Glossary of Genetic Terminology
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MARKER CHROMOSOME:
A chromosome, or part of a chromosome, usually
small, of unknown origin.
MATERNAL SERUM TESTING:
A test which assesses the risk of fetal abnormalities
such as neural tube defects and Down syndrome by analysing a number of biochemicals in the mother's
blood during pregnancy (see Genetics Fact Sheet 17B).
MEIOSIS:
The special cell division which only takes place in the sex cells of females and males, resulting in egg and sperm cells that contain 23 chromosomes (the haploid number).
MENDELIAN INHERITANCE:
This refers to the inheritance of single genes and
follows specific patterns: autosomal
dominant, autosomal recessive and X- linked inheritance (see Genetics Fact Sheets 8, 9 and 10).
Conditions included under the 'umbrella' term of mental illness are schizophrenia, depression and bipolar disorder (manic depression). There may be an inherited susceptibility (see Genetics Fact Sheet 58).
METABOLISM:
The physical and chemical processes by which energy is made available for
essential body functioning, growth and development.
METACENTRIC:
Refers to a chromosome which has its centromere
in the middle and the short ('p') and long ('q')
arms are of equal length.
MISCARRIAGE:
Loss of a baby before the twentieth week of pregnancy.
MITOCHONDRIA:
These structures or organelles in the cell are the
main energy source: they are often called the powerhouse of the cell. The
mitochondria also contain their own DNA and therefore genes; mitochondrial genes follow
maternal inheritance (see Genetics Fact Sheet 12).
MITOCHONDRIAL DNA:
The genetic material contained in the circular genome found in mitochondria.
MITOSIS:
The process of cell division in all cells except the
reproductive cells. Mitosis results in 'daughter' cells which are genetically
identical to the parent cells.
MOLECULAR GENETICS:
The branch of genetics that studies the function and structure of genes at the molecular level.
The smallest unit of a compound or substance that can exist by itself, and still have all of its chemical properties.
MONOCLONAL:
A group of cells that are all identical copies of an
original cell.
MONOGENIC:
A characteristic which is due to the information contained in a single gene.
MONOSOMY:
Where one chromosome is represented once only
instead of twice; for example, girls with Turner syndrome have only one X
chromosome instead of the usual two copies (Monosomy X; see Genetics Fact Sheet 32).
MONOZYGOTIC TWINS:
Please see 'Identical twins'.
MOSAIC:
A situation where some cells have an abnormal or
unusual genetic or chromosomal make-up while the rest of the cells in the body
have the usual genetic or chromosomal constitution. For example, a person who
is mosaic for trisomy 21 (see Down syndrome) would have some cells which have 47 chromosomes with
an extra chromosome number 21 and other body cells which have the usual 46
chromosome complement. The number of cells with abnormal genetic or chromosomal
content will determine the level of severity of the condition.
mRNA:
An abbreviation for messenger RNA which
is the chemical that transfers the genetic DNA message to the ribosomes where it is translated into proteins.
MULTIFACTORIAL
INHERITANCE:
A pattern of inheritance which results from the interaction of one or more genes with environmental factor(s) (see Genetics Fact Sheet 11).
MUTAGEN:
A physical or chemical agent that causes a permanent change (mutation) in a
gene. It may or may not be a carcinogen.
MUTATION:
A permanent change in a gene. If the mutation occurs
in the germ line cells, it is then able to be inherited. Mutations in somatic cells cannot be inherited. Mutations can occur naturally and
spontaneously or they may be due to exposure to mutagens.