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 in a male?

Explanation:

Male fruit flies have only one X chromosome. If a recessive mutation occurs on the X chromosome, it will be expressed in males because there is no second X chromosome with a normal allele to mask it. Females have two X chromosomes, so for a recessive mutation on the X chromosome to be expressed in females, it would need to be present on both X chromosomes (homozygous), which is less likely to occur spontaneously compared to the single - copy situation in males.

Answer:

Male fruit flies have only one X chromosome. A recessive mutation on the X chromosome will be expressed in males (as there is no second X to mask it), while in females, two copies (homozygous) of a recessive X - linked mutation are needed for expression, which is less likely to occur spontaneously.