Full, partial or mosaic?

Trisomy 18. What is it? Most people can grasp pretty quickly that it means that you have three copies of chromosome 18.

Most of us have 22 pairs of chromosomes, each pair numbered 1-22, and a pair of sex chromosomes (XX for a girl, XY for a boy). Babies with a trisomy have three of one of these chromosomes instead of the pair. Babies with trisomy 18 therefore have three copies of chromosome 18.

See this image for a picture of the chromosomes of a boy with T18.

Full Trisomy 18 (FT18)

Most babies with trisomy 18 have the 'full' variant. This means that they have an entire extra copy of chromosome 18 in every cell in their body. It is caused by an error in cell division at the time of conception.

Mosaic Trisomy 18 (MT18)

Some babies have mosaic trisomy 18. This means that they have an extra copy of chromosome 18 in only some of their body's cells; the remaining cells have the normal pair. This is caused by a error in cell division at a later point than conception.

The effect that mosaic T18 has on a child depends on how many cells are affected. In occasional cases it is an incidental finding in an otherwise healthy adult. Research shows that most children with MT18 do have learning difficulties on the mild-to-moderate spectrum, although some do have IQs in the average range. Children with MT18 may have a range of physical disabilities. It is unclear whether children with MT18 have a restricted life expectancy. It seems likely that it depends on the extent of their physical disabilities.

Partial Trisomy 18 (PT18)

Partial T18 is rarer than both full and mosaic trisomy 18, and is harder to understand.

Chromosomes have a short and a long arm. The short arm is labelled p; the long arm is labelled q.

People with partial trisomy 18 have a duplication of one part of the chromosome, so either p or q. Sometimes they may have a duplication of part of the long arm and&nbsp part of the short arm.

People with a duplication of the short arm of the chromosome (trisomy 18p) have less severe medical problems as a group than people with a duplication of the long arm of the chromosome (trisomy 18q).

It is also possible to have a duplication of only part&nbsp of the short or long arm of chromosome 18. For example, if you had a duplication of part of the short arm, you would be described as having partial trisomy 18p. Your difficulties may be fewer than someone with full trisomy 18p. However, both of you in the spectrum of trisomy 18 would be described as having partial trisomy 18.

Make sense? No? See here for a clearer explanation from people more knowledgeable than me.

The most important thing about partial trisomy 18 is that it can be passed from parent to child. Say a father has a balanced translocation. Part of his chromosome 18 has attached itself to another chromosome (let us say chromosome 4), but helpfully, in exchange, that chromosome (4) has attached part of itself to chromosome 18, so all is good, fair and balanced. The father doesn't have any problems: all his genetic material is present.

However, when he goes onto have a baby, his chromosome 4 (with its extra bit of chromosome 18 attached) is passed on to his child, but (as it happens) he passes on his 'normal' chromosome 18 and not the one with the extra bit of chromosome 4 attached. So now the child has two full chromosome 18s, and an extra bit of chromosome 18 attached to chromosome 4. The child is said to have an unbalanced translocation, which constitutes a form of partial T18.

Balanced and unbalanced translocations can happen randomly, so your child may be the first to have one. Your child can have partial trisomy 18 without it being inherited, but if your child has partial T18, both parents should be offered testing to see whether either of them do have a balanced translocation and to discuss the risks of PT18 to any future children.

So how do you know which kind of T18 your child has?

On CVS, amnio or post-birth blood results, it should be clear that your child has either:

• A partial copy of chromosome 18. You will then be offered genetic testing. Your child has partial trisomy 18.
• Three copies of chromosome 18 in some of the cells that were tested but not others. Your child has confirmed mosaic trisomy 18.
• Three copies of chromosome 18 in every cell tested. Your child has either full trisomy 18 (most likely) or mosaic trisomy 18.

It is impossible to exclude mosaicism by antenatal testing, as insufficient cells are tested. After birth, a full test can be performed and then you can say that it is highly likely that your child has full T18. However, as you cannot possibly test every cell in the body, you cannot absolutely rule out mosaicism in any child.