Chromosome loss or gain (aneuploidy)

When a cell is dividing, sometimes a whole chromosome can end up in the wrong cell, so that the cell has the wrong number of chromosomes.  If there is an extra chromosome, the cells have extra DNA and if there is one missing there will be less. This type of chromosome error is called aneuploidy.

You may be familiar with Down syndrome that is caused by having an extra copy of chromosome 21. This gives 3 copies in total and Down syndrome is sometimes called trisomy 21.

Trisomy

T21 female karyotype
This image shows the chromosomes of a person affected by Down syndrome.
 

This image shows the chromosomes from a failed pregnancy (miscarriage) with trisomy 16; i.e. 3 copies of chromosome 16.
 
This chromosome abnormality is one of the most common found in miscarriage material.
Triploidy

Triploid female karyotype
This image shows the chromosomes of a pregnancy that is affected by Triploidy, which has 69 chromosomes instead of the usual 46 chromosomes. Such a pregnancy typically ends in miscarriage.

 

Chromosome (structural) rearrangements

Pieces of chromosomes can sometimes be rearranged, so that one piece of a chromosome is stuck on another (translocation). If you have a translocation it would not affect you, because you would still have all the information you need, just in a different order.

Reciprocal translocation 

translocation t(2;22) karyotype
This image shows the chromosomes of a normal individual who carries a structural chromosome rearrangement referred to as a translocation. However, such a person may have fertility problems such as difficulty falling pregnant or miscarriage, or may give birth to abnormal child.

When you produce sperm or eggs, which only have half the number of chromosomes as the rest of your cells, the matching chromosome pieces could get separated during the cell divisions. You could end up with the right combination if the sperm or egg receives one of each pair with the rearrangement or one of each pair not rearranged.

However, you could end up with the wrong combination if the sperm or egg receives one of the pair with the rearrangement and one of the pair not rearranged. Then the egg or sperm wouldn’t have all the information it needs. The wrong dose of chromosomes will usually result in either failed implantation, miscarriage, or an abnormality in the baby.