Final answer:
The false statement regarding familial Down syndrome is that if one parent is a translocation heterozygote, there is a 50/50 chance of having a normal child or a child with Down syndrome. The risk varies and is not a simple 50/50 chance.
Step-by-step explanation:
The statement regarding familial Down syndrome that is false is:
d. If one parent is a translocation heterozygote for chromosomes 14/21, a child born to this parent is as likely to have Down syndrome as he is to be normal.
In familial Down syndrome, which accounts for about 5% of all Down syndrome cases, the condition is often due to a Robertsonian translocation, typically involving chromosome 21 and another acrocentric chromosome like chromosome 14. Parents who are heterozygous carriers of the translocation can pass the translocation on to their children, who may then either be carriers themselves or, if they inherit an extra translocated chromosome 21, be affected by Down syndrome. However, the risk of having a child with Down syndrome does not equate to a 50/50 chance as implied by statement d; it varies depending on the specific translocation and which parent is the carrier.
Additionally, the risk of bearing a child with Down syndrome due to nondisjunction, which is the most common cause, increases in females over 36. This is primarily due to the aging of oocytes, as the likelihood of nondisjunction in sperm is considerably lower. It is important to note that lifestyle factors also play a role in the associated risk of Down syndrome, though age is the primary factor.