B1 - You and Your Genes

B1 - You and Your Genes

1. Different versions of the same gene are called alleles, and these can determine features like eye colour, and the inheritance of disorders such as cystic fibrosis

2. DNA

   • 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

3. Variation

   • 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

4. 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

5. 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 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.
         • 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:
         • females are XX males are XY

6. 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 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.
   • 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
           • 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
       • 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

7. Alleles

   • 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
     • 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
           • 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

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

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

10. 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. 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.

11. Huntington’s disorder

    • 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.
      • 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.
        • 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

12. testing

    • 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 tests
      • Genetic testing involves analysis of a person’s DNA to see if they carry alleles that 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
        • 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.

13. 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.

14. ethical issues

    • 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
    • Access to our genetic profile
      • Some people believe employers should have access to our genetic profile when we apply for a job. In the future, they may be able to use this information to determine how much time we are likely to take off due to illness, and use this information to help decide whether or not to offer us a job.
        • Insurance companies may also want access to our genetic profile. They could use this information to see how long we are expected to live, and possibly refuse to give us insurance or only provide it at an increased cost.
      • Both of these are examples of values that science cannot address. Genetic testing is an example of what is technically feasible, ie what can be done. But the decisions we take on how to use this information are an example of values, and cannot be ‘answered’ by science. A common argument when discussing ethical issues like this is that the right decision is the one that leads to the best outcome for the majority of those involved.
        • Society is going to have to decide who has access to our genetic profile. For example, should we all be on a DNA database to help the police catch criminals? Or is this a step too far, because in the future it could allow employers and insurance companies to access to data that we would rather they did not see?

15. Clones

    • Clones are genetically identical individuals. Bacteria, plants and some animals can reproduce asexually to form clones that are genetically identical to their parent. Identical human twins are also clones: any differences between them are due to environmental factors.
      • Asexual reproduction only requires one parent, unlike sexual reproduction, which needs two. Since there is only one parent, there is no fusion of gametes, and no mixing of genetic information. As a result, the offspring are genetically identical to the parent, and to each other - so they are clones
    • Plants
      • Asexual reproduction in plants can take a number of forms. Many plants develop underground food-storage organs that later develop into the following year’s plants. Potato plants and daffodil plants do this
        • Some plants produce side branches with plantlets on them. The Busy Lizzie plant does this. Others, such as strawberry plants, produce runners with plantlets on them
      • Animals Asexual reproduction in animals is less common than sexual reproduction but it does happen in sea anemones and starfish, for example
    • Natural cloning. Twins are genetically identical because they are formed after one egg cell is fertilised but splits to form two embryos. They have the same genes. As the genes came from both parents they are not clones of either parent, but they are natural clones of each other.
    • Artificial cloning It is possible to make clones artificially. The cloning of animals has many important commercial implications. It allows an individual animal that has desirable features, such as a cow that produces a lot of milk, to be duplicated several times
    • If you were to receive medical treatment with cells grown from stem cells, your body’s immune system would recognise the cells as foreign, and they would be rejected and die. But this would not happen if you received cells with the same genes as your own
      • This could be done by cloning one of your cells to produce an embryo, then taking stem cells from this. This is called therapeutic cloning. Here are the steps involved
        • nucleus taken out of a human egg cell
          • 2.nucleus from a patient's cell put into the egg cell
            • 3.egg cell stimulated to develop into an embryo
              • 4.stem cells taken from the embryo
                • 5.stem cells grown in a container of warm nutrients
                  • 6.stem cells treated to develop into required cell types

16. stem cells

    • adult stem cells - these are unspecialised cells that can develop into many (but not all) types of cells
    • embryonic stem cells - these are unspecialised cells that can develop into any type of cell
    • During the development of an embryo, most of the cells become specialised (cells with modifications to structure according to the task they have to perform). They cannot later change to become a different type of cell.
      • But embryos contain a special type of cell called stem cells. These embryonic stem cells can grow into any type of cell found in the body so they are not specialised. Stem cells can be removed from human embryos that are a few days old, for example, from unused embryos left over from fertility treatment
    • Here are some of the things stem cells could be used for
      • making new brain cells to treat people with Parkinson’s disease
      • rebuilding bones and cartilage
      • repairing damaged immune systems
      • making replacement heart valves

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