Glossary

N.B. Some definitions contain words in italics ( like this ). You will find these words defined elsewhere in the glossary .

A

Allele: one of two or more alternative forms of a gene that exist at a specific gene location on a chromosome and which give rise to alternative inherited characteristics. For example, many different alleles are known at the location of the gene associated with cystic fibrosis; some pairs of alleles give rise to the disease, while other pairs do not.

Amino acids: simple organic units in living things which link together to make proteins. There are 20 amino acids which are commonly found in the proteins of living organisms.

Amniocentesis: the removal during pregnancy of a small quantity of the amniotic fluid surrounding the fetus in the womb. This fluid contains cells from the fetus, and analysis of these cells can give doctors specific information about how the fetus is developing (for example, whether it has Down's syndrome).

Autosome: any chromosome which is not a sex chromosome. There are 22 pairs of autosomes in the nucleus of each human cell and two sex chromosomes, making 23 pairs (or 46 chromosomes) in total.

Autosomal: affecting the autosomes (i.e. all chromosomes except the sex chromosomes).

C

Carrier: a person who has one faulty and one working copy of the gene for a recessive genetic disorder or for a characteristic. Carriers of most recessive disorders are themselves unaffected because the working copy of the gene overrides the faulty copy, but they can pass a copy of the faulty gene on to their children.

Chromosomal condition/ disorder: when a whole chromosome (rather than a single gene) is missing or perhaps parts of whole chromosomes are re-arranged, giving rise to specific conditions or disorders (eg Down’s Syndrome).

Chromosome: A thread-like structure made of DNA which is found in the nucleus of animal and plant cells. Most human cells contain 46 chromosomes (23 pairs), but eggs and sperm (the sex cells) contain only 23 unpaired chromosomes. There are thought to be approximately 100,000 pairs of genes spread out along the 46 chromosomes which make up the human genome.

D

Dermatologist: a person specialising in the diagnosis and treatment of skin diseases.

DNA: deoxyribonucleic acid. Chromosomes are made up of long strands of DNA and genes are segments of this DNA. DNA molecules are made up of two sugar-phosphate strands held together by pairs of organic bases and twisted into the shape of a double helix. The order of the four bases (or nucleotides) - adenine (A), cytosine (C), guanine (G) and thymine (T) - determines the order in which amino acids are joined together to make the many tens of thousands of proteins your body needs to develop and work properly.

Dominant: If a faulty gene is dominant, it will show an effect even though there is a working copy of the gene on the other chromosome. Huntington's disease is an example of a dominantly-inherited genetic disorder. A person only needs to inherit a faulty gene for Huntington's from one parent to develop the disease.

E

Embryo: a developing organism. In humans, the word embryo is used until about eight weeks after fertilisation. [See foetus.]

Enzyme: a protein which helps chemical reactions to take place in cells.

Eugenics: an attempt to influence human evolution through programmes aimed at encouraging the passing on of 'desirable' characteristics and preventing the transmission of 'undesirable' ones.

F

Fertilisation: When a sperm cell penetrates the outer layer of an egg cell and joins with it to form a new life. Fertilisation can take place insides the body (as a result of the sperm meeting the egg after sexual intercourse has taken place) or sometimes in a laboratory (where it is known as in vitro fertilization, or IVF. Some people refer to babies conceived in this way as ‘test-tube babies’.)

Foetus: in humans, the developing embryo in the womb from about eight weeks after fertilisation until birth.

G

Gamete: a sex cell (i.e. sperm or egg cell) containing 23 unpaired chromosomes.

Gene: a segment of DNA which carries coded instructions for amino acids, the building blocks from which proteins are made. Passed on form one generation to the next in the chromosomes, genes are responsible for determining our inherited characteristics.

Gene Therapy: an experimental technique in which doctors introduce working copies of a missing or faulty gene into living cells in the hope that the new genes will help the affected cells to produce the protein they need to function properly.

Genetic Code: segments of DNA (or genes) contain coded instructions for the production of amino acids and therefore of proteins. [See DNA].

Genetic Condition/Disorder: a condition resulting from the genetic make-up of that individual. The word ‘disorder’ implies unwanted consequences (for example disability or disease) arising from the genes which one has inherited.

Genome: all the genetic material of an organism. Scientists are have mapped the human genome (i.e. to found the relative positions of all the genes on each chromosome).

Germ Cell: a sex cell (i.e. sperm or egg cell).

Germ-line gene therapy: this is currently against the law. Germ-line gene therapy involves replacing or repairing faulty genes in germ cells and would therefore have an effect on the next generation.

H

Helix: see DNA

Human Genome: see Genome

I

Infertile: a person who is infertile cannot have children. This could be because he/she does not produce enough healthy sperm /egg cells to achieve fertilisation, or perhaps the tubes down which the sperm/egg travel are blocked or damaged.

