- acrocentric chromosomes are those with the centromere very close
to one end, giving the chromosome a "V" shaped appearance. The
short (p) arms are very short and usually have small dot-like appendages
on stalks, known as "satellites". The acrocentric chromosomes
are 13, 14, 15, 21 & 22.
- one member of a pair of genes at a given region of DNA.
- a complex material composed partly of proteins which is present
in the body of people suffering from several diseases.
- the occurrence of a genetic disorder at earlier age and/or at greater
severity in successive generations.
Dominant - determined by a gene on one of the chromosomes other
than the sex chromosomes, requiring only one member of the gene pair
to be altered to express the condition.
- the 22 pairs of chromosomes not including the two sex chromosomes.
- the series of darkly and lightly stained stripes across a chromosome
produced by a variety of chemical treatments. Each chromosome
has its own characteristic banding pattern which allows specific identification
of chromosomes or parts of chromosomes.
division - the mechanism by which cells multiply during the growth
of tissues or organs. The type of cell division involved in
the growth of the body is known as "mitosis". The cell division
which produces sperm or ova in the testis or ovary is known as "meiosis".
- the constricted or "pinched in" part of the chromosomes separating
the short (p) arm from the long (q) arm.
arm - see centromere.
number - in humans, the chromosome number is 23 pairs giving a
total of 46. Half of the chromosomes are inherited from the
mother and half from the father.
- chromosomes contain genetic material and are in every cell of the
- thread-like structures consisting of DNA which "package" genetic
information within the nucleus of most cells of the body. Human
cells contain 46 chromosomes in 23 pairs. The 46 chromosomes
contain all the genetic information inherited from both parents, thus
one of the pair is inherited from the mother, the other from the father.
- the study of the number, structure, behaviour and effects of chromosomes.
- loss of a whole chromosome or part of a chromosome. The term
usually refers to loss of part of a chromosome (partial deletion).
A terminal deletion refers to breakage and loss of the end of a chromosome;
interstitial deletion is the loss of material from within the chromosome,
between the ends.
- a cell with a normal chromosome number including two sets of parental
chromosomes (23 from each parent). See triploidy and tetraploidy.
- meaning two bodies. Refers to a normal chromosome pair.
(deoxyribonucleic acid) - the long double-stranded chemical molecule
found in the nucleus of cells and packaged into chromosomes.
DNA contains the "genetic code" (genes) which we inherit from our
- a double copy of part of a chromosome resulting in an extra (abnormal)
dose of the duplicated material.
IX - blood coagulation factor.
- stands for Fluorescence In-Situ Hybridisation - the technique whereby
small pieces of DNA (probes) which "recognise" particular chromosomes
or parts of chromosomes are labelled with a coloured fluorescent dye
and then used to highlight "target" DNA in chromosomes.
- the important red-cell enzyme defect, glucose 6-phosphate dehydrogenase
deficiency, is particularly prevalent in parts of the Middle East,
Mediterranean, South East Asia and in people of African decent.
- the germ cell (sperm or ovum) continuing a single haploid set of
(23) chromosomes from one parent.
- see band. The common banding produced with Giemsa stain and
used to routinely identify chromosomes.
- a sequence of genetic information (DNA code) which is responsible
for a particular bodily function or characteristic.
- the chromosome number (23) found in the parental gamete. i.e.
half the diploid number.
- an individual with two different alleles of a gene region.
- a condition which leads to elevated levels of plasma cholesterol.
- where gene expression is influenced by the parental origin.
- the "memory" held by a chromosome as to which parent it was
inherited from. The memory is chemically "stamped" into the
DNA and can result in chromosomes behaving differently, depending
on the parent of origin.
- any part of a chromosome between the ends (telomeres).
- the breakage of a chromosome in two places followed by a 180 switch
of the segment between the breaks. There is no loss or gain
of material but the sequence of the inverted segment is reversed.
- the International System of Cytogenetic Nomenclature used to describe
- refers to genetic factors which are inherited together, because
they are closely located to one another.
- a chromosome, usually small, of unknown origin and unidentifiable
from its G-banding pattern.
- the process of cell division leading to formation of eggs and sperm
with halving of the chromosome number.
- following the patterns of inheritance proposed originally by Gregor
- a metacentric chromosome has its centromere in the middle and the
p and q arms are of approximately equal length.
- see cell division.
- see cell division.
number - the chromosome count characteristic of cells from a particular
individual (usually 46 in a normal human).
- a single copy of a chromosome rather than a pair (disomy).
- the co-existence of genetically different cells within the same
individual or tissue. e.g. a mixture of both normal cells and cells
with a chromosome abnormality.
- determined by multiple genes and also by non-genetic factors.
- the visible expression of the action of a particular gene; the
clinical picture resulting from a genetic disorder.
- determined by multiple genes.
- Literally many shapes. Variations of chromosome appearance
which are considered to be normal. These "normal variants" are
passed on within families without causing any problems in development.
- a small fragment of DNA designed to attach to specific parts of
chromosomes. The probe can be labelled with a coloured stain
and used to confirm the presence or absence of the target material
or to identify unknown chromosomal material.
translocation - the breakage and exchange of material between
two chromosomes without any gain or loss of genetic material.
Such a reciprocal translocation is said to be balanced. Where
translocated chromosomes are passed on to offspring in a way which
results in partial trisomy or monosomy, the translocation is said
to be unbalanced.
chromosome - a chromosome which has broken at each end, lost the
terminal fragments and curled around to join at the ends to form a
ring. Formation of a ring results in the loss or deletion of
the terminal segments.
translocation - a type of translocation exclusive to the acrocentric
chromosomes, in which two chromosomes join at or near their centromeres.
This is effectively a fusion between two whole chromosomes rather
than an exchange of material as in reciprocal translocations.
Sex Chromosome - the X and Y chromosomes which are responsible
for sexual development. In humans, females have two X chromosomes
while males have an X and a Y.
- chromosomes with centromeres which are not centrally placed.
The chromosome arms are not of equal length. This description
does not apply to the acrocentric chromosomes which have an almost
Repeat Sequence - a series of identical DNA sequence lying adjacent
to each other in the same orientation, within a much larger DNA sequence.
- the terminal segment of each chromosome arm.
- the presence of four, instead of two, of a particular chromosome.
- the presence in a cell, of four haploid sets (23) of parental chromosomes,
giving a total chromosome count of 92.
- the fusion or exchange of material between chromosomes.
When there is no gain or loss of material, the translocation is said
to be balanced; when there is gain or loss, resulting in trisomy or
monosomy for particular chromosome segments, it is said to be unbalanced.
- the presence in a cell of three haplid sets (23) of parental
chromosomes, giving a total chromosome count of 69.
- the presence of three copies of a chromosome, i.e. trisomy 21.
Three copies of part of a chromosome is a partial trisomy.
Uniparental disomy - where both members of a chromosome pair
are contributed by the one parent rather than one from each parent.
X-chromosome - see sex chromosome.
Y - chromosome - see sex chromosome.
Zygote - the single cell with 46 chromosomes resulting from
the fertilisation of an ovum (23) by a sperm (23). Through cell
division (mitosis), the zygote develops into a multicellular embryo
and then into the foetus.