Glossary of terms
Genetics is a fascinating but often complicated subject. We hope our A-Z of terms will help!
Alleles determine the colour of your eyes
Photo: DISCOVER magazine
Allele –A person inherits two copies of any one gene: one from each parent. The DNA sequence of the different copies may be slightly different. Each one is called an allele. Even simple characteristics, like eye colour, are determined by at least one pair of alleles.
Autosome – Any chromosome, apart from the sex chromosomes (X and Y). We have 22 pairs of autosomes.
Autosomal dominant inheritance - A type of inheritance where just one faulty copy of a gene is all it takes to cause a genetic disorder. Somebody with a condition with this type of inheritance has a 50% chance of passing it to each of his/her children.
Autosomal recessive inheritance – A type of inheritance where both copies of a gene must be faulty for a genetic disorder to occur. One copy on its own isn't 'strong' enough. For a recessive condition to happen, someone has to inherit two copies, one from each parent. People who have only one copy of an unusual allele (these people are often called ‘carriers’) are usually healthy and have no symptoms of the condition.
Carrier – Someone who has an allele that could cause a specific genetic condition or disease. In autosomal recessive conditions, carriers have one normal gene and one altered gene and often wouldn't show any symptoms of the condition. We are all carriers of around 6-8 recessive genes.
Cell – The basic structural and functional unit of life. As humans, we are made of approximately 50 trillion cells!
A complete set of male chromosomesChromosome – A long threadlike strand of DNA that carries a set of genes (see diagram on right).
Congenital – A condition or characteristic that is present from birth.
De novo mutation – also called a ‘sporadic mutation’ – a “new” alteration to a gene that is seen for the first time in a family. These new mutations are not inherited from a parent, but someone who has it can then pass it down to their children and so on.
Conditions caused by sporadic mutations are due to an alteration to a gene that occurs out of the blue in either the egg or sperm near the time of conception, or just afterwards.
Diploid – A cell in which all the chromosomes occur in pairs. All cells in the human body are diploid, and carry 23 pairs of chromosomes for a total number of 46, with the exception of the mature sperm and egg cells, which are haploid.
Deoxyribonucleic acid (DNA) – Found within the nucleus of a cell, the primary carrier of genetic information.
Dominant allele – This is an allele which will completely hide the effects of a recessive allele.
Enzyme – Enzymes speed up biological reactions.
Expressivity – The amount someone is affected by one of their genes. For example. two children might have the same faulty gene but how much they're affected could be completely different.
Gamete – A sperm or an egg.
Gene – The fundamental unit of heredity which carries the instructions for how the body grows and develops.
Gene therapy – Treatment of a genetic condition by putting a new gene into the affected cell. The extra gene makes up for the gene that is not working properly. The gene that's inserted into the cell makes some form of protein and this allows the cell to work properly again. Read how gene therapy is helping 4 year old Teigan to fight infection.
Genotype – The genetic information carried by someone’s genes. We can't see the genotype but it determines a particular characteristic, like the colour of our eyes.
Haploid – A cell that has a single set of unpaired chromosomes. Mature sperm and egg cells are haploid, each carrying only 23 single chromosomes.
Heterozygous –Possessing two different alleles of a particular gene.
Homozygous – Possessing two identical alleles of a particular gene.
Chris Madden www.chrismadden.co.uk
Human Genome Project – A major research project started in 1990 and finished in 2003, to map each human gene and its location on the chromosome. Knowing where genes are located is the first step in being able to develop treatments and, eventually, cures for genetic disorders.
Inheritance – The passing down of genetic information from parents to children.
Karyotype – How many chromosomes somebody has and any abnormalities them may have. A karyotype is also a diagram of somebody’s chromosomes, like the one above.
Locus – The precise location on a chromosome where a gene is found.
Mutation – A permanent alteration to a gene where part of the DNA within the gene is different from what it should be. There may be an extra or missing part for example. Mutations may affect the proper growth or development of a person. They can have either positive or negative effects on an individual.
Nucleus – The control centre of a cell. Chromosomes and therefore DNA are found within the nucleus.
Penetrance – The likelihood that a gene will actually result in a genetic condition.
Phenotype – Your appearance based on your genotype i.e what you look like.
Protein – One of the basic chemicals of life, proteins make up the structure and determine the function of a cell.
Recessive allele – This allele is only expressed if there are two of them present.
Sex chromosomes - The two chromosomes, X or Y, in each cell which determine the genetic gender of an individual. Males have an X and a Y chromosome and females have two X chromosomes.
Stem cells – Cells in the body that are capable of renewing themselves and becoming any number of different cell types. Stem cells are made in the bone marrow.
Bone marrow, and the stem cells within it, can be transplanted from a healthy donor into a patient who can't make their own white blood cells to fight infection, for example.
The stem cells can then become white blood cells, which work to fight off infection.
Syndrome – A group of symptoms that helps a doctor diagnose a particular condition or disease. For example, an oddly-shaped head, slanting eyes, a single crease on the palm, and delayed mental and social skills are all symptoms of Down Syndrome.
X-linked recessive inheritance – A type of inheritance where a recessive gene on the X chromosome is passed down to children. Generally men will display full symptoms of the condition whereas women will often have either have only mild symptoms or none at all.
This is because women have two X chromosomes. Usually only one of them has the recessive gene. The other, unaffected X chromosome, can generally compensate for the recessive gene in the first X, masking any symptoms.
Men display the full symptoms because they have one X and one Y chromosome - the Y chromosome can't compensate for the recessive genes that are carried on the X chromosome. The blood disorder, haemophilia is inherited this way.
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