Question

Imagine a genetic counselor working with a couple who have just had a child who is suffering from Tay-Sachs disease. Neither parent has Tay-Sachs, nor does anyone in their families. Which of the following statements should this counselor make to this couple? Oa. "Because no one in either of your families has Tay-Sachs, you are not likely to have another baby with Tay-Sachs. You can safely have another child." b. "Because you have had one child with Tay-Sachs, you must each carry the allele. Any child you have has a 50% chance of having the disease C. "Because you have had one child with Tay-Sachs, you must each carry the allele. Any child you have has a 25% chance of having the disease. d. "Because you have had one child with Tay-Sachs, you must both carry the allele. However, since the chance of having an affected child is 25%, you may safely have three more children without worrying about having another child with Tay-Sachs."

Answer 1

Answer:

C. "Because you have had one child with Tay-Sachs, you must each carry the allele. Any child you have has a 25% chance of having the disease

Explanation:

Tay-Sachs disease is an autosomal recessive disorder that means a child will acquire the phenotype or we can say disease only when both the alleles which cause disease are present in the child.

Here, in this condition neither parent has Tay-Sachs but they gave birth to a child who is suffering from the disease then it simply means that both the parents are carrier of this disease i.e. they both are heterozygous.

The cross and probability of healthy and diseased child is shown as under:

                          Mother        Father

Parents                    Tt               Tt

                              /        |          |     \

Off-springs         TT      Tt       Tt      tt

Out of these 4 probable off-springs only 3 with genotype TT, Tt & Tt  will be healthy while the 1 with genotype tt will be diseased. So, the probability of diseased child is 1/4 which is 25%.

Also, the birth of children is an independent event so if one child has already been born with this disease then it doesn't mean that the next 3 children will be healthy for sure i.e. birth of first child cannot influence the next child. In short we can also say that in the next birth, there is still 25% chance of the child being born with this disease because previous birth has nothing to do with next birth.