Final answer:
Individuals with familial Down syndrome have a unique form of the condition where part of chromosome 21 is translocated to another chromosome. They usually have 46 chromosomes in total, but with altered gene dosage due to the extra chromosome 21 material, leading to characteristic symptoms of Down syndrome.
Step-by-step explanation:
Down Syndrome and Chromosomal Translocation
Individuals with familial Down syndrome carry an abnormal chromosome complement due to a translocation. This means that part of chromosome 21 breaks off and reattaches to another chromosome. As such, these individuals still have the genetic material that causes Down syndrome, but instead of having three full copies of chromosome 21 (as in the more common Trisomy 21 form), part of one chromosome 21 is attached to another chromosome. They typically have a total of 46 chromosomes, including two normal copies of chromosome 21 and the additional chromosome 21 material attached elsewhere, leading to an altered gene dosage. This can result in characteristic physical features such as a flat facial profile and upward slanting eyes, as well as developmental delays and various health issues. However, the symptoms and overall health can vary widely among individuals. Familial Down syndrome differentiates from the non-familial form because it has a hereditary component and can be passed down from parent to child.