b1 revision notes

Genes are found in our chromosomes,which parents pass on to offspring in their sex cells in reproduction. 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.
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
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
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&lifestyleIdentical 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 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.
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. 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.
Treating/testing different genetic diseases.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.
.Genetic terminology
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
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.
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.
Ideas about science
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?
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.
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
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
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 time