B1 - You And Your Genes

• Genes are found in our chromosomes
• Parents pass on to off spring in their sex cell during reproduction
• Different versions of the same gene are called alleles
• Alleles determines features like eye colour and the inheritance of disorders such as cystic fibrosis and huntingtons 
• 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 DNA
• 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 -used for growth and repair or functional proteins such as enzymes- seed up chemical reactions e.g. amylase

• 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.
• 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.
• 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
•  Male sex cells are called sperm

• A pair of chromosomes carries 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.
• Females have the combination of **
•  Males have the combination of XY
• When sex cells form, the pairs of sex chromosomes (** and XY) are separated. Remember that females carry **, males XY. This means that all normal egg cells produced by a human ovary have an X chromosome and 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.
• 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.
• 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.
• 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 is caused by a recessive allele.
• People with CF produce thick and sticky mucus in their lungs and airways
• 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.
• 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.
• Huntington’s disorder is caused by a dominant allele.
• The symptoms usually develop in middle age, and include tremors, clumsiness, mood changes, memory loss and the inability to concentrate.
• You only need to inherit one copy of the faulty allele to have Huntington’s disorder.
• You can inherit Huntington’s disorder if one or both of your parents carry the faulty allele, because it is a dominant allele

• 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.
• 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.
• 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.
• 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.

• There are two types of 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.

• 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:
• 1. 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.

• The cloning of human embryos is an issue which raises many questions and arguments.      You need to be able to use your ideas about science to:
• Clearly state what the issues are. For example, some human embryos will be destroyed during the cloning process
• Summarise different views that might be held. For instance, some people think that destroying human embryos this way is murder. Others believe it is furthering our knowledge of science for the benefit of everyone
• Identify and develop arguments based on the idea that the right decision is the one that has the best outcome for the majority of the people involved. For example, even though a few embryos will be destroyed, many people will be free from suffering and cured of diseases that are currently incurable
• Identify and develop arguments based on the idea that certain actions are very hard to justify because they are considered unnatural and wrong. For example, some people think that no matter what the benefits are of curing a disease, we should not clone embryos because destroying human life is always wrong.

Embryo transplants

• A developing embryo is removed from a pregnant animal at an early stage, before its cells have had time to become specialised.
• The cells are separated, grown for a while in a laboratory then transplanted into host mothers.
• When the offspring are born, they are identical to each other and to the original pregnant animal.
• They are not identical to their host mothers because they contain different genetic information.
• Fusion cell cloning- Fusion cell cloning involves replacing the nucleus of an unfertilised egg with one from a different cell.
• The replacement can come from an embryo.
• If it is from an adult cell, it is called adult cell cloning.

• 'Dolly the sheep' was the first mammal to be cloned using adult cell cloning. She was born in the UK in 1996 and died in 2003. Here is how she was produced:
• 1.An egg cell was removed from the ovary of an adult female sheep, and its nucleus removed
• 2.The nucleus from an udder cell of a donor sheep was inserted into the empty egg cell
• 3.The fused cell then began to develop normally, using genetic information from the donated DNA
• 4.Before the dividing cells became specialised, the embryo was implanted into the uterus of a foster mother sheep. The result was Dolly, who was genetically identical to the donor sheep.