The Term "Tay-Sachs Disease"
The term "Tay-Sachs disease" refers to a hereditary condition caused by the death of nerve cells in the brain and spinal cord. In specifically, a gene mutation in the HEXA genes, which are located on chromosome 15, causes Tay-Sach illness (Hussein, Weng, Kai, Kleijnen & Qureshi, 2015).
Autosomal Recessive Inheritance
An autosomal recessive mode of inheritance means that a person gets the illness from his or her parents. The genetic mutation disrupts the function of the fatty substance-breaking enzyme beta-hexosaminidase, which leads to an accumulation of the GM2 ganglioside molecule inside cells and ultimately to toxicity that harms central nervous system cells. To have a child diagnosed with the disease, a mutated copy of the gene must be included in his DNA from both parents. Hence, assuming that the disease affects only the homozygous recessive individuals, for an individual to get affected, he or she must have two recessive alleles got from each parent (Mistri et al., 2012).
Punnet Square Analysis
To determine the chances of a child having the Tay-Sachs disease given that it only affects homozygous recessive individuals and both a mother and a father are carriers (Tt, heterozygous) of the disease, a Punnet Square is utilized as below:
Male Parent
T
t
Female Parent
T
TT
Tt
t
Tt
tt
Interpretation:
T - other trait
t - recessive trait
TT - children who do not get the recessive gene from either parent (1 out of 4)
Tt - children who are carriers of the recessive gene (2 out of 4)
tt - children with the Tay-Sachs disease (1 out of 4)
Based on the above results, the chances of the child having Tay-Sachs disease is 25% since the probability of a child getting a recessive gene from each parent is one out of four pregnancies. On the other hand, the probability of the child not getting any recessive gene from the parents is also 25%, while the probability of getting only one recessive gene from either of the parents is 50%, indicating that two children out of four pregnancies will be carriers of the recessive gene.
References
Hussein, N., Weng, S.F., Kai, J., Kleijnen, J., & Qureshi, N. (2015). Preconception risk assessment for thalassaemia, sickle cell disease, cystic fibrosis and Tay‐Sachs disease. The Cochrane Database of Systematic Reviews, 12(8):CD010849.
Mistri, M., Tamhankar, P.M., Sheth, F., Sanghavi, D., Kondurkar, P., Patil, S., … & Sheth, J. (2012). Identification of novel mutations in HEXA gene in children affected with Tay Sachs disease from India. PloS One, 7(6), e39122.
