Chromosomal disorders can result from changes in either the number or structure of the chromosomes. Changes in the number of chromosomes happen when there are more or fewer copies of a particular chromosome than usual. Changes in chromosome structure happen when the material in an individual chromosome is disrupted or rearranged in some way.
Changes in chromosome number
Usually, every cell in a person’s body contains 46 chromosomes. Sometimes, however, a baby is born with either too many or too few chromosomes. The baby, therefore, has too few or too many genes or instructions. One of the most common examples of a genetic condition caused because of an extra chromosome is Down syndrome. People with this condition have 47 chromosomes in their cells instead of 46. This is because there are three copies of chromosome number 21 instead of the usual two.
Changes in chromosome structure
Changes in chromosome structure happen when the material in an individual chromosome is broken and rearranged in some way. This may involve the addition or loss of chromosome material. This may happen in a number of ways which are discussed below. Changes in chromosome structure may be very subtle and hard to detect by scientists in the laboratory. Even when the change in structure is found, it is often hard to predict what effect the change will have on an individual child. This can be frustrating for parents who are keen to have as much information about their child’s future as possible.
When there is a deletion part of the chromosome has been lost or deleted, which you can see in the diagram below. A deletion can happen on any chromosome, and along any part of the chromosome. The deletion can be any size. If the material (genes) that has been deleted contains important instructions for the body, that person may have a learning disability, developmental delay, and/or health problems. The seriousness of these depends on how much of the chromosome has been deleted, and where the deletion is.
A chromosome has duplicated part of itself so that there is too much chromosome material present, which can be seen in the diagram below. This additional chromosome material may mean there are too many instructions for the body to process, and this may result in learning disability, developmental delay and/or health problems in a child.
As can be seen in the diagram below, one portion of a chromosome has moved to a different part of the chromosome (intrachromosomal) or to a different chromosome altogether (interchromosomal). Chromosomal translocations can be reciprocal when segments from two different chromosomes are exchanged or Robertsonian when an entire chromosome attaches to another. The effect these translocations have depends on whether any chromosome material has been lost or gained. When this is the case, translocations may lead to miscarriage or the birth of children with symptoms including learning disabilities and physical disabilities.
The ends of a chromosome have joined together in a ‘ring’ shape, as you can see in the diagram below. This usually happens when the two ends of the same chromosome are deleted. The remaining ends of the chromosome are ‘sticky’ and join together to make a ring shape. The effect this has on the person usually depends on how much chromosome material, and therefore ‘information’, was deleted before the chromosome formed a ‘ring’.
In the diagram below, part of a chromosome has turned so that the sequence of genes in the chromosome is the opposite orientation (inverted). In the majority of cases, this does not cause any health problems to the person carrying the inversion.
If a parent has an unusual chromosome rearrangement, will he or she always pass it on?
Not necessarily, there are several possibilities for each pregnancy:
- The child may inherit an entirely normal chromosome arrangement.
- The child may inherit the same chromosome rearrangement as the parent.
- The child may be born with a learning disability, developmental delay, and/or health problems.
- The pregnancy may end in miscarriage.
Therefore, it is usually possible for a person who carries a chromosome rearrangement to have healthy children, and many do. As each rearrangement is unique, carriers would have to discuss their particular situation with a genetic specialist. A child can be born with a rearrangement although both parents’ chromosomes are normal. This is called a de novo (from Latin) or new rearrangement. In this case, the chance that the parents will have another child with the same rearrangement is usually very low.
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