Most cells in your body have a Nucleus and its the nucleus that contains your genetic material. The human cell nucleus contains 23 pairs of chromosomes. They're all well known and numbered. Chromosomes carry genes. Different genes control the development of different charaxteristicas e.g hair colour.
A gene is a short lenght of the chromosome which is one moleclue of DNA. The different versions versions of the same gene are called alleles.
Sperm and Egg cells have half the normal amounut of DNA. The sex cells contrain 23 single chromosomes, instead of 23 pairs. The fertalised egg then has now got 23 pairs of chromosomes just like the normal human body.
Girl or Boy
Your chromosomes control whether you're male or female. There are 23 pairs of chromosomes in every human body cell. the 23rd pair are lebelled XY. All men have X and a Y chromosome: XY The Y chromosome causes male characteristics. All Women have two X chromosomes: XX the lack of Y chromosome causes femal characteristics.
One Gene Determines which sex organs you develop. The gene that makes an embroy a male causes a protein called testis determing factor (TDF) to be produced.
When the embryo's reproductive system begin to develop, TDF causes the development of testes.
The testes produce a male sex hormones which makes you male. In females there is no TDF and so the embryo develops ovaries and the rest of a female reproductive system.