In Drosophila melanogaster, the yellow body gene is X-linked. The yellow body phenotype is recessive to brown body phenotype. A yellow body female fruit fly was mated to a brown body male. The vast majority of the male progeny are yellow body. However, a rare brown body male was found in the progeny (1 out of 2060 males). What is the likely karyotype of this brown body son? XXY XX XYY XO XXX Solution Sex inheritance by modified Mendelian mechanism: X-linked inheritance means that the gene causing the trait or the disorder is located on the X chromosome. X-linked recessive inheritance is a mode of inheritance in which a mutation in a gene on the X chromosome causes the phenotype to be expressed in males who are necessarily hemizygous for the gene mutation because they have one X and one Y chromosome. Carrier females who have only one copy of the mutation do not usually express the phenotype, Females are XX (homogametic) Males are XY (heterogametic) Body color inherited by single gene, two allele system, brown dominant to yellow But, sex and body color not inherited independently Body color is linked to sex Body color gene on X chromosome Therefore, males have only one body color allele: Haploid and females have two eye color alleles: Diploid. Normally all the sons and none of the daughters show the recessive sex-linked characters of the mother when the father carries the dominant allelomorph. For example, a yellow body female mated to a brown body male produces yellow body males and brown body females, but rarely also a yellow body female or a brown body son. The production of these exceptions by a normal XX female must be due to an aberrant reduction division at which the two X chromosomes fail to disjoin from each other. In consequence both remain in the egg or both pass into the polar body. In the latter case an egg without an X chromosome is produced. Such an egg fertilized by an X sperm produces a male with the constitution XO. These males received their single X from their father and therefore show the father\'s characters. But that the Y does play some positive role is proved by the fact that all the XO males have been found to be absolutely sterile. Answer is d) XO.

1. In Drosophila melanogaster, the yellow body gene is X-linked. The yellow body phenotype is
recessive to brown body phenotype. A yellow body female fruit fly was mated to a brown body
male. The vast majority of the male progeny are yellow body. However, a rare brown body male
was found in the progeny (1 out of 2060 males). What is the likely karyotype of this brown body
son? XXY XX XYY XO XXX
Solution
Sex inheritance by modified Mendelian mechanism:
X-linked inheritance means that the gene causing the trait or the disorder is located on the X
chromosome.
X-linked recessive inheritance is a mode of inheritance in which a mutation in a gene on the X
chromosome causes the phenotype to be expressed in males who are necessarily hemizygous for
the gene mutation because they have one X and one Y chromosome. Carrier females who have
only one copy of the mutation do not usually express the phenotype,
Females are XX (homogametic)
Males are XY (heterogametic)
Body color inherited by single gene, two allele system, brown dominant to yellow
But, sex and body color not inherited independently
Body color is linked to sex
Body color gene on X chromosome
Therefore, males have only one body color allele: Haploid and females have two eye color
alleles: Diploid.
Normally all the sons and none of the daughters show the recessive sex-linked characters of the
mother when the father carries the dominant allelomorph. For example, a yellow body female
mated to a brown body male produces yellow body males and brown body females, but rarely
also a yellow body female or a brown body son. The production of these exceptions by a normal
XX female must be due to an aberrant reduction division at which the two X chromosomes fail
to disjoin from each other. In consequence both remain in the egg or both pass into the polar
body. In the latter case an egg without an X chromosome is produced. Such an egg fertilized by
an X sperm produces a male with the constitution XO. These males received their single X from
their father and therefore show the father's characters. But that the Y does play some positive
role is proved by the fact that all the XO males have been found to be absolutely sterile.
Answer is d) XO