In vitro fertilisation (IVF): see Fertilisation.

M

Metabolic disorder: when a person is born with a vital enzyme missing or not working properly and a metabolic blockage occurs. The blockage usually means that chemicals are unable to get through to where they are needed and so build up abnormally on one side of the blockage, which can have very serious consequences for health.

Miscarriage: a pregnancy which ends before the fetus is 24 weeks old. About 1 in 6 pregnancies ends in early miscarriage. (i.e., before the 12 th week of pregnancy). If a woman has a miscarriage, her baby usually dies, although sometimes it can be saved by a special care unit in hospital.

Mutation: a change in the structure of DNA. Such changes can be passed on to the next generation. A few mutations are beneficial, but others can cause genetic disorders.

N

‘Nature v. nurture’: the debate about the degree to which human psychology and behaviour are influenced by genes and/or environmental factors is sometimes referred to as the ‘nature versus nurture’ debate. It is likely that both nature (ie, our genetic inheritance) and nurture (i.e. the way we are brought up, the food we eat, our expectations etc.) influence our behaviour, but calculating the precise influence of each is a hugely complex task.

Neurological: a neurological genetic condition is an inherited disorder affecting the nervous system.

Nucleus: the part of the cell which contains the chromosomes.

Nurture: see ‘nature v nurture’

O

Orphan diseases: rare diseases which could be treated but for which no treatment is likely to be developed. This is because drug companies are unwilling to invest huge sums of money in developing new treatments for a limited market. Of the 4500 or so known genetic disorders, those which affect relatively small numbers of people could be classed as orphan diseases.

Orthopaedic clinic: a place where doctors study, and try to cure, problems arising from disease and injury to bones, particularly in childhood.

Ovum: another word for the female sex cell (egg).

P

Pattern of inheritance: see Dominant, Recessive and X-linked.

Pre-implantation diagnosis: a technique being developed to help families affected by specific genetic disorders who wish to avoid passing on a faulty copy of the gene which causes the disorder to their children. Eggs are taken from the woman and fertilised in vitro. At a very early stage in the development of each fertilised egg, a single cell is removed and its genetic material (DNA) analysed. Only embryos which do not contain copies of the faulty gene are re-implanted in the woman’s womb where it is hoped that one embryo will go on to develop into a baby. Pre-implantation diagnosis (PID) is currently available in only a few countries around the world and for a limited number of genetic conditions.

Prenatal tests: tests carried out when a woman is pregnant to check if the fetus is developing normally.

Proteins: molecules made up of many amino acids. Proteins are an important part of all living cells and play vital roles in the chemical reactions which enable organisms to function.

R

Recessive: If a faulty gene is recessive, it will usually show little or no effect unless the same recessive gene is faulty in both of a pair of chromosomes. Sickle cell anaemia is an example of a recessively-inherited disorder. A person who inherits two faulty copies of the gene (one from each parent) will have sickle-cell anaemia. A person who only inherits one faulty copy of the gene will be a carrier and could pass the faulty gene onto any children he/she may have.

Replicate: to make an exact copy of something. Chromosomes (which are made up of very long strands of DNA) replicate before cell division takes place.

S

Screening: genetic screening is when doctors test everyone within a population or subset of that population (ie, all women of childbearing age, or all male newborn babies) to see if they have a particular gene or predisposition towards a specific genetic disorder. The people being screened may have no family history of the disorder or reason to suspect that they might have this gene or predisposition. [See Testing].

Sex cell: another way of describing the sperm (male sex cell) or egg (female sex cell). [See also Germ cell.]

Sex chromosomes: these are responsible for determining the sex of an individual. In humans they are know as X and Y chromosomes; females have two x chromosomes in most body cells, while males have an X and a Y chromosome.

Sex determination: the process whereby the inheritance of the sex chromosomes decides the sex of an embryo.

Sex-linked: see X-linked.

T

Termination: another word for abortion (i.e. the ending of a pregnancy by medical means).

Testing: genetic testing is when doctors test an individual with a known family history of a particular genetic disorder to see whether that person has, or does not have, a copy of the faulty gene associated with that disorder. [see screening]

Translocation: when parts of chromosomes transfer themselves to other chromosomes which are not of the same pair. This can lead to problems and abnormalities in the developing fetus.

U

Unaffected carrier: see carrier

X

X chromosome: the female sex chromosome.

X-linked: refers to the inheritance of a particular characteristic or disorder from a gene carried on the X (or female sex) chromosome. Also known as sex-linked inheritance.

XYY male: a male with an extra Y chromosome in most of his body cells.

Y

Y chromosome: the male sex chromosome.

Z

Zygote: a fertilised egg.

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