Also known as heart-hand syndrome this condition has a lot of different permutations. People with HOS often have upper limb growth abnormalities which can range from being very minor to having parts of arms / fingers / thumbs missing. Often the collar bone is also quite small. In about 75% of cases the heart is also affected, this can be in a variety of ways, ranging from normal function to VSD / ASD, conduction disorders, holes in the heart etc.
Holt-Oram is listed as a rare disease and is estimated to affect 1 in 100000 individuals. HOS is caused by a mutation of the TBX5 gene, this mutation can occur in people with no history of the mutation in their family. HOS is an autosomal dominant disorder which means that only one copy of the mutated gene needs to be passed on in order for a child to inherit the condition, this makes it a 50% chance that the condition will be passed on to children.

This information has not been put together by a health professional, this is simply my understanding of the condition I have. If you feel that there are mistakes in what I have written please let me know and if you want more detailed information please see one of the links.