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R

RECESSIVE:
Every cell contains two copies of each gene. Each gene contains the information for a particular gene product, such as a protein. If a gene is mutated, the gene no longer codes for the gene product. Where an individual has one gene copy or allele mutated and the other copy 'correct', the cell will only be producing half the amount of gene product. If this does not result in any condition for the individual, the mutation is described as being hidden or 'recessive' to the correct copy of the gene. An individual with this genetic constitution is said to be a 'carrier' of a recessive gene mutation. For a recessive gene mutation to result in a particular characteristic or a condition, both copies of the genes must be mutated (see Genetics Fact Sheet 8).

RECURRENCE RISK:
The risk that an inherited condition will occur again in a family.

REGULATORY GENE:
A gene containing information for the regulation (switching on or off) of other genes.

REPLICATION:
The identical duplication of DNA or of a cell.

RESTRICTION ENZYME:
Enzymes that can cut DNA into strands at specific places along its length.

RIBOSOMES:
Small components in a cell that are composed of ribosomal RNA. They are important in the reading of the DNA messages in a cell. See also mRNA.

RISK:
Chance that a specific process or event may occur. 'Genetic risk' refers to the likelihood or probability that a genetic characteristic or condition will occur or recur in a family, based on an understanding of the pattern of inheritance.

RFLPs (RESTRICTION FRAGMENT LENGTH POLYMORPHISMS):
The fragments of DNA which result when it is cut by special enzymes called restriction enzymes. The patterns of these fragment lengths are used to indicate the presence or absence of mutations in particular genes.

RIBONUCLEIC ACID:
Please see 'RNA'.

RING CHROMOSOME:
This occurs as a result of the fusion of the two ends of the same chromosome; there is a consequent loss of genetic material.

RNA (RIBONUCLEIC ACID):
An abbreviation for ribonucleic acid, a chemical similar to DNA which has an important role in protein manufacture. There are several types of RNA (see mRNA).

ROBERTSONIAN TRANSLOCATION:
A type of translocation exclusive to the acrocentric chromosomes (13, 14, 15, 21 and 22) in which two of these chromosomes join at or near their centromeres. This is effectively a fusion between two whole chromosomes.