Question

The recessive genes located on X-chromosomes of humans are always

• A. Lethal
• B. Sub-lethal
• C. Expressed in males
• D. Expressed in females

Answer 1

The Correct Option is C

To understand why the recessive genes located on X-chromosomes of humans are always expressed in males, we need to consider the genetic makeup of males and females as it pertains to the X-chromosome.

Humans have a pair of sex chromosomes that determine their sex. Females possess two X-chromosomes (XX), whereas males possess one X-chromosome and one Y-chromosome (XY). This fundamental difference plays a crucial role in how sex-linked traits are expressed.

1. Expression of X-linked Recessive Genes in Males:
   • Males have only one X-chromosome. Thus, any recessive gene present on the X-chromosome will be expressed, as there is no second X-chromosome to mask its expression with a dominant allele.
   • For example, consider a gene responsible for a condition like hemophilia, which is recessive and located on the X-chromosome. If a male inherits this recessive allele from his mother on his X-chromosome, he will express the condition because he does not have a second X-chromosome with a possible dominant healthy allele to counteract it.
2. Expression of X-linked Recessive Genes in Females:
   • Females have two X-chromosomes. For a recessive gene to be expressed in females, they must inherit the recessive allele from both parents (i.e., the gene must be homozygous recessive).
   • Thus, recessive X-linked genes are less frequently expressed in females compared to males.
3. Analysis of Options:
   • Lethal, Sub-lethal: Recessive X-linked genes are not inherently lethal or sub-lethal; their effects depend on the specific gene involved.
   • Expressed in females: Recessive X-linked genes are expressed in females only if they are homozygous recessive, which is less common than expression in males.
   • Expressed in males: As explained, because males have only one X-chromosome, recessive genes on this chromosome are always expressed. Thus, this is the correct answer.

Therefore, the correct answer is Expressed in males.