Genes And Inheritance
The genetic information that makes your baby the person she is comes from you and your partner.The sperm and egg each contain 23 chromosomes. When the sperm and egg fuse, the fertilized egg then contains 23 pairs of chromosomes. Each chromosome contains thousands of genes that determine everything from eye and hair colour to intelligence, personality and physical health.
What is a chromosome?
Chromosomes are elongated structures made up of thousands of genes, threaded together like beads. Every cell in your body contains 46 chromosomes, except for an egg cell or a sperm cell, which contain 23 chromosomes each. As a result, when the sperm and egg cells come together, they form a new cell that contains 46 chromosomes, in 23 pairs. This one cell then divides over and over again as it develops into a baby, and every cell in your baby's body contains the same genetic information as the very first cell.
Because your baby inherits half of her genes from you and half from your partner, she will have a unique combination of genetic material. As a result, your baby could inherit your eye colour but your partner's nose shape, your short legs but your partner's mathematical ability. Although the environment in which your child grows up, and the experiences she has, will contribute to the person she becomes, her genetic make-up is determined at the moment of conception.
Dominant and recessive genes
In most cases, the two sets of genes, one from each parent, blend together so that your baby becomes a unique combination of genetic characteristics. However, some genes are'stronger'and take priority over others: these are called dominant genes. Weaker genes are called recessive genes. For example, the gene for brown eyes is dominant while that for green eyes is recessive. As a result, if you have brown eyes and your partner has green eyes, your baby's eyes are more likely to be brown because your genes will dominate.
To complicate matters, both you and your partner each carry genes from each of your parents, who carried genes from their parents, and so on. These genes are passed on from generation to generation, but may remain recessive for a long time until they suddenly pop up years later. For example, you may have blonde hair and your partner black hair, but you could still have a baby with red hair. How does this happen? At some time in the past, someone in each of your families must have had red hair. The gene for red hair is recessive so, although it is been passed from generation to generation, a more dominant gene, such as black or brown hair, has always dominated and no one has had red hair. Now, you and your partner both pass on a gene for red hair. The gene is recessive but when two recessive genes come together your baby ends up with red hair.
Genetically inherited conditions
Just like the genes that determine appearance, genes for inherited conditions can be dominant or recessive. Diseases such as cystic fibrosis, thalassaemia and sickle-cell anaemia are passed on through a recessive gene. This means that many people can carry the gene for the disease without actually suffering from the disease, as a dominant normal gene takes precedence over the disease gene. If two people who each carry a gene for a condition have a child together, that child has a 25 per cent chance of inheriting two disease genes, in which case they will suffer from the condition.
If you, your partner or any member of your family has an inherited condition, you should discuss this with your doctor before you try to conceive. You may be referred for genetic counselling or, if you are already pregnant, you may be offered tests to determine whether your baby is affected by the condition.