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Inherited Disorders and Birth Defects

Syndactyly

07/05/2006

Question:

My daughter has the second and thrid toe on both of her feet joined up to the first knuckle. My dad`s is joined up to the toenail. I do not have this trait. My dad`s brother and nine sisters had the same trait. He has a nephew with this trait.  Neither one of his children has it, but his grandaughter does. Is it normal to skip a generation? Both my cousin and I, have sons but neither of them have syndactly. We have been told that it was a sign of inbreeding. Is this true? I know that my grandparents were first cousins and that one set of their parents was first cousins. Any help would be appreciated.

Answer:

Syndactyly or webbing of the fingers and toes is a very common finding which can be a normal variation or minor anomaly. If the webbing only goes about a third of the way up between the toes, it is said be a variation of normal. If the webbing goes further than this – it is said to be a minor abnormality. Syndactyly can be sporadic - that is, it happened just in one child in a family, is not inherited from either parent and geneticists do not know why it happened. It can also run in families – where a parent has a gene that codes for syndactyly and can pass that gene on to any of his or her children. Finally, syndactyly can also be seen in many genetic disorders, but these children have multiple problems, not just syndactyly.

In your case, it sounds like your family has a gene that codes for syndactyly and is probably inherited in what is called autosomal dominant inheritance. For all of our genes – there are 2 copies of each. Autosomal dominant inheritance describes a trait or disorder in someone who has a gene mutation (or change) in one of the two copies for that specific gene. That gene mutation specifically refers to a gene on one of the 22 pairs of autosomes (non-sex chromosomes).

Anyone with an autosomal dominant inherited trait or disorder has a 50-50 chance of passing that gene on to any child that person has. Autosomal dominant traits can be seen in both males and females equally. Usually, when someone has this gene mutation for a trait, such as syndactyly, you see it in multiple generations. However, some genes are not 100% penetrant. Penetrance is the proportion of people with a gene mutation causing a particular trait or disorder who actually show the trait.

In your case, you most likely have the gene for syndactyly, but because the gene is not 100% penetrant, you do not show it. This is why the gene may look like it has “skipped” a generation. Syndactyly is not a sign of inbreeding. However, if there are family members who are related, there may be an increased chance that a gene that codes for syndactyly will be seen more frequently in that family.

Related Resources:

National Society of Genetic Counselors Resource Center

For more information:

Go to the Inherited Disorders and Birth Defects health topic, where you can:

Response by:

Anne   Matthews, RN, PhD Anne Matthews, RN, PhD
Associate Professor of Genetics
School of Medicine
Case Western Reserve University