MadSci Network: Genetics

Re: where can i find a punett square for tay-sachs disease?

Date: Wed Dec 13 13:42:14 2000
Posted By: Jeff Buzby, Research Scientist, Molecular Immunology
Area of science: Genetics
ID: 976597199.Ge

Dear Nick,

I found the following example for heterozygous parents in a Biology Take-home Assignment from the Univ. of Illinois @ Urbana-Champaign.

