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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 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
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Sex cells and chromosomes

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.

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Determination of gender

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 why some people think we should be able to choose the sex of our children, while others do not.

The Y chromosome - higher

The Y chromosome carries a gene called the ‘sex-determining region Y’, or SRY for short. The SRY gene causes testes to develop in an XY embryo. These produce androgens: male sex hormones. Androgens cause the embryo to become a male. Without them, the embryo develops into a female.

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Individuals differ in all sorts of ways, even when they are offspring of the same parents. These differences are called variation.

Most characteristics, such as height, are determined by several genes working together. They are also influenced by environmental factors. These include:

  • climate
  • diet
  • physical accidents
  • culture
  • lifestyle

For example, an individual might inherit a tendency to tallness, but a poor diet during childhood will result in poor growth and a shorter individual.

Identical twins

Identical twins are genetically the same. They are a good example of the interaction between inheritance and the environment. For example, an identical twin who takes regular exercise will have better muscle tone than one who does not. All of the differences that you see between identical twins, for example, in personality, tastes and aptitude, are due to differences in their experiences or environment.

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The chromosomes in a pair carry the same genes in the same places. But there are different versions of the same gene.

Different versions of the same gene are called alleles, which is pronounced al-eels. For example, the gene for eye colour has an allele for blue eye colour and an allele for brown. For any gene, a person may have the same two alleles, or two different ones.

Recessive or dominant alleles

Alleles may be either recessive or dominant.

  • A recessive allele only shows if the individual has two copies of it. For example, the allele for blue eyes is recessive. You need two copies of this allele to have blue eyes.
  • A dominant allele always shows, even if the individual only has one copy of it. For example, the allele for brown eyes is dominant. You only need one copy of it to have brown eyes. Two copies will still give you brown eyes.

individual A is heterozygous and has one allele for blue eyes (recessive). B is homozygous and has two alleles for brown eyes (dominant). C is homozygous recessive and has two alleles for blue eyes (recessive)  (

Individuals A and B have brown eyes - only individual C has blue eyes

Only individual C will have blue eyes, because the allele for blue eyes is recessive.

Individual A is called a carrier because, even though they have brown eyes, they still carry the allele for blues eyes and can pass this allele on to future generations.

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Cystic fibrosis

Cystic fibrosis

Cystic fibrosis (CF) is caused by a recessive allele. In the genetic diagram below, it is written as f.

People with CF produce abnormally thick and sticky mucus in their lungs and airways. As a result, they are more likely to get respiratory infections. Daily physiotherapy helps to relieve congestion, while antibiotics can fight infection. CF also affects the gut and pancreas, so food is not digested efficiently.

Inheriting copies of the allele

You need to inherit two copies of the faulty allele to be born with CF. If you have just one copy, you are a carrier, but will not experience any symptoms. If two carriers have a child together, there is a one in four chance of it inheriting the disorder.

The genetic diagram shows why.

father and mother have one normal allele and one cystic fibrosis allele. the four possible combinations for a child of theirs are: two CF alleles, one normal and one CF, one CF and one normal, two normal alleles  (

Inheritance of cystic fibrosis

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Huntington’s disorder

Huntington’s disorder

Huntington’s disorder is caused by a dominant allele, written as H. The symptoms usually develop in middle age, and include problems with controlling your muscles and forgetfulness.

Inheriting copies of the allele

You only need to inherit one copy of the faulty allele to have Huntington’s disorder, unlike cystic fibrosis, where you need to inherit both copies. You can inherit Huntington’s disorder if one or both of your parents carry the faulty allele, because it is a dominant allele.

You can show inheritance of the disorder using genetic diagrams.

H is the Huntingdon's allele. Father has two h alleles, mother has one H and one h allele. The four combinations are: Hh, Hh, hh, hh.  (

In this example, one parent, the mother, carries one copy of the Huntington’s allele. She has the disorder. The father does not carry the Huntington’s allele, so he does not have the disorder. There is a 1:2, or 50 per cent, chance of them producing a child with the disorder. Note that in an individual family, by chance, all of the children may inherit the disorder, or none at all.

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Inheriting the Huntington's allele

Inheriting the Huntington's allele

H is the Huntingdon's allele. Both father and mother have one H and one h allele. The four combinations are: HH, Hh, Hh, hh.  (

In this example, both parents carry one copy of the Huntington’s allele. Both have the disorder. There is a 3:4, or 75 per cent, chance of them producing a child with the disorder. Note that in an individual family, by chance, all of the children might inherit the disorder. It is also possible for none of them to inherit the disorder, although this is less likely than in the first example.

H is the Huntingdon's allele. Father has two H alleles, mother has one H and one h allele. The four combinations are: HH, HH, Hh, Hh.  (

In this instance, one parent, the mother, carries one copy of the Huntington’s allele. The father carries two copies. Both have the disorder. All of their children will have it, too.

Ideas about science - values

Scientists are now able to test adults and foetuses for alleles which can cause genetic diseases. However, the scientific information produced raises many issues that science cannot address. For example, should a couple with a one in four risk of having a child with cystic fibrosis take the gamble, or decide not to have any children at all? If a woman becomes pregnant with a child that is going to have cystic fibrosis, should she have the child, or choose to have an abortion? These are questions about values that science cannot answer. Different people will have different views.

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