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

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 exercise. 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.

When an egg and sperm cell come together, the now fertilised egg contains 23 pairs of chromosomes. Sex chromosomes are responsible for certain genetic traits.

Sex cells and chromosomes

Human body cells each contain 23 pairs of chromosomes. Parents pass on theirgenes 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 each chromosome within the pair. However, there are different versions of a gene called alleles. These alleles may be the same (homozygous) on each pair of chromosomes, or different (heterozygous), for example, to give 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; they are a pair. The longer sex chromosome is called the X chromosome, the shorter one the Y chromosome.

• females are **
• males are XY.

When sex cells form, the pairs of sex chromosomes (** and XY) are separated. Remember that females carry **, males XY. This means:

• all normal egg cells produced by a human ovary have an X chromosome
• half 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 can have more or less of each sex, but in a large population there will be roughly equal numbers of boys and girls. If you toss a coin many times you will get roughly equal numbers of ‘heads’ and ‘tails’ for the same reason.

Ideas about science - choosing gender

Some societies prefer to have male children. It is now possible in some countries to choose the sex of a child using IVF. Some people think parents should be able to choose the sex of their future children, especially if they had a child that died, or already have 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 tier

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 tier

The Y chromosome carries a gene called the ‘sex-determining region Y’, or SRYfor 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.

Different versions of the same gene are called alleles. They can be either recessive or dominant. Genetic testing can determine whether a person is carrying the alleles that cause genetic disorders. But there are limits to the testing, and the subject raises a number of ethical issues.

Alleles

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 (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.

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.

Genetic terminology - Higher tier

When describing an organism it is important to distinguish between the genotype and phenotype.

Genotype describes the genetic make-up of an organism (the combination of alleles).

Phenotype describes the observable, physical characteristics that an organism has. This is often related to a particular gene.

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. The disease blocks tubes that take enzymes to the gut meaning food is not digested properly, leaving the person short of essential nutrients.

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 passing on the disorder.

The genetic diagram shows why.

Inheritance of cystic fibrosis

Huntington’s disorder is caused by a dominant allele, written as H. The symptoms usually develop in middle age, and include tremors, clumsiness, mood changes, memory loss and the inability to concentrate.

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.

In this example (represented on a Punnett square), one parent - the mother - carries one copy of the Huntington’s allele and has the disorder. The father does not carry the Huntington’s allele, so he does not have the disorder. There is a 1 in 2 or 50 per cent chance of the couple producing a child with the disorder.

Note that in any individual family with a carrier parent, by chance, all the children may inherit Huntington's disorder or none at all.

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 in 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.While it is also possible that none of the children will them to inherit Huntington's this is much less likely than in the first example where only one parent carries the Huntington’s allele.

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 their children will have it, too.

Ideas about science - values

Scientists are now able to test adults and foetuses for alleles that 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 consider having an abortion? These are questions about values that science cannot answer. Different people will have different views.

Genetic testing involves analysis of a person’s DNA to see if they carry allelesthat cause genetic disorders. It can be done at any stage in a person’s life. There are several types of genetic testing, including testing for the purpose of medical research.

Antenatal testing

This is used to analyse an individual’s DNA or chromosomes before they are born. At the moment, it cannot detect all inherited disorders. Prenatal testing is offered to couples who may have an increased risk of producing a baby with an inherited disorder. Prenatal testing for Down’s syndrome, which is caused by a faulty chromosome, is offered to all pregnant women.

Neonatal testing

Neonatal testing involves analysing a sample of blood taken by pricking the baby's heel

This is used just after a baby has been born. It is designed to detect genetic disorders that can be treated early. In the UK, all babies are screened for phenylketonuria, congenital hypothyroidism and cystic fibrosis. Babies born to families that are at risk of sickle cell disease are also tested.

Pre-implantation genetic diagnosis

Pre-implantation genetic diagnosis (PGD) is a procedure used on embryos before implantation. Fertility drugs are taken by the female so that several eggs are released and collected by a doctor. These eggs are then fertilised in a Petri dish by sperm, either from the father or a donor. This is known as in vitro fertilisation (IVF). Once the embryos have reached the eight-cell stage, one cell is removed from each.

The cells are tested for the allele posing a risk (for example the Huntington’s allele). This is known as PGD. Embryos that don’t contain the unwanted allele are then implanted into the uterus to hopefully create a lower risk, full-term pregnancy.

Carrier testing

This is used to identify people who carry a recessive allele, such as the allele for cystic fibrosis. Carrier testing is offered to individuals who have a family history of a genetic disorder. It is particularly useful if both parents are tested, because if both are carriers there is an even greater risk of producing a baby with a genetic disorder.

The cells are tested for the allele posing the risk (for example the Huntington’s allele). This is known as PGD. Embryos not containing the unwanted allele are then implanted into the uterus.

Predictive testing

This is used to detect genetic disorders where the symptoms develop later in life, such as Huntington’s disorder. Predictive testing can be valuable to people who have no symptoms but have a family member with a genetic disorder. The results can help to inform decisions about possible medical care.

Limits of genetic testing

Genetic tests are not available for every possible inherited disorder. And they are not completely reliable. They may produce false positive or false negative results, which can have serious consequences.

False positives

A false positive occurs when a genetic test has wrongly detected a certain allele or faulty chromosome. The affected individual or family could believe something is wrong when it is not, which may lead them to decide against starting a family or considering an abortion in order to avoid having a baby with a genetic disorder.

False negatives

A false negative happens when a genetic test has failed to detect a certain allele or faulty chromosome. The affected individual or family would be wrongly reassured, which may lead them to decide to start a family or continue with a pregnancy that they otherwise would have avoided.

You will need to use your ideas about science to:

• Distinguish questions that can be answered using a scientific approach from those that cannot. For example, science can answer the question, 'What are the chances of my child having cystic fibrosis?'. However, it cannot answer the question, 'Should I have my pregnancy terminated?'
• Clearly state the issue at the heart of any debate. For instance, in a debate about treating genetic diseases with gene therapy, some people think that altering our DNA is against nature or God. The ethical issue is whether scientists should be allowed to use gene therapy.
• Summarise the different views that different people might hold. For example, if gene therapy saves lives it can only be a good thing. Or, we should not use gene therapy because we do not know the long-term outcomes.
• Identify arguments that are based on the right decision, ie a decision that produces the best outcome for most of the people involved. So, if a certain type of gene therapy involves a risk of causing harm but 90 per cent of the people who have the therapy are cured, is it worth taking the risk?
• Identify when certain actions are very hard to justify because they are considered unnatural or wrong. For example, we could wipe out cystic fibrosis in one generation if we made sure that anyone who was a carrier was not allowed to have children. However, most people would consider this action to be immoral and wrong.