Meiosis and variation

Mitosis and meiosis compared

In Year 8 (2J) you met the two kinds of cell division. Here is how they compare, now that you know what a chromosome is made of.

Both begin with DNA replication, so each chromosome is two sister chromatids. In meiosis I the homologous pairs separate, one chromosome of each pair going to each new cell. In meiosis II the sister chromatids separate, as in mitosis. The halving happens in meiosis I.

Three sources of variation

Sexual reproduction mixes genetic material in three ways.

• Independent assortment. In meiosis I each homologous pair lines up independently of the others, so which of your mother's or father's chromosomes ends up in a gamete is random for each pair. With 23 pairs that gives 2²³, about 8.4 million, possible combinations in each gamete. You explored this in Year 8.
• Crossing over. Early in meiosis I, homologous chromosomes pair up tightly. Non-sister chromatids can break at matching points and swap sections. The crossing point is called a chiasma (plural chiasmata). Each human chromosome pair usually forms one to three chiasmata. Crossing over makes recombinant chromosomes: new combinations of alleles that did not exist in either parent.
• Random fertilisation. Any one of millions of sperm can fertilise the egg, so 8.4 million × 8.4 million gives about 70 trillion combinations for one couple, before crossing over is counted.

Linked genes and gene maps

Genes on different chromosomes are inherited independently. Genes on the same chromosome tend to be inherited together: they are linked. Only crossing over can separate them, and it is more likely to separate two genes the further apart they are, because there is more room for a chiasma to form between them.

In 1910 Thomas Hunt Morgan's team at Columbia University noticed that some fruit fly traits were usually inherited together. In 1913 his undergraduate student Alfred Sturtevant realised the percentage of offspring with recombined traits could be used as a measure of distance. He made the first genetic map, placing six genes in order along the fruit fly X chromosome. One map unit (a centimorgan) is the distance that gives 1% recombinant gametes.

When meiosis goes wrong

Sometimes a homologous pair fails to separate in meiosis I, or sister chromatids fail to separate in meiosis II. This is called nondisjunction. One gamete ends up with an extra chromosome (n + 1) and another with one missing (n − 1). If an n + 1 egg is fertilised by a normal sperm, the zygote has three copies of that chromosome, a trisomy, with 47 chromosomes. An n − 1 gamete gives a monosomy with 45.

Most embryos with the wrong number of chromosomes do not survive: chromosome errors are the most common cause of early miscarriage. The ones that can survive involve the smallest chromosomes (13, 18 and 21) or the sex chromosomes (1B). Nondisjunction in eggs becomes more common as women get older. This is partly because every egg a woman will ever produce starts meiosis before she is born, then pauses for decades, and the proteins holding the chromosome pairs together slowly weaken.