Tay-Sachs disease is an autosomal recessive disorder caused by a mutation in the HEXA gene that results in progressive deterioration of the nervous system. Affected individuals usually die by age 4. Let D represent the wildtype allele and d the disease-causing allele. What are the three genotypes and their respective phenotypes (i. e., affected or unaffected) ?

DD:Dd:dd:

DD: Unaffected

Dd: Unaffected

dd: affected (diseased)

Explanation:

Tay-Sachs disease is an autosomal recessive disorder as mentioned in the question.

Autosomal recessive disorders are those in which the allele causing the disease rests on autosomes (chromosomes other than sex chromosomes) and are only expressed when both disease causing alleles are present in the genotype (i. e. homozygous for disease causing allele).

If D is the wildtype allele (i. e. codes for normally functioning phenotype) and d is the disease causing allele then an individual will have Tay-Sach's disease when both disease causing alleles are present in the genotype i. e. if the genotype is dd. If the individual has one wild-type and one disease-causing allele i. e. Dd then he will be a carrier for the disease but will not have Tay-Sach's disease as it is an autosomal recessive disease and such diseases require both alleles to be present.

If an individual is DD then he will be homozygous for the wild-type allele and will be unaffected. He will not be a carrier for the disease as he does not possess the disease-causing allele.

Therefore, the following genotypes will have the following phenotypes.

DD: Unaffected

Dd: Unaffected

dd: affected (diseased)