Chromosome X is
**Core Concept**
Chromosome X is one of the 23 pairs of chromosomes found in humans, specifically the sex chromosome that determines female sex. It is a crucial component of the human genome, carrying genes that contribute to various traits and conditions.
**Why the Correct Answer is Right**
Chromosome X is a large chromosome, approximately 155 million base pairs in size, and contains around 1,500 genes. It plays a significant role in the development and function of the female reproductive system, as well as other bodily systems. The X chromosome is also responsible for the inheritance of certain genetic disorders, such as hemophilia and Duchenne muscular dystrophy.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because chromosome X is not one of the autosomal chromosomes (chromosomes 1-22).
**Option B:** This option is incorrect because chromosome Y is the sex chromosome that determines male sex, not chromosome X.
**Option C:** This option is incorrect because chromosome 21 is a small autosome that is associated with Down syndrome, not chromosome X.
**Clinical Pearl / High-Yield Fact**
One important fact about chromosome X is that it is subject to X-chromosome inactivation, a process that randomly silences one of the two X chromosomes in female cells to avoid a doubling of gene expression. This process is essential for the proper development and function of female tissues.
**Correct Answer:** D.