Glossary of Genetic Terminology
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P
'P' ARM:
Each chromosome is divided into two parts, joined by the centromere. The 'p' arm is the shorter of the two segments and is at the top of the chromosome. The longer segment is called the 'q' arm.
PARKINSON DISEASE (PD):
The second most common adult-onset neurological condition with three major symptoms: tremor, stiffness of the muscles and slowed movements. There may be a genetic component to the cause of the condition (see Genetics Fact Sheet 46).
PATAU SYNDROME:
Also known as trisomy 13, is a chromosomal condition that is the result of an extra copy of chromosome 13. It is a severe condition where children with the condition may have small wide-set eyes, cleft lip and palate, extra fingers and toes, scalp defects, malformed low-set ears, microcephaly (smaller head), severe intellectual impairment, kidney defects and heart defects (see Genetics Fact Sheet 29).
PEDIGREE:
A diagrammatic representation of a family health history tree.
PENETRANCE:
The probability of detecting the presence or clinical expression of a gene or combination of genes when they are present. If the penetrance of a particular condition is less than 100%, not all individuals who carry a mutation in the gene or genes responsible for the condition will develop symptoms of the condition it causes. Such a genetic condition is said to have reduced or incomplete penetrance.
PHARMACOGENETICS/PHARMACOGENOMICS:
The study of how an individual's genetic make-up affects their response to medicines. The terms pharmacogenomics and pharmacogenetics tend to be used interchangeably. Pharmacogenetics is generally thought of as the study of genetic differences that give rise to differing responses to medications and drugs, while pharmacogenomics is the broader application of genomic technologies to new drug discovery and further characterisation of older drugs. Pharmacogenetics considers one or at most, a few, genes of interest, while pharmacogenomics considers the entire genome (see Genetics Fact Sheet 25).
PHENOTYPE:
The physical and/or biochemical characteristics of a person, an animal or other organism which are determined by their genetic make-up and/or environment.
PLACENTA:
The structure that provides the foetus with nourishment during development prior to birth. It is attached to the wall of the uterus and connects to the foetus through the umbilical cord.
PLASMID:
Small circular DNA molecule used to transfer genes from one organism to another.
PLURIPOTENT:
Cells that may still differentiate into various types of specialised tissue in the body.
POLYGENIC:
A condition or characteristic that is caused by many different genes acting together.
POLYMORPHISMS:
Changed genes, DNA sequences or chromosome structures which occur naturally in the population and do not cause any harm to the individual.
PREDICTIVE TESTING:
A form of genetic testing performed on a person with a family history of a particular genetic condition, but who does not have any symptoms of the condition at the time of testing. This testing determines if that person has inherited the mutation (present in their family). If testing for this mutation reveals that it is present in the person, then they have an increased predisposition to developing the condition that was tested for. The detection of a specific mutation does not necessarily mean the individual will definitely develop the condition. Familial breast cancer is an example of a condition where predictive testing is used (see Genetics Fact Sheet 48).
PREDISPOSITION:
A situation in which a person, due to their inherited genetic make-up, may have a particular susceptibility to a condition if exposed to the correct environmental triggers.
PREIMPLANTATION GENETIC DIAGNOSIS (PGD):
An adjunct to the IVF process where the embryo undergoes genetic testing before it is transferred (implanted) into to uterus.
PRENATAL DIAGNOSIS:
The detection of foetal abnormalities during pregnancy (see Genetics Fact Sheets 17, 17A and 17C).
PRENATAL SCREENING:
Tests during pregnancy to assess the possibility that a foetus is affected with a particular condition. These tests are not 100% accurate (see Genetics Fact Sheet 17B).
PRESYMPTOMATIC TESTING:
A form of genetic testing performed on a person with a family history of a particular genetic condition, but who does not have any symptoms of the condition at the time of testing. This testing determines if that person has inherited the mutation (present in their family). If testing for this mutation reveals that it is present in the person, then they will most likely develop symptoms of the genetic condition it causes at some stage of their life. Huntington disease is an example of a genetic condition where presymptomatic testing is used (see Genetics Fact Sheet 44).
PREVALENCE:
The proportion of a whole population affected by a certain condition.
PROBE:
A small segment of DNA of known origin, manufactured in the laboratory, which is designed to recognise the DNA on specific parts of chromosomes. A coloured stain can be attached to the probe and used to confirm the presence or absence of a particular gene or mutation.
PROPHYLACTIC SURGERY:
A surgical procedure designed to prevent the development of a disease. For example, prophylactic mastectomy is the removal of one or both breasts in an effort to prevent breast cancer in individuals who carry a mutation in a breast cancer gene and are therefore at high risk of developing breast cancer.
PROSTATE:
A small gland of the male reproductive system that secretes a fluid that makes up part of the ejaculate (seminal fluid).
PROTEIN:
Substances which are major components of the body structure, essential to body function. They are made up of smaller units called amino acids.
PROTO-ONCOGENE:
Genes that are part of a person's usual genetic make-up. They have a role in various aspects of cell division. If these genes are changed in some way, they may give rise to oncogenes that can lead to cancer.