A chromosome that differs in structure or appearance from other chromosomes, particularly sex chromosomes like X and Y that look distinct.
From hetero- (different) + chromosome (colored body). Genetics term from early 20th century describing chromosomes with different morphology.
Your sex chromosomes are heterochromosomes—the tiny Y chromosome looks completely different from the large X, and the Y is actually shrinking over evolutionary time, having lost most genes it once shared with X.
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