Eric and Janelle are carriers of the recessive allele causing Tay-Sachs disease which is lethal in early childhood.

Question

Eric and Janelle are carriers of the recessive allele causing Tay-Sachs disease which is lethal in early childhood. If they have a healthy child what is the probability that this child will not be a carrier of Tay-Sachs disease 

Answer 1

Since both Eric and Janelle are carriers of a recessive allele, (let's call the healthy allele 'X' and the Tay-Sachs allele 'x') their genotypes are both Xx. Doing a punnet square, we can see that the distribution for offspring is %25 XX, 50% Xx, 25% xx. However, since Tay-Sachs is lethal in early childhood, we can assume that a healthy child is not xx. Since based on our Punnett square, their child is twice as likely to be a carrier (Xx) than a healthy non-carrier (XX), we can say there is a 33.333% chance that the child is not a carrier of Tay-Sachs.