1q21.1 deletion syndrome happens when someone is missing piece of chromosome 1, one of the body’s 46 chromosomes. Chromosomes are structures in our cells that house our genes.

We inherit chromosomes from our parents. When the sperm from the father joins the egg from the mother, they form a single cell with 46 chromosomes — 23 from the mother and 23 from the father. This cell then makes many copies of itself.

Dominant inheritance

Children have a 50% chance of inheriting the genetic change.

Some people inherit a genetic change from a parent. In other people, small mistakes can occur when genes are being copied. Parts of the chromosomes can break off, make extra copies, or end up in a different order than expected. When this happens, it is called a “de novo”, or new, change. The child can be the first in the family to have the genetic change.

No parent causes their child’s 1q21.1 deletion syndrome. We know this because no parent has any control over the chromosome changes that they do or do not pass on to their children. Please keep in mind that nothing a parent does before or during the pregnancy causes this to happen. The chromosome change takes place on its own and cannot be predicted or stopped.

Each family is different. A geneticist or genetic counselor can give you advice on the chance that this will happen again in your family.

The risk of having another child who has 1q21.1 deletion syndrome depends on the chromosomes of both birth parents.

• If neither birth parent has the same chromosome change found in their child, the chance of having another child who has the syndrome is on average 1 percent. This 1 percent chance is higher than the chance of the general population. The increase in risk is due to the very unlikely chance that more of the mother’s egg cells or the father’s sperm cells carry the same change in the gene.
• If one birth parent has the same chromosome change found in their child, the chance of having another child who has the syndrome is about 50 percent.

For a symptom-free sibling, a brother or sister, of someone who has 1q21.1 deletion syndrome, the risk of having a child who has the syndrome depends on the symptom-free sibling’s genes and their parents’ genes.

• If neither parent has the same chromosome change found in their child who has the syndrome, the symptom-free sibling has a nearly 0 percent chance of having a child who has 1q21.1 deletion syndrome.
• If one birth parent has the same chromosome change found in their child who has the syndrome, the symptom-free sibling has a small chance of also having the same chromosome change.
• If the symptom-free sibling has the same chromosome change as the child who has the syndrome, the symptom-free sibling’s chance of having a child who has 1q21.1 deletion syndrome is about 50 percent.

For a person who has 1q21.1 deletion syndrome, the risk of having a child who has the syndrome is about 50 percent.

Not necessarily. Some people do not have any symptoms. Some people may not learn that they have this genetic change until it is found in their children.

The content in this guide comes from published studies on 1q21.1 deletion syndrome. Below you can find details about each study, as well as links to summaries or, in some cases, the full article.

• Bernier R. et al. Genetics in Medicine: Official Journal of the American College of Medical Genetics, 18, 341-349, (2016). Clinical phenotype of the recurrent 1q21.1 copy-number variant www.nature.com/articles/gim201578
• Buse M. et al. Italian Journal of Pediatrics, 43, 61, (2017). Expanding the phenotype of reciprocal 1q21.1 deletions and duplications: a case series www.ncbi.nlm.nih.gov/pmc/articles/PMC5518118