形成細胞內基因組或整個遺傳物質的單體。染色體是通過纏繞蛋白質而被包裝、壓縮而成的遺傳物質或DNA長鏈。染色體的數量和長度隨物種而不同。人類有23對染色體（每對染色體中的兩個單體分別來自父母雙方）。一對稱為性染色體因為它包含決定性別的基因。攜帶決定男性的基因的染色體稱為Y染色體，攜帶決定女性的基因的染色體稱為X染色體。其余的染色體稱為常染色體。1號染色體最大，22號染色體最小。每條染色體有兩臂稱為p和q。

One of the physically separate segments that together form the genome, or total genetic material, of a cell. Chromosomes are long strands of genetic material or DNA that have been packaged and compressed by wrapping around proteins. The number and size of chromosomes varies from species to species. In humans, there are 23 pairs of chromosomes (a pair has one chromosome from each parent). One pair are called the sex chromosomes because they contain genes that determine sex. The chromosome carrying the male determining genes is designated Y and the corresponding female one the X chromosome. The remaining pairs are called autosomes. Chromosome 1 is the largest and chromosome 22 the smallest. Each chromosome has two “arms” designated p and q.