Tay-Sachs disease is an inherited disorder that mainly affects infants after their third month after birth. This disease affects the central nervous system by destroying nerve cells in the brain and the spinal cord. This results in the loss of motor skills meaning the infant is not able to turn over, support their neck, crawl or even sit down. The condition worsens as the infant grows with seizures, loss of hearing and vision and general paralysis with intellectual disability setting in.
Tay-Sachs is a very rare condition often found among people with Jewish heritage but some cases have been documented in Quebec, Pennsylvania and Louisiana. This condition results from mutations in the HEXA gene. This is the gene that is responsible for the coding of a particular enzyme beta-hexosaminidase A found in lysosomes of spinal and brain nerve cells. This enzyme plays a role in the breakdown of fatty toxins called GM2 ganglioside. Mutation of this gene results in impaired production of B-hexo A which in turn results In the accumulation of the ganglioside inside the nerve cells. The accumulation results in the damage of the affected nerve cells and is responsible for the symptoms associated with Tay-Sachs disease (Genetics Home Reference).
Inheritance of the mutation responsible for Tay-Sachs disease occurs in an autosomal recessive pattern. This implies that both parents are carriers of the mutation in a heterozygous manner. If the parents gave birth to four children, only one would suffer from this condition with two others being carriers and one absolutely normal. With this information, I would advise the couple to make an effort of getting another child. However, the fetus would have to get gene mapping done intrauterine to prevent propagation of the mutation to future generations.
Genetics Home Reference. 11 January 2016. 16 January 2916. Retrieved from http://ghr.nlm.nih.gov/condition/tay-sachs-disease.