What are X and Y chromosomes?
The 23rd pair of genes determines the sex of a baby. These are X and Y chromosomes. A woman has two X chromosomes, while a man has one X and one Y chromosome. When the chromosomes join together in fertilization, if the sperm contains an X chromosome, it will combine with the X chromosome of the female to produce XX — a girl. If the sperm contributes the Y chromosome, the baby will have one X and one Y chromosome and will be XY — a boy.
What is a mutation?
Mutations are changes in the DNA inside a cell that affect the genes and chromosomes. Mutations arise because of errors in the DNA molecule when it divides and re-forms in the new cells. When this happens in sperms or egg cells, it causes changes that can be passed on to a child. The appearance of haemophilia is an example of a mutation in a single gene.
What are dominant genes?
Although genes from both parents are mixed together at fertilization, some genes have a more powerful effect than others. These dominant genes override the effects of others which are called recessive genes. For example, if a child has a gene for brown eyes from one parent and a gene for blue eyes from the other, the child will always have brown eyes. This is because the gene for brown eyes is a dominant gene.
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