Final answer:
A possible reason for Thomas Hunt Morgan's observation of a naturally occurring mutant fruit fly involving a sex chromosome and being found in a male is the hemizygous nature of X chromosomes in males, leading to direct expression of any recessive mutations.
Step-by-step explanation:
A possible reason that the first naturally occurring mutant fruit fly observed by Thomas Hunt Morgan had a gene mutation on a sex chromosome and was found in a male is due to the hemizygous nature of the X chromosome in males. Since Drosophila males only have one X chromosome (XY), any recessive mutations like the white eye color (Xw) will be expressed, as there is no second copy of the gene that could potentially mask the mutation. Females, with two X chromosomes (XX), can carry a recessive mutation on one of the X chromosomes without it being expressed since the dominant allele on the other X can mask it. This explains why X-linked traits are more frequently observed and expressed in males.