The Same Set in Every Cell

You are an organism. You have cells, your cells have nuclei, and the nuclei contain chromosomes. So if we're going to think about chromosomes, we might as well start by thinking about you and your chromosomes.

You are made up of billions and billions of cells. Every non-sex, or somatic, cell has 46 chromosomes sitting in its nucleus. Sex cells are sperm and ova. They are special.

Here's the important point: the 46 chromosomes that are sitting in any one of your somatic cells are identical to the 46 that are sitting in every other of your somatic cells.

It's not like your skin cells have one set of chromosomes and your kidney cells have another. All of your cells have exactly the same set of chromosomes in their nuclei.

The same is true of every other organism in the whole world. Think about your best friend. His (or her) chromosomes are different from yours, that's for sure. But the 46 chromosomes in any one of his cells is absolutely identical to the 46 chromosomes that are in every other one of his cells.

Think about your dog, cat, turtle, or hamster. Think about some tree on your block. Think about any organism you like, and this will always be true; within any organism, the chromosomes that are sitting in one cell's nucleus are identical to the set that's sitting in every other cell's nucleus.

An Exception

Within any individual, all cells have identical chromosomes. That's true. Two different individuals do not have the same sets of chromosomes. That's almost always true. The exception is: identical twins. Each identical twin has exactly the same chromosomes as the other.