Which of the following statements regarding familial Down sy

Which of the following statements regarding familial Down syndrome is false? Heterozygous carriers of the Robertsonian translocation that causes familial Down syndrome can have children who are phenotypically normal and are not carriers of the translocation. Heterozygous carriers of the Robertsonian translocation that cause familial Down syndrome can have children who are also carriers of the translocation. It accounts for approximately 5% of all Down syndrome cases. 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.

Solution

Whenever we need to answer a question about wrong option, we eliminate the ones which are right. Robertsonian translocation is the translocation of chromosomes between chromosomes 13, 14, 15 and 21. Among these, a translocation between chromosome 14 and 21 may lead to familial Down syndrome (extra 21 chhromosome material). However, if one parent is translocation heterozygote for chromosome 14/21, there is still chance that the children will be normal and not carriers of translocation because they may inherit the normal chromosome from the parent. So option a is correct. Rarely, if both heterozygous carriers contain the same translocation, then the children may appear normal even after having translocations because they inherit the similar translocations from their parents. So, option b is conditionally right. Familial Down syndrome accounts for 4.5% of all Down syndrome cases. So, option c is correct. Now if one parent is translocation heterozygote for chromosome 14/21, then depending on the parent gender, child born to the parent will have different chances of having Down syndrome. The result is a 15% chance of having down syndrome when mother is affected and less than 5% chance when father is affected. Therefore, the chances of child to be normal is higher than the chances of him being affectted by Down syndrome. So, option d or the last option is wrong.

As we were asked about the false option, the d or the last option is the answer.