Genes are found in our chromosomes and parents pass these on to offspring in their sex cells. Different versions of the same gene are called alleles, and these can determine features such as eye colour, and the inheritance of disorders such as cystic fibrosis. DNA
You will remember from your Key Stage 3 studies that the nucleus controls the activities of a cell. The instructions for how an organism develops are found in the nuclei of its cells Chromosomes
Chromosomes are structures found in the nucleus of most cells. They consist of long strands of a substance called deoxyribonucleic acid, or DNA for short. A section of DNA that has the genetic code for making a particular protein is called a gene. The proteins can either be: * structural proteins such as the ones found in muscles and hair * enzymes, such as proteases and other digestive enzymes
Nucleus, chromosome and gene
Sex cells and chromosomes
Human body cells each contain 23 pairs of chromosomes.
Parents pass on their genes to their offspring in their sex cells. * female sex cells are called egg cells, or ova
* male sex cells are called sperm
Process of fertilisation
A pair of chromosomes carry the same genes, in the same place, on the chromosome. However, there are different versions of a gene called alleles. These alleles may be the same on each pair of chromosomes, or different, for example, blue eyes or brown eyes. Sex cells only contain one chromosome from each pair. When an egg cell and sperm cell join together, the fertilised egg cell contains 23 pairs of chromosomes. One chromosome in each pair comes from the mother, the other from the father. Which chromosome we get from each pair is completely random. This means different children in the same family will each get a different combination. This is why children in the same family look a little like each other and a little like each parent, but are not identical to them. Sex chromosomes
A set of chromosomes can be separated from its cell, spread out on a microscope slide and magnified many thousands of times. When stained and photographed, they look like this:
Chromosomes from a female
Chromosomes from a male
The highlighted pair of chromosomes are called the sex chromosomes. The longer sex chromosome is called the X chromosome, the shorter one the Y chromosome. * females are XX
* males are XY
Determination of gender
When sex cells form, the pairs of sex chromosomes (XX and XY) are separated. Remember that females carry XX, males XY. This means: * all normal egg cells produced by a human ovary have an X chromosome * half of the sperm carry an X chromosome, and half a Y
So a human baby’s gender is determined by the sperm that fertilises the egg cell. The baby will be a girl if it carries an X chromosome. It will be a boy if the fertilising sperm carries a Y chromosome. Study the animation below to test your understanding of this.
Notice that half of the babies should be male, and half female. Individual families often have more, or less, boys than girls, but in a large population there will be roughly equal numbers. This is for the same reason that if you toss a coin many times you will get roughly equal numbers of ‘heads’ and ‘tails’. Ideas about science - choosing gender
Some societies prefer to have male children. It is now possible to choose the sex of a child using IVF. Some people think parents should be able to choose the sex of their children, especially if they have a child that dies, or have had three or four children of the same sex. Other people think we should not be able to choose, because this could affect the balance of males and females in society, or because they believe it is against God or nature. Different values - higher only
Decisions of this kind are called values. Science can provide information and data, but it cannot answer questions about values. Values often result in different people coming to different decisions. This is...