A to Z of Genetics


Some of the terms doctors will use when they talk about genetics are baffling. Here is an A to Z of Genetics for the most common terms you will hear


A person inherits two copies of almost every gene: one from each parent.  The DNA sequence of the different copies of the gene may be slightly different.  Each one is called an allele.  So alleles are different versions of the same gene.  For example, there is a gene for blood type and there are three different versions of this gene called A, B and O, these are the different alleles of that gene. 

We will each inherit two of these in different combinations from our parents.


Amino acids are the individual chemical building blocks of proteins. There are 20 different amino-acids that can be put together in different combinations to make proteins. The sequence of amino acids is determined by the genetic code.

Amniocentesis (Amnio)

This is a procedure carried out in pregnancy to test the developing baby for a genetic condition. It can usually be performed from about 15 weeks of pregnancy. Under ultrasound guidance, a fine needle is passed through the mother’s abdomen into the womb and a small sample of the amniotic fluid from around the developing baby is collected. This contains cells from the baby which can be grown in the laboratory and tested for chromosome abnormalities and specific other genetic conditions that have already been identified in the family


If a condition is described as autosomal, this means the altered gene causing the condition is located on one of the autosomal chromosomes (autosomes), chromosomes 1 to 22.  

Autosomal dominant inheritance

A type of inheritance where just one altered copy of a gene is sufficient to cause the genetic condition.  Somebody with a condition which is inherited in this way has a 50% chance of passing the condition on to each child they have regardless of whether the child is a girl or a boy.

Autosomal recessive inheritance

A type of inheritance where both copies of a gene must be altered for the genetic condition to occur.  Someone with a recessive condition will have inherited two altered copies of the gene, one from each of their parents.

People who have only one copy of an altered recessive gene are usually healthy with no symptoms of the condition.  They are often described as ‘carriers’.


Any chromosome, apart from the sex chromosomes (X and Y). We have 22 pairs of autosomes and these are the same in males and females.


A unit in the chemical structure of DNA. There are 4 different bases in DNA called adenine (A), thymine (T), guanine )G) and cytosine (C). The sequence or order of these bases in the DNA, for example CGA, is the genetic code.


Someone who has one altered copy of a gene that could cause a specific genetic condition.  In autosomal recessive conditions, carriers have one working copy of the gene and one altered copy.  Carriers usually do not show any symptoms of the condition.  We are all carriers of around 5-7 recessive genetic conditions.


The basic structural and functional unit of our bodies.  As humans, we are made of approximately 50 trillion cells! There are different types of cell, for example, hair cells, skin cells and muscle cells. The majority of cells each contain a complete copy of our DNA.

Chorionic Villus Sample (CVS) Test

This is a procedure carried out in pregnancy to test the developing baby for a genetic condition. It can usually be performed from 11 to 13 weeks of pregnancy. A fine needle is used to take a tiny sample of tissue from the placenta. This process is guided by an ultrasound scan. The needle is usually passed through the mother’s abdomen, or occasionally her vagina. The cells from the placenta have the same genetic make-up as the baby and can be tested for chromosome abnormalities and specific other genetic conditions that have already been identified in the family. 


A long threadlike strand of DNA that carries a set of hundreds of genes (see diagram on right) We have 46 chromosomes arranged in 23 pairs.


A condition or characteristic that is present from birth.


Cytokines can increase or decrease the activity of certain genes. Our immune system is made up of cells with the ability to make and release lots of different cytokines. These can travel via blood vessels to stimulate a response from other groups of cells in the body.

De novo mutation – also called a ‘sporadic mutation’

a “new” alteration to a gene that is seen for the first time in the 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.


A cell in which all the chromosomes are present in pairs.  The majority of  cells in the human body are diploid, having 23 pairs of chromosomes, a total number of 46 chromosomes.   Mature sperm and egg cells are different, they have a single set of chromosomes with one from each pair, a total of 23 chromosomes.  This is known as haploid.

Deoxyribonucleic acid (DNA)

The molecule inside a cell that carries genetic information and is passed from one generation to the next. It contains the genetic code and is famous for its double helix structure. DNA is found in almost all cells.

Dominant allele

A version of a gene which always shows even if you only have one copy of the allele.  For example, with eye colour the allele for brown eyes is dominant, you only need one copy of this allele to have brown eyes.  It dominates over other alleles such as an allele for blue eyes.


A specialist protein that speeds up a biological reaction.  For example, enzymes in your stomach speed up the breakdown of food as part of digestion. Each enzyme has a very specific structure and a small change in the genetic instruction that encodes an enzyme can alter the function of the enzyme resulting in a genetic condition.


This is the part of our entire genetic make-up (genome) that  contains the instructions to make proteins. It comprises only about 1 – 2% of the genome. Any genetic changes found in these protein coding sections of our genome are thought to be more likely to cause genetic conditions.  


The degree to which someone is affected by one of their genes.  For example. two children might have the same altered gene but how much they’re affected could vary widely.
Gamete – A sperm or an egg cell.


The fundamental unit of heredity consisting of a segment of DNA which carries an instruction for how the body develops and functions. We get our genes from our parents and we all have a differentcombination of genes making us individual and unique.

Gene therapy

Treatment of a genetic condition by putting a new working copy of the gene into the affected cell. The extra gene makes up for the gene that is not working properly.  


This is the entirity of someone’s genetic make-up. It is all of their DNA, along all chromosomes including all genes and all the genetic material in-between genes.


The specific genetic information carried by someone’s genes. It is distinguished from someone’s physical appearance or symptoms, which is referred to as the phenotype. We can’t see the genotype but it determines specific characteristics, like the colour of our eyes.


A cell that has a single set of unpaired chromosomes.  Mature sperm and egg cells are haploid, each carrying only 23 single chromosomes.


Possessing two different alleles of a particular gene.  For example, with the gene for eye colour, someone who is heterozygous might have one allele for brown eyes and one for blue eyes.


Possessing two identical alleles of a particular gene.  For example, with the gene for eye colour, someone who is homozygous might have two alleles for blue eyes.

Human Genome Project

A major research project started in 1990 and completed in 2003, to work out the order of all the 3 billion letters (bases) that make up the entirity of the genetic information in humans (the human genome). DNA samples from a small group of people was used to get a sort of average DNA sequence. This has enabled many more genes to be identified and their precise locations on the chromosomes to be mapped.  Identifying genes and where they are located is the first step in being able to develop treatments and, eventually, cures for genetic conditions.  


This is the rate new cases for a condition occur within a given population.


The passing down of genetic information from parents to children.


Karyotyping is a test used to check for chromosome abnormalities.  It tells you your karyotype which describes the number of chromosomes someone has, whether they are genetically male (XY) or female (XX) and whether their chromosomes have any abnormalities that may cause health problems, such as a small piece of a chromosome missing or extra.
A karyotype is also a diagram of somebody’s chromosomes, like the one above.


The precise location on a chromosome where a gene is found.


The Austrian monk Gregor Mendel discovered the basic underlying principles of genetic inheritance after conducting experiments cultivating garden peas. Mendelian refers to his laws of heredity and these remain the basis of genetics today. Mendelian genetic conditions are genetic conditions caused by alterations in a single gene and can be inherited in recessive, dominant or X-linked patterns.


These are tiny structures found within cells. They produce the chemical energy needed to power the cell. Mitochondria contain a small amount of their own DNA, called mitochondrial DNA (mtDNA).

Mitochondrial inheritance

Mitochondria contain their own DNA and genes. Genetic misprints in mitochondrial genes are responsible for several genetic conditions, for example Leber hereditary optic neuropathy. Generally, mitochondria and therefore mitochondrial genes are inherited only from the mother.  


A permanent alteration to a gene where part of the DNA within the gene is different from it’s usual state.  There are lots of different ways a gene can be altered, for example there may be a small extra or missing part of the gene.  Mutations can cause genetic conditions or can be just part of normal variation.


The control centre of a cell. Chromosomes and therefore DNA are found within the nucleus.


The likelihood that a gene alteration which is associated with a genetic condition will actually result in someone developing symptoms of the condition. For example, 80% of people with a Van der Woude syndrome gene alteration have signs and symptoms of the condition and 20% do not, despite having the same gene alteration.  The penetrance of the gene alteration is therefore 80%.    


This describes your physical appearance, behaviour and how well your body functions including any symptoms you have.  It is the outcome of your genetic make-upp (genotype) interacting with environmental influences.


One of the basic chemicals of life, proteins are large molecules composed of building blocks called amino-acids . Our genes encode proteins and it is the proteins themselves that make up the structure and carry out the functions of a cell.

Recessive allele

This type of allele is only expressed when there are two of them present.
Human X and Y chromosomes Photo: Indigo® Instruments

Sex chromosomes

There are two types of sex chromosomes called X and Y. Generally we all have one pair of sex chromosomes and they  determine our genetic sex. Males have one X and one Y chromosome and females have two X chromosomes.


DNA sequencing is a technique used in a laboratory to determine the order of bases or ‘letters’ that make up a segment of DNA. The DNA base sequence is the genetic code which carries the information acell needs to assemble proteins.

Stem cells

Cells in the body that are capable of renewing themselves and becoming any number of different cell types.  Blood stem cells, for example, 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,. The stem cells can then become white blood cells, which work to fight off infection.


A group of signs and symptoms that tend to occur together. For example, an unusual-shaped head, slanting eyes, a single crease on the palm, and delayed learning and social skills are all symptoms of Down syndrome.


Our chromosomes usually come in pairs, one from each parent. Sometimes a mistake happens by chance around the time of conception resulting in an embryo with three copies of a particular chromosome.  This situation is known as trisomy, there are three copies of a chromosome rather than the usual two. Down syndrome, for example, is due to trisomy of chromosome 21.

X chromosome

Everyone has at-least one X chromosome. If you have two you are female. If you have one X chromosome and one Y chromosome, you are male.

X-linked recessive inheritance

A type of inheritance where an altered or “faulty” copy of a gene on the X chromosome causes a genetic condition in males. As males have just one X chromosome they only have one copy of each of the genes on this chromosome. If one of these genes has an alteration preventing if from working properly, they will usually develop full symptoms of the condition.

Females can be carriers of X-linked recessive genetic conditions with one altered copy of the gene on one of their two X chromosomes. The other X chromosome with a working copy of the gene is usually able to compensate for the altered copy. Carrier females do not usually develop symptoms of the condition but occasionally mild symptoms can occur. The blood condition, haemophilia is inherited in this way.

Useful Links

Talking Glossary of Genetic Terms from the US National Human Genome Research Institute

Merriam-Webster’s Medical Dictionary by Merriam-Webster Inc on Medline Plus