Final answer:
Individuals with familial Down syndrome are trisomic, meaning they have three copies of chromosome 21 and 47 chromosomes in total, which contributes to characteristic physical features and developmental delays.
Step-by-step explanation:
Individuals with familial Down syndrome have an extra copy of chromosome 21, making them trisomic. Trisomy 21 is the most common trisomy among viable births and is also known as Down syndrome. These individuals have 47 total chromosomes instead of the usual 46.
The characteristic feature of individuals with familial Down syndrome is that they are trisomic. This means they have an extra copy of chromosome 21, making a total of three copies instead of the usual two. Therefore, people with Down syndrome have 47 chromosomes in total, one more than the typical 46 found in humans. This extra genetic material affects physical and cognitive development, often leading to characteristic facial features, developmental delays, and various health issues.