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A single cell made you: the same genes in every cell

You have a unique mixture of genes. No one else has the same pattern of genes as you - unless you are an identical twin.

Genes are the instructions that tell your cells how to make you. One set of your genes came from the sperm cell and one set from the egg cell; half from your father, half from your mother. At fertilisation a single cell formed that had a complete set of genes.

You grew by this first cell copying its genetic material then splitting into two cells; these grew, copied their genetic material and split into four cells, the same into eight cells, and so on... After 10 days a ball of hundreds of cells was formed, this was an embryo. Because the cell copies the genetic material before it splits into two and because you started off as one cell, every cell in your body has the same genetic material.

Identical twins.

If you have an identical twin then you will both have identical genetic material. Identical twins can form at any time in the 14 days after fertilisation if the ball of dividing cells splits into two totally separate embryos. Because these two embryos started off from this first single cell they will share the same genetic material.