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Chromosome 1
Each human cell nucleus has one pair of chromosome 1, the largest of all the human chromosomes. Its coiled-up DNA contains as many as 247 million nucleotide base pairs, which makes up about 8 percent of the cell's DNA. Researchers who made size-based predictions of chromosome 1's gene count initially placed it at about 3,000 genes; however, newer studies on its sequence show the count to be closer to 4,000. Deletion of multiple genes in this chromosome has been linked to certain types of cancerous tumors and other structural abnormalities.
Chromosome 21
Chromosomes generally fall under two classes: autosomes and sex chromosomes. Of the 23 chromosome pairs in the human cell, 22 are autosomes. Of these, chromosome 21 is the smallest, spanning 47 million DNA building blocks, or another 1.5 percent of the cell's total DNA. Chromosome 21, being the the smallest autosome, was the second human chromosome to be sequenced completely, after chromosome 22. It contains an estimated 300 to 400 genes. Problems with this chromosome or with the genes it contains have been linked to disorders such as Down syndrome and some forms of leukemia.
The X Chromosome
The X chromosome is the first and bigger of the two sex-determining chromosomes, containing about 155 million base pairs, or 5 percent of the cell's total DNA. A normal human female has two X chromosomes in each cell, while a normal male has one. The X chromosome is estimated to contain 1,400 to 2,000 genes. Issues with the X chromosome's genes can cause unusual facial features, neurological abnormalities and skin defects. Other disorders associated with the chromosome or with the genes it contains include Klinefelter syndrome, Turner syndrome and triple X syndrome.
The Y Chromosome
The Y chromosome, the smaller of the two human sex chromosomes, spans an estimated 58 million nucleotide base pairs, or 2 percent of the total DNA contained in the nucleus. It contains as few as 86 genes, nearly all of which play vital roles in male sex determination and development. Issues with the Y chromosome and with the genes it contains often manifest as testicular disorders, testosterone reduction, infertility, skeletal abnormalities and decreased IQ.