Meiosis.

Step 1 of 5

Half a set

At fertilisation an egg and a sperm join into one cell, the fertilised egg, and the whole body is built from it. Sex cells, gametes, are not made by mitosis. They are made by another kind of division, meiosis, which happens only in the testes and ovaries.

The cell in the Scene is such a cell, in a testis or an ovary, and has already copied its DNA. Move Stage from the start to the end, and notice how many times the cell divides.

Step 2 of 5

The first division

In prophase I something happens that does not happen in mitosis: each chromosome finds its homologous partner, and the two lie together along their whole length. At that moment they swap stretches, in crossing over: in the Scene you see a blue tip on a pink chromosome, and a pink tip on a blue one.

Then the pairs line up on the equator, and in anaphase I it is not the sisters that part but the partners: one whole chromosome of each pair to each pole. The cell pinches in two.

Step 3 of 5

The second division

Each of the two cells divides again, this time without copying the DNA first. The second division is like mitosis: the chromosomes line up, and the sisters of each one part. Stage is fixed until you choose an answer.

Step 4 of 5

The sex cells

At the end of meiosis each cell is haploid: it holds $n$⁠ chromosomes, one copy of each pair. At fertilisation two such cells join and bring the number back to $2n$⁠.

	Mitosis	Meiosis
Divisions	one	two
Cells at the end	$2$⁠	$4$⁠
Chromosomes in each cell	$2n$⁠	$n$⁠
The cells at the end	identical to the mother cell	different from each other
Where	all over the body	in the testes and ovaries

Step 5 of 5

New combinations

At metaphase I each pair lines up on the equator one of two ways: pink above and blue below, or the other way round. Each pair’s way round does not depend on the others, like coins being tossed. So each sex cell gets a different mix of chromosomes from the mother and the father, and that, together with crossing over, is what makes brothers and sisters differ.