Glossary of Genetic Terminology
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IDENTICAL
TWINS:
Twins that arise from a single egg fertilised by a single sperm. These twins are therefore genetically identical. They are also
referred to as monozygotic twins.
IMMUNE SYSTEM:
The body's defence system that destroys potentially harmful, foreign
substances or organisms that enter the body. It is made up of specialised cells.
IMPLANTATION:
The process whereby a fertilised egg (embryo) is embedded in the lining of a woman’s uterus.
IMPRINTING:
The 'memory' held by a chromosome as to whether
it was inherited from the mother or the father. The memory is chemically
'stamped' into the DNA and can
result in chromosomes or the genes located on the
chromosomes behaving differently, depending on the parent of origin.
INBORN ERROR OF
METABOLISM:
A congenital condition which results from a change in a gene which causes a
deficiency in the presence or activity of particular enzymes important for the
functioning of the body's metabolism (see Genetics Fact Sheet 20).
INCIDENCE:
The number of new cases of a condition, detected annually, per unit of the
population. For genetic conditions, the incidence is quoted as the number of
affected individuals per 1,000 births whether detected at birth or not.
INCOMPLETE PENETRANCE:
Please see 'Penetrance'.
INHERITED:
The transmission of genetic information from a parent to a child.
INSERTION:
The addition of a piece of chromosomal material into a chromosome in a place where it is not normally found. This may result in a condition,
because the genetic code may then be read or
translated incorrectly.
INTRON:
The part of the genetic sequence that is not translated into the final gene product or message.
INVERSION:
Where there are two breaks in a chromosome, the
segment may flip over and rejoin, that is, become inverted. This results in the genes being in the reverse order along the chromosome. This may cause the genetic code to be read or translated incorrectly.
IN VITRO FERTILISATION (IVF):
The process whereby an egg is fertilised with sperm in the test tube and then transplanted into a woman's uterus.
ISOCHROMOSOME:
A chromosome in which the arms ('p' and 'q') are of equal length and the information in
each of the two arms is genetically identical.
IVF:
Please see 'In vitro Fertilisation'.