Question

1. Tay-Sachs is an autosomal recessive disorder, which is the inability for infants to breakdown gangliosides in the CNS, which overtime build-up and destroy neurons. As the disease progresses, the child loses muscle control. Eventually, this leads to blindness, paralysis and death. If you have a family history of Tay-Sachs disease or if you're a member of a high-risk group and plan to have children, doctors strongly recommend genetic testing and genetic counseling. Scott and Tina are a young couple who plan to start a family, but Tina's family has had a couple of family members that have had children that suffered from Tay-Sachs. As a matter of fact, Tina's brother Ray has had a Tay-Sachs child as did Tina's great-grandmother. Tina's sister, Kim, has three children and known of her children have Tay-Sachs. Tina is very concerned and comes to you, a genetic counselor to ask for help. Using the above scenario answer the following questions.

A. Create a complete pedigree diagram using the above information. Underneath each symbol, give the genotype. Use the symbols T and t to represent the allele. If an individual can be either homozygous dominant or heterozygous use the symbol T/-. You do not need to give the genotypes of Kim and her three children, but you should give the genotype of Scott's parents. Remember, use all given information. Draw out the pedigree using the appropriate symbols.

B. What is the assumption of all the people who marry into the family, but are not shown in the pedigree? (21 Points)

C. Indicate the proband. (4 Points)

D. Is there any irrelevant information in the problem as stated? If so, what is it? If not, state there is no irrelevant information. (5 Points)

E. What is the probability that Scott and Tina first child will have Tay-Sachs? Remember, you need to determine the probability that Scott, Tina and both Scott and Tina could pass on the genetic trait. (6 Points)

Answer 1

Tay Sachs disease is an autosomal recessive disease and any person in the pedigree with homozygous recessive alleles will get the disease (tt).  

ㄒㄤ 下ヒ (fkoBAN b鬮ㄧㄒㄧ-O ㄤㄤ 了 kin ㄒㄧ- T)- "TI-

Since tina's great grand mother had a child with the disease so the great grand mother and great grand father both must be heterozygous for the disease. The grand father will be the affected child and so the grand mother could be either heterozygous or homozygous dominant for the disease.

Since Kim has no child with the disease so kim's husband must be homozygous dominant whereas Kim should be homozygous dominant or heterozygous. Kim's mother could be homozygous dominant or heterozygous.

Since Ray has a child with the disease so ray and his wife both should be heterozygous. Tina could be either homozygous dominant or heterozygous for the disease. Since nothing is mentioned about Scott's family so Scott and his parents could be either heterozygous or homozygous dominant for the disease.

C) Proband is usually that member of the family who seeks medical attention for the disease in the family for the first time. So in this case it will be the Grandfather of Tina as he was the first ancestor in the family to be diagnosed for the disease.

D) No irrelevant information has been given for the following pedigree.

E) If Scott is homozygous dominant and Tina is also homozygous dominant then the probability of first child being affected will be zero.

rua

If Scott is homozygous dominant but Tina is heterozygous then also the probability of first child being affected is zero.

Suo ya

If Scott is heterozygous and Tina is homozygous dominant then again the probability will be zero.

七

But, if Scott and Tina are both heterozygous then the probability of the first child being affected is 1/4.

Suotr ) ha も17t L4