.

Step 2: Fruit fly

Drosophila melanogaster is 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.

Step 3: Reason for the occurrence of the first mutation in sex chromosome

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.