How many chromosomes do humans have?
Humans normally have 46 chromosomes, arranged in 23 pairs. One set of 23 comes from the mother and the other set comes from the father.
Chromosomes are long, tightly coiled strands of DNA found in the nucleus of most cells. They contain genes, the instructions for building and running the body. Of the 23 pairs, 22 are called autosomes, numbered 1 to 22 roughly by size. The 23rd pair are the sex chromosomes.
Females usually have two X chromosomes, and males usually have one X and one Y. Egg and sperm cells each carry only 23 chromosomes, so when they combine at fertilization, the new cell has the full set of 46. Red blood cells are an exception, because they have no nucleus.
Chromosome differences
Sometimes a person has an extra or missing chromosome. Down syndrome is caused by an extra copy of chromosome 21. The number of chromosomes does not reflect how complex an organism is: dogs have 78, and some ferns have more than 1,000.
Did You Know?
Scientists confirmed that humans have 46 chromosomes only in 1956. Before then, many textbooks said there were 48.
This answer is general information, not medical advice. For a personal health question, talk to a doctor.