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
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.
- 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.
The longer sex chromosome is called the X chromosome, the shorter one the Y chromosome.
- females are XX
- males are XY
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 chance for getting either a boy or a girl will always be 50/50
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:
- physical accidents
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 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.
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.
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.
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.
Inheriting the Huntington's allele
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.
Carry on ...
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.