Before you read this, I suggest you read post 21.9.
In post 21.9, we saw how cells can divide to produce copies of themselves during growth or to replace old cells in a process called mitosis. During mitosis, chromosomes appeared in the cell nucleus. These chromosomes appeared in pairs, so we can say that each cell contains 2n chromosomes where n is an integer (whole number). In humans n = 23, so a human cell has 46 chromosomes.
Each chromosome in a pair is almost identical, except in men who have one pair of non-identical XY chromosomes. So, we could consider that men have 2(n – 1) + XY chromosomes. Then women have 2(n – 1) + XX chromosomes.
A few people are exceptions to what I have written in the previous two paragraphs. But I am going to concentrate on the majority of people so that we can understand meiosis and its consequences.
Meiosis is a form of cell division that produces specialist cells called gametes that have n chromosomes. The male gamete is called the sperm and can have (n – 1) + X or (n – 1) + Y chromosomes. The female gamete is called the ovum and has (n – 1) + X.
When a sperm fertilises an ovum, a new cell is produced. This new cell could have
(n – 1) + X + (n – 1) + X = 2(n – 1) + XX
chromosomes, so it is a female cell. Or it could have
(n – 1) + Y + (n – 1) + X = 2(n – 1) + XY
chromosomes, so that it is a female cell.
These new cells then divide, by the process of meiosis, to produce a foetus (sometimes spelt fetus) and finally an embryo that is more recognisably human, before the birth of a girl (starting from a 2(n – 1) + XX cell) or a boy (starting from a 2(n – 1) + XY cell).
According to this simple model the probability of a baby being a boy is equal to its probability of being a girl, 50%. We would then expect equal numbers of boys and girls to be born. But other factors could influence the result; there could be a difference in the ability of X and Y sperm to fertilise an ovum or the embryo of one sex could be more viable than the other. But we would still expect about 50% of babies to be boys and about 50% to be girls – which is what we observe.
There are now two strong reasons to suspect that chromosomes contain genes: (1) they determine whether a baby will be a boy or a girl and (2) the chromosomes in a baby are a mixture of those from the father and the mother.
The picture above shows the stages involved in meiosis. Stage 1 starts with a cell about to divide so that the chromosomes appear in its nucleus. I have drawn two chromosomes – one red, the other blue. In stage 1, these chromosomes are converted into two chromatids, just as in mitosis. In stage 2, the two chromatids exchange some of their material – in a process called crossing-over. At the same time, the cell and its nucleus divide into two (not shown). Finally, in stage 3, these cells and the nuclei they contain, divide to produce four gametes, each with one chromosome. I have simplified the description of meiosis to emphasise the underlying principles and to aid comparison with mitosis as described in post 21.9. Crossing over leads to the production of more types of chromosomes; there are now red/blue and blue/red as well as the original red and blue.
Most of these ideas were developed by the American biologist Thomas H Morgan (1866-1945) who performed selective breeding experiments on the fruit-fly (Drosophila melanogaster). Why did he do experiments on fruit-flies? Because they breed quickly, enabling him to study many generations in a short time. He could explain the sex and eye colour of his flies if chromosomes contained genes and crossing-over occurred. Note that his experiments did not prove that chromosomes contain genes because experiments cannot prove that a hypothesis is true – only that it is false, as explained in post 16.3. But the best explanation of our observations (see post 16.2) is that chromosomes contain genes.