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Chromosomes |
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What are chromosomes?
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| Chromosomes are the structures in which our hereditary material (DNA) is packed.
A healthy person has 46 chromosomes (23 pairs).
One chromosome of each pair (i.e. a total of 23 chromosomes) is donated by each parent to
the fetus.
Twenty-two pairs are called autosomes and the other pair are the sex chromosomes (X and Y).
Females have two X chromosomes and males have one X and one Y.
The chromosome pairs are arranged and numbered by size and shape, and each has different genes.
Because there are two copies of each chromosome, there is effectively a duplicate gene
system (one maternal and the other paternal), each identical in sequence to the other.
Disorders in the number and structure of the chromosomes may cause defects, genetic
syndromes and/or developmental retardation.
Almost any insertion or deletion of chromosomal material (mainly from autosomes) may manifest in
mental retardation.
The best known defect is an extra chromosome 21, so that there are three copies instead of two,
and this causes Down syndrome.
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Chromosome analysis (karyotype) |
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An examination of all the chromosomes can be carried out
on blood cells and also in a prenatal test
(amniocentesis, chorionic villus sampling, cordocentesis).
In this examination, the number of chromosomes and the structure of each are analyzed.
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Chromosome disorders (general terminology) |
| Trisomy - a condition in which there is an extra chromosome,
with three of the same type being present rather than two.
For example: trisomy 21 ( Down syndrome ), trisomy 18, etc
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| Monosomy - a condition in which an entire chromosome
is missing, with only one being present rather than two.
For example: monosomy X ( absence of an X chromosome, Turner syndrome ).
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Translocation - the exchange of chromosomal material between
two chromosomes of different pairs. A translocation is said to be balanced when a change occurs in the
location of certain genes, but there is no gain or loss of genetic material. If a person carries a balanced
translocation, with no change in the amount of genetic material and no damage to the genes has occurred,
he will be healthy, but is at increased risk of having a child with an unbalanced chromosome arrangement.
In such a child, the chromosome arrangement will include gains or losses of genetic material that result
from the parental translocation. Some fetuses that have an unbalanced chromosome arrangement abort
spontaneously at the beginning of pregnancy, so that some of the couples where one partner carries
a balanced translocation have recurrent miscarriages. It is important to note that a parent carrying
a balanced translocation can also have healthy children without a translocation or children carrying
a balanced translocation like that of the parent. In these cases, where the chromosome arrangement is
identical to that of the affected parent, there are no clinical manifestations.
This can be diagnosed during pregnancy by amniocentesis or chorionic villus sampling.
When a balanced translocation is diagnosed in a fetus in a prenatal test, and when the chromosome
arrangement in each of the parents is normal (without a translocation), this is described as
a "de novo" translocation. In these cases there is an increased empirical risk of defects and mental
retardation in the fetus. The risk is approximately 7% higher than the standard risk in the general population,
which is approximately 2% - 3%. The exact risk varies according to the type of translocation and the results
of other tests such as an ultrasound scan.
There are two types of translocations: Robertsonian and reciprocal.
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| Robertsonian translocation - this is a condition in
which there is almost complete fusion between two chromosomes. This type of translocation is only possible
with chromosomes 13, 14, 15, 21 and 22 because of their shape - they have only long arms and no short arms.
If a translocation of this type is identified in the fetus and in a balanced form in one of the parents,
the child will be healthy in about 98% of cases, but nevertheless it is important to check for conditions
that may be associated with anomalies in the fetus, even though the translocation in the fetus is apparently balanced.
The main condition to look for is uni-parental disomy (UPD). Here the fetus receives two chromosomes of
the same pair from a single parent, instead of the normal situation in which it receives one of each pair from
each parent. This phenomenon can be diagnosed using molecular genetics involving testing at the DNA rather than
the chromosome level. |
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Reciprocal translocation - this is a condition in which there is an exchange
of chromosomal material between two chromosomes of different pairs.
Breaks occur in the arms of each of the chromosomes concerned, and the broken-off part of one chromosome
attaches itself to the other chromosome and vice versa. When the break occurs between the genes and the
genes are not damaged, there are no clinical consequences. However, when as a result of such a translocation,
damage is caused to an essential gene, a mutation that can cause impaired functioning may arise.
This means that a de novo translocation carries a risk for defects and mental retardation that is
about 7% higher than the regular risk. On the other hand, if one of the parents is found to have a
break at the same location and he or she is healthy, this proves that no damage associated with a
disease has occurred at the translocation site.
Inversion -
in this condition there is inversion of a segment of one of the chromosomes that results in neither gain
nor loss of genetic material.
This means that a person carrying an inversion is almost always healthy.
There is a relatively small risk, however, of having a child with an unbalanced chromosome arrangement
where there may be gains or losses of DNA.
A significant number of such pregnancies abort spontaneously at the beginning of the pregnancy.
An inversion can be detected during pregnancy by amniocentesis or chorionic villus sampling.
There are a number of very common inversions in the population, especially in chromosomes 2, 9 and 17,
which in most cases are transmitted unchanged from parent to child.
In these cases there is usually no reason to examine the fetus, because the likelihood that there
will be a problem in the fetus is very small.
Mosaicism -
in this condition a change occurs in the structure or shape of the chromosome or in the chromosome
arrangement usually after the fetus is formed.
Here the fetus starts developing in the uterus with a normal chromosome arrangement, but when
it reaches the stage of multiple cells, one cell might develop a chromosome anomaly.
From then on, all the cells that are derived from this one cell will also contain the same abnormal chromosome
arrangement. This means that in certain body organs the person has a combination of cells, some with a normal
genetic structure and others with an abnormal genetic structure. This situation is known as mosaicism.
The clinical significance of mosaicism depends on the number of abnormal cells and their function, i.e.
what they are responsible for. In most cases a chromosome disorder resulting from mosaicism is milder than
one that occurs when all the cells have the same abnormal chromosome arrangement.
Because mosaicism is present in only some of the body organs, tissues and cells, there is a
possibility that it will not be diagnosed in chromosome tests performed on blood or amniotic fluid.
Microdeletions of chromosomes - there are a number of
syndromes that are caused by microdeletions (deletions that are so small they are barely, if at all,
detectable microscopically) in chromosomes, e.g. Williams syndrome,
which is caused by a deletion of a small segment of chromosome 7; VCF syndrome, caused by a deletion of
a small segment of chromosome 22; cat cry (cri du chat) syndrome, caused by a small deletion of chromosome 5,
and many others.
A very small number of these deletions can be identified using classic cytogenetic diagnostic methods,
but most can be diagnosed only after birth and only after clinical suspicion of their existence has been established.
Diagnosis is made by carrying out fluorescent staining (FISH - fluorescent in-situ hybridization) or by
molecular methods.
Deletions in chromosome Y -
in cases where there is a deletion of part of the short arm of chromosome Y, a female phenotype is found
because of the deletion of a gene called SRY that is responsible for testicular development.
This condition is rare.
Deletions in the long arm of chromosome Y that do not cause a loss of the normal male phenotype but
that may be associated with the absence of sperm formation are more common. In most of these cases the
deletion is in a region that does not encode genes and thus has no clinical significance.
If a short Y chromosome is identified, the father should be examined. When the father's Y chromosome is
identical to that found in the fetus, the deletion has no significance, but when the father does not have an
identical Y chromosome structure, and paternity is not in doubt, the deletion in the fetus may have consequences
for the offspring's final height and sterility (because of a lack of sperm cells), although these
conditions can be treated.
Some men who are sterile have deletions in certain regions of the long arm of the Y chromosome.
These can be detected using molecular methods.
Examples of common numerical disorders in the autosomes (the non-sex-determining chromosomes)
Down syndrome, trisomy 18 trisomy 13.
Numerical disorders of the sex chromosomes (X and Y)
Disorders in the number of sex chromosomes (trisomies and monosomies) differ from similar problems in the
autosomes. Whereas disorders in the number of autosomes entail such significant defects as mental
retardation etc., in sex chromosome disorders the problems are less severe in comparison but of greater scope.
Sometimes the level of intelligence is almost normal, but there can be severe problems of fertility, stature
and mental development. When a couple seeks genetic counseling because one of these conditions has been
detected in the fetus, they often face severe dilemmas.
Examples of numerical disorders in sex chromosomes: Turner syndrome (XO),
Klinefelter syndrome (XXY), XXX (triple X) syndrome
and XYY syndrome.
The risk for recurrence of chromosome disorders
In order to evaluate the risk for the recurrence of a chromosome disorder, it is important to know what
disorder the first fetus had. It is important to note that even when the risk of recurrence is not
significantly high relative to that in the general population (which is about 1:300), it is advisable that
every couple with a history of a chromosome disorder in pregnancy should undergo prenatal testing for fetal
chromosome analysis in every future pregnancy.
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In cases of numerical chromosome disorders (trisomy or monosomy), the risk of recurrence is 1% higher
than that of other women of the same age. The risk is slightly less when the fetus has a numerical
disorder involving the sex chromosomes. The maternal age is also a factor that affects the incidence of
numerical chromosome disorders.
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When one of the partners has a numerical disorder in one of the sex chromosomes,
there is a theoretically increased risk of having an infant with a similar sex chromosome disorder.
In reality, however, this is usually not the case unless intra-cytoplasmic sperm injection
(ICSI) is performed.
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When the chromosome disorder in a previous pregnancy was structural (e.g. translocation, inversion,
minimal deletions or insertions) and the chromosomal structure of both parents is normal,
the risk of recurrence is not significantly higher than that in the general population.
However, the risk is not zero, because it can happen that the chromosomal structure of a parent
looks apparently normal, but there is an abnormality in the parent's gametes (mosaicism).
This can be transmitted to the fetus. However, as noted, this condition is rare, and the empirical
risk is not significantly greater than that in the general population.
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When the chromosome disorder is structural and has been demonstrated in one of the parents in a
balanced or unbalanced form (translocation, inversion, marker chromosome, etc.),
there is a significant risk for recurrence in future pregnancies. The empirical risk depends on two
parameters: the chromosomes involved and whether the problem is in the husband or wife.
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When the chromosome disorder in the fetus is of mosaic type and the parents have normal chromosomes,
the risk of recurrence is not significantly increased because the disorder is not in the sperm or
ovum that made the fetus and therefore is not present in the parents' chromosomes, but rather occurred in
the fetus after fertilization.
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Chromosome to DNA
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