Question

A single recessive trait, which can express its effect should occur on
A) Any autosome
B) A chromosome
C) X chromosome of female
D) X chromosome of male

Answer 1

Generally humans contains 23 pairs of chromosomes which is present in each cells, and the chromosomes of normal cells are called as Autosomes, while the chromosomes of the sex cells are called as allosomes or sex chromosomes, so out of 23 pairs, 22 pairs act as autosomes and one pair act as allosomes.

Complete answer:
The chromosome contains genes and the genes present on the chromosomes are of two types, where they can be dominant or recessive.
Each chromosome has a gene where a gene contains a pair of alleles which represent the trait, where the expression is dependent on the type of alleles, which means that should be dominant.
In general dominant ones are expressed in phenotypes, where the recessive genes are expressed only in the case of homozygous recessive condition.
As we know that autosomes are present as homologous pairs, but to express the recessive trait, where it should carry both the copies, but this can not happen in autosomes so by that Option A and B are wrong.
Where as in sex chromosomes, each offspring obtain one from each parent, and the females have XX chromosomes and males have XY chromosomes.
In heterozygous conditions the chromosomes contain both dominant and recessive genes, but only the dominant one is expressed as its mask the recessive gene expression.
In females as they have two X chromosomes the dominant may mask the recessive one, but in Males only One X chromosome so that whether it’s dominant or recessive it is expressed.

So, the correct option is D.

Note: In females, they get X chromosome from the mother as well as from father, while in males they get X chromosome from mother and Y from father, so they have only one X chromosome, and whatever the gene that present will be expressed as it do not have a pair to mask the expression.