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.
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.
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.
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.
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 Fybrosis part 1
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.
Cystic Fibrosis part 2
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 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.
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.
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.
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.
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 (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.
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.
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 gentic 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.
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.
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.
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.
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.
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
- nucleus from a patient's cell put into the egg cell
- egg cell stimulated to develop into an embryo
- stem cells taken from the embryo
- stem cells grown in a container of warm nutrients
- stem cells treated to develop into required cell types.
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:
- An egg cell was removed from the ovary of an adult female sheep, and its nucleus removed
- The nucleus from an udder cell of a donor sheep was inserted into the empty egg cell
- The fused cell then began to develop normally, using genetic information from the donated DNA
- 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.