Final answer:
Familial Down syndrome can be caused by a translocation involving chromosome 21, where part of it gets attached to another chromosome. This can result in a hereditary form of Down syndrome, which can recur in families and is not associated with maternal age.
Step-by-step explanation:
Cause of Familial Down Syndrome
A cause of familial Down syndrome is a translocation involving chromosome 21, specifically when part of chromosome 21 is translocated onto another chromosome. Trisomy 21 is when there are three copies of chromosome 21 and is the primary cause of Down syndrome, typically due to nondisjunction during meiosis.
However, familial Down syndrome may also occur when there is a translocation between chromosome 21 and another chromosome, such as chromosome 14. This type of Down syndrome is less common but can be hereditary.
Down syndrome is characterized by various degrees of physical and intellectual disabilities, as well as distinctive facial features. The occurrence of Down syndrome due to nondisjunction increases with maternal age, especially in women over 36, because nondisjunction events are more frequent in eggs than in sperm. In contrast, translocation-related Down syndrome does not have this age-related association and can recur in families.