Читать книгу Need to Know Fertility, Conception and Pregnancy - Harriet Sharkey - Страница 25

All cells in the body contain DNA (deoxyribonucleic acid), the substance that makes up your genetic code. For this reason, DNA is sometimes referred to as the ‘building blocks’ of life. This is what makes you what you are, the blueprint for the structure and function of your body. Everyone except identical twins has a different genetic code. Your genetic code is made up of thousands of genes that are carried on ‘chromosomes’. Each of us has 23 pairs of chromosomes in every cell in our body – a total of 46 chromosomes per cell. Of the 23 pairs of chromosomes in every cell, 22 pairs are ‘general’ chromosomes and one is a pair of ‘sex’ chromosomes.

Sperm and eggs are an exception to this general rule. An egg from a female has only 23 chromosomes; 22 of these are ‘general’ chromosomes and one is a ‘sex’ chromosome. The sperm from the male also has only 23 chromosomes: 22 general chromosomes and one sex chromosome. When conception occurs and the sperm fertilizes the egg, the egg and sperm cells fuse. A fertilized egg therefore has the full complement of 46 chromosomes and the baby that grows from the egg has 46 chromosomes in every cell.

The sex of the baby conceived is determined by the ‘sex’ chromosomes – referred to as ‘X’ and ‘Y’ chromosomes – and the way these combine determines whether we are male or female. All girls have two X chromosomes and no Y chromosome, while a boy has one X and one Y chromosome. It is the sex chromosome that comes from the father’s sperm that determines the sex of your baby. The mother has no power to influence the sex of the baby because she can contribute only an X chromosome, whereas the father can contribute either an X or a Y chromosome.