Question

Mike and Margaret are expecting their first child. They are Ashkenazi Jews and Margaret’s OB recommended they get tested to see if they are carriers for genetic disease causing mutations that have a high frequency in the Ashkenazi Jewish population. The results are that Mike is heterozygous for a mutation in HexB while Margaret is heterozygous for a mutation in HexA. Should they worry about their child? Explain why or why not.

A couple has a child with Down’s syndrome. Genetic testing reveals that the child’s mother is a carrier of a Robertsonian translocation between chromosomes 14 and 21. How can this explain the incidence of Down’s in her child? If the couple wants to have another child what are their chances of having a normal, healthy baby?

Answer 1

Hex A and Hex B gene products are involved in synthesizing enzyme Beta- hexosaminidase A. This enzyme is found in lysosomes. It has 2 subunits. one alpha subunit and two beta subunit. Alpha subunit is encoded by Hex A gene and beta subunit is encoded by Hex B gene.When there is mutation in Hex A gene it causes Tay-sachs disease. When there is mutation in HexB gene it causes Sandhoff disease.

But both diseases are AUTOSOMAL RECESSIVE DISORDERS. When both the copies of gene are mutated then only it will cause disease. So basically Margaret is carrier for Tay- sachs disease and martin is carrier for Sandhoff disease.

Tay- sachs disease probability in their children will be 0. But 1/4 carrier and 3/4 children will be normal.

Sandhoff disease probability in their children will be 0. But 1/4 will be carrier and 3/4 will be normal.

Probability of having carrier for both disease will be 1/16.

So chances of having normal children is 3/4 and diseased is 0. So they do not need worry much more. Chances of carrier are there but it will not be diseased condition.

Q.2 As mother is a carrier has a robertsonian translocation for chromosome number 14 and 21. Genetic make up of her gamete is translocated 14/21. Father is having normal chromosome 21. When these 2 are fused together they will create a zygote containing tranlocated 14/21 + 21. So it will be an offspring with down syndrome. So this couple if they want to have another child as female gamete are affected, her offsprings will suffer from Down syndrome. Chances of having healthy baby are extremely less.