Genes carry the instructions that control how we develop and function. They do this by telling the cells to make proteins needed for the body to work.
Each gene if a section of a very long molecule of a chemical called DNA (deoxyribonucleic acid).
Lengths of DNA are coiled and packed into structures called chromosomes. These are found in the nuclei of the bodies cells.
Strands of DNA are made up of four chemicals called bases, as well as phosphate groups and sugar molecules.
The order of the bases in a DNA strand determines the order of amino acids in a protein.
Proteins fall under two groups:
- Functional proteins enable the body to function. e.g. enzymes, antibodies and hormones.
- Structural proteins give the body structure, rigidity and strength. e.g. Collagen in ligaments and keratin in skin.
The human genome project has identified the location of all the genes on human chromosomes.
Genome: The complete gene set of an organism.
The project will help us to understand how genes control our characteristics and development, and can lead to certain diseases.
The project has ethical implications , e.g. some drug companies want to own genes. They could then charge other scientists money to investigate the genes, which could restrict research.
Our characteristics are controlled by our genes and our environment or a combination.
We inherit our genes from each of our parents, so we are similar but not identical to each parent.
Differences in genes produce variation in offspring.
Some characteristics are controlled by several genes working together. These charateristics will show continuous variation throughout the population e.g. the continuous range of eye colours and different heights.
Phenotype and Genotype
For a particular characteristic, you can describe a person by their genotype or phenotype.
Genotype: A persons genetic make up. Usually written as two letter e.g. DD.
Phenotype: Describes a persons observable features. It depend on a persons genes but also how the interact with the environment.
Identical twins have identical genotypes because they develop after a fertilised egg splits in two.
Studies of identical twins, especially those that have been seperated can help us to study what effects environment has on a person phenotype.
Chromosomes are organised into pairs. Human cells have 23 pairs of chromosomes (a total of 46 chromosomes).
Sex cells are eggs (ova) and sperm. These have 23 chromosomes one from each pair.
At fertilisation an egg and a sperm join together to form a zygote. The zygote has a full set of 46 chromosomes, 23 from the father and 23 from the mother.
Pairs of chromosomes have genes for the same characteristic at identical positions on each chromosome of the pair.
Changes to DNA sometimes occur called a mutation. This can take place when sex cells are being made or after fertilisation.
One type of mutation is a chromosome mutation. This results in individuals having extra chromosomes. e.g. a person with an extra chromosome 21 will have downs syndrome.
Variation in offspring
The combination of chromosomes in an egg or sperm will allways be different. e.g. in an egg chromosome 1 could have been inherited from the mother wereas chromosomes 2 and 3 were inherited by the father, etc, so the combination of chromosomes (and genes) will be unique to that person- unless he or she is an identical twin.
Environmental effects will add to the variation.
Pairs of alleles
Alleles: the different form in which the genes controlling a characteristic can occur.
If two of the of the alleles of a gene are identical, the person is said to be homozygous (for that characteristic).
If the two alleles are different the person is said to be heterozygous (for that characteristic).
Traits are passed on from parents to their offspring through genes on the chromosomes.
Genes for a particular trait are found at the same place on each chromosome of the chromosome pair.
The different forms of a gene that control a certain trait are called alleles.
In most cases alleles for a trait can be dominent or recessive.
Dominent alleles are written with upper case letters in genetic diagrams.
Recessive alleles are written with lower case letters in genetic diagrams.
If at least one dominent allele is present the trait shown will be the dominant one.
For the recessive trait to be shown there must be two of the recessive alleles present.
You can use a punnet square to:
- Show genetic crosses
- Finding the probability of two types of parents producing different types of offspring.
You can show the inheritance of a trait in a family over several generations using a family tree diagram.
A family tree diagram is very useful when tracing a genetic disorder such as huntingtons disease over generations.
The 23rd pair of chromosomes determines our sex.
A female has two X chromosomes, written XX.
A male has an X and a Y chromosome, written XY.
Egg and sperms have 23 chromosomes:
- Each egg cell produced by a female will have an X chromosome.
- Half of the Sperm cells produced by a male will have an X chromosome, half will have a Y chromosome.
- So in theory 50% of females should be male and 50% female.
It's the presence of a Y chromosome- the sex determining gene- that determines the sex of the embryo. In the absence of the Y chromosome ovaries develop.
Because of the shape of sex chromosomes there are parts of the X chromosome that can have no matching alleles on the Y chromosome. If a defective gene is found on this part of the X chromosome, this can result in a sex linked disorder.
Sex linked diseases such as haemophilia and red green colour blindness, are far more likely to be present in males.
Some disorders are caused by defective or faulty alleles.
Huntingtons is a dominent disorder. The presence of a single dominent allele will cause the disorder. It occurs in middle age and causes tremors, memory loss, inability to concentrate and mood changes.
Both alleles on a chromosome must be recessive for a person to get a recessive disorder, such as cystic fibrosis. Symptoms include the production of thick gluey mucus that affects the lungs and makes digesting food difficult, breathing problems and chest infections.
For recessive single gene disorders, a person with a normal and a deffective gene will be normal, but they will be a carrier.
If both parents are carriers they can give birth to a child with the disease.
Genetic testing is screening is used to check for a particular disorder, even when there is no history of it in the family. It is hoped this will minimise the damage such disorders can cause. e.g. the heel ***** 'blood spot test' is used on most newborn babies to diagnose rare genetic disorders.
Genetic testing of individuals is used when a genetic disease such as cystic fibrosis runs in the family. This may allow people to get treatment for the disease or plan for the future.
Genetic testing- ethical questions e.g. If a person has huntingtons disease should they tell their employer, insurance company and family?
Genetic testing during pregnancy may involve cell sampling by:
- amniocentesis (collecting cells from the developing fetus which are present in amniotic fluid).
- Chorionic villus sampling (testing a sample of cells taken from the placenta)
Both tests carry a risk- 1% of babies are micarried as a result of genetic testing.
Production of embryos by IVF allows doctors to check the genetic make up of the embryos prior to implantation. This is embryo screening.
Embryo screening is used to investigate families with a known history of a disorder e.g. cystic fibrosis. It allows doctors to remove any embryos suffering from a disorderand implant normal embryos.
Screening embryos prior to implantation and only using healthy embryos is called pre-implantation genetic diagnosis (PGD).
PGD and embryonic research are carefully monitered in the UK.
Parents likely to pass on a genetic abnormality:
- May decide to not have a family.
- May decide whether to carry on with the pregnancy or terminate it.
Types of genetic tests may sometimes give wrong results.
Clones are individuals with identical genes.
Asexual reproduction: one parent is involved, offspring has identical DNA to parent. Bacteria, some plants and simple animals use this.
Plants can reproduce asexually by:
- Using runners- shoots sent out that grow into identical plants.
- Producing bulbs.
Identical twins are human clones produced when a fertilised egg splits.
Any difference between clones and their parents are caused by environment not genetics.
Advantages of clones:
- Sucessful characteristics seen in offspring.
- Asexual reproduction is useful where plants and animals live in isolation.
Disadvantage: no genetic variation, if conditions change or there is a disease the entire population could be wiped out.
Clones have been produced by artificial animal cloning e.g. Dolly the sheep and Snubby the dog.
- Nucleus from body cell is extracted and inserted into egg cell that has had its nucleus removed. This gives the egg cell a full set of genes without being fertilised.
- The embryo is implanted into a suitable surrogate mother.
It is illegal to create human clones in many countries, including the UK.
A human embryo develops from a single cell. This cell divides over and over again as a baby develops. Most of these cells become specialised, so do different jobs (differentaition).
- After 5 days the embryo is a ball of cells containing embryonic stem cells.
- These cells are unspecialised, they divide and develop into different types of cells in the human body.
As adults some stem cells, adult stem cells remain in the human body.
Adult stem cells can repair or replace certain cell types. e.g. bone marrow cells can develop into different types of stem cells.
Adult stem cells are used to treat certain diseases but are of little use.
Embryonic stem cells can develop into other cell types so have huge potential, but their use is controversial:
- They are usually taken from unused embryos following fertility treatment.
- Their use involves the destruction of the embryo.
Recent research is working on programming adult body cells into stem cells and collecting cells from the umbilical cord blood when a baby is born.
Stem cells could be used in:
- The testing of new drugs.
- Understanding how stem cells become specialised in the early stages of human development by the switching on and off of particular genes.
- Renewing damaged or destroyed cells in spinal injuries, heart disease, Alzheimers disease and Parkinsons disease.