Which of the following is the smallest autosome?
**Core Concept**
The human genome consists of 22 pairs of autosomal chromosomes, each with distinct sizes, and two sex chromosomes (X and Y). The smallest autosome refers to the shortest chromosome among the 22 pairs.
**Why the Correct Answer is Right**
The correct answer is chromosome 21, which is the smallest autosome. Chromosome 21 is a member of the group C (acrocentric) chromosomes, which are characterized by their short arms. The short arm of chromosome 21 is particularly small, making it the smallest autosome. The size difference among the chromosomes arises due to variations in the number of nucleotides and the presence of heterochromatic regions.
**Why Each Wrong Option is Incorrect**
* **Option A:** Chromosome 1 is the largest autosome, not the smallest. It has a long arm and a short arm, making it the largest among the autosomes.
* **Option B:** Chromosome 2 is the second-largest autosome, but it is not the smallest. Chromosome 2 is also a member of the metacentric group, with a relatively balanced size between its long and short arms.
* **Option D:** Chromosome Y is a sex chromosome and is not considered an autosome. It is also much smaller than chromosome 21, but its size is due to its unique structure and function.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the size of the chromosomes can be an important factor in genetic disorders. Chromosomal abnormalities, such as trisomy 21 (Down syndrome), can arise from errors in chromosome replication or segregation, highlighting the importance of accurate chromosome size and structure in genetic diagnosis.
**Correct Answer:** D. Chromosome 21