TRISOMY
13/18
Why
Trisomy 13 or Trisomy 18 ?
Babies have been born with trisomy13 and trisomy18 since
the beginning of time, but it was only 1n 1960 that Dr.
Patau and Dr. Edwards identified the respective features
of these syndromes. Both syndromes are the result of an
extra 13th or 18th chromosome, respectively, being present
in each cell.
We
know what happens, but in the majority of cases nobody
knows why a baby is conceived with trisomy 13 or 18. Parents
should not feel that anything they did or failed to do
was responsible for their baby’s chromosomal disorder.
One
in four of all pregnancies ends in miscarriage and a high
proportion are thought to be caused by a chromosomal disorder.
A high proportion of trisomy 13 and trisomy 18 pregnancies
end in miscarriage within twelve weeks of conception.
It is therefore quite rare for such a pregnancy to continue
to full term. The incidence of trisomy 13 is 1 in 5,000
of total births while that of trisomy 18 is about 1
in 2,500 of total births. In Ireland both North and South,
there are approximately seven babies born with trisomy
13 and seventeen babies born with trisomy 18 every year.
As the female X chromosome is more viable, there are more
girls than boys.
The
risk of conceiving a baby with a chromosomal disorder
increases with age. The average age of a mother giving
birth to a baby with trisomy 13 or trisomy 18 is 32 years.
However, as most babies are born to mothers in their twenties
and early thirties, many babies with trisomy 13 and 18
are born to women of this age.
Unless
there is a genetic problem, it is extremely rare to have
a second baby with the same disorder. However, if you
are considering having another baby it is worthwhile to
seek the advice of a genetic counsellor.
CHROMOSOMES
Chromosomes are minute thread-like structures found inside
the cells of our bodies. Each chromosome consists of two
parts: the short arm “p” and the long arm
“q”. On the chromosomes there are between
50,000 and 100,000 genes. These genes determine the individual
hereditary characteristics of each person i.e. height,
eye colour, blood group. See Fig 1
A
human cell contains 46 chromosomes and these are arranged
into 23 pairs. A human egg or sperm contains only one
chromosome from each pair. Therefore, there should be
23 of the mother’s chromosomes in each egg and 23
of the father’s chromosomes in each sperm. , the
resultant fertilised egg will contain 46 chromosomes (23
pairs); one chromosome of each pair came from the mother
and the other chromosome from the father. This is the
unique “blueprint” for the individual baby.
See Fig 2
Karyotypes
A karyotype or picture of chromosomes is prepared from
a sample of blood. The blood cultures are grown for three
days and the thread-like chromosomes in the cell are released
on to a slide and stained with special dyes before being
photographed, cut out and arranged in order of size to
make study possible. See Fig 3
Trisomy
- How it occurs
The word trisomy is from the Greek word tri (three) and
means that there are three identical chromosomes present
in each cell instead of the usual pair. In trisomy 13
(Patau’s syndrome) an extra chromosome number 13
is present in each cell. See Fig 4
In
trisomy 18 (Edwards’ syndrome) an extra chromosome
number 18 is present in each cell. See Fig 5
The
extra chromosome may come from either the mother’s
egg or from the father’s sperm during cell formation,
or when the cells of the fertilised do not divide correctly
during cell division. See Fig 6
Although
the genes in the three chromosomes are normal, too much
or too little genetic material in a cell affects every
stage of the development of the baby. Therefore the blueprint
for development is altered from the moment of conception.
Related
Disorders
In addition to trisomy 13 and trisomy 18, there are a
number of related disorders as follows:
Partial Trisomy
A very small percentage of babies are born, not with a
complete trisomy but with a partial trisomy. This means
that an extra piece of a chromosome is present. Babies
with a partial trisomy may show some or all of the characteristics
of the particular trisomy. This depends on the precise
amount and nature of the additional chromosomal material.
Balanced
Translocation
In a balanced translocation, the normal number
(46) of chromosomes are present in each cell but some
or all of the material from one chromosome may be located
on to a different chromosome. There is no loss of chromosomal
material. Babies with balance translocations are completely
normal and their chromosomal rearrangement should have
no implications regarding health or physical appearance.
However, their children would be at risk of inheriting
an unbalanced translocation.
Unbalanced
Translocation
This means that either
(a) an extra piece of chromosomal material is present
in each cell, usually attached to another chromosome
(b) that a piece of chromosomal material is missing.
Balanced
or Unbalanced Translocations can arise spontaneously,
both parents having normal chromosomes. They can also
derive if one or other parent has a balanced translocation.
The chance of another pregnancy with either trisomy 13
or 18 is much greater than it would otherwise be. The
unaffected siblings may also be carriers of the balanced
translocation.
Deletion
A deletion is a condition where some chromosomal material
is missing from the normal chromosome. In the case of
a condition denoted by 13q-, the long arm “q”
of chromosome 13 is missing. In the case of 18p- the short
arm “p” of chromosome 18 is missing.
Mosaicism
This condition is a less common form of trisomy. It results
from an incorrect separation of chromosomes after the
first normal division of the fertilised egg. Therefore,
some cells have an extra chromosome (47), while other
cells have the normal number of chromosomes (46). The
number of cells having the extra chromosome varies from
baby to baby and therefore each baby with mosaicism is
unique. These babies are often less severely affected
compared to those with an extra chromosome in every cell.
Ring
Formation
Here the two ends of a chromosome have broken off. Rejoining
of the broken ends in the formation of a ring chromosome
and the loss of two chromosome fragments from each end.
Ring chromosomes are rare.
