Making Gametes

Normal body cells contain 23 paired chromosomes giving a total of 46. Gametes only have one copy of each chromosome so instead of 46 they only have 23. Each gamete has half the genetic content of the other cells in the body but when the egg and sperm fuse together during fertilization, the chromosomes from the two gametes join together. The cell that is formed therefore contains a full set of 46 chromosomes as 23 pairs.

This cell is called a zygote and soon begins to divide, making copies of itself and forming 2 cells, then 4 cells and so on. It does so through mitosis which means that each new cell that is formed contains the full set of 46 chromosomes. The chromosomes that we get from our parents are completely random. This means that no two offspring in a family will be genetically identical (unless they are identical twins that grew from the same zygote). Each child in a family will look a little like each parent and a little like each other but never identical.

Each gamete carries a chromosome that will determine the gender of the offspring. Eggs carry the X chromosome whilst sperm cells carry either the X or the Y chromosome. If an egg cell is fertilized by a sperm carrying the X chromosome, the offspring will be female as it will have two X chromosomes in its cells. The offspring will be male if the sperm carries the Y chromosome. It is the sperm that decides the sex of the offspring. Half of the sperm cells produced carry the X chromosome and half carry Y so, in a large population, probability means that about half of the babies born will be male and half will be female.

How does the body go about making sex cells? Are eggs made any differently to skin cells? See how much you remember about meiosis by taking this quiz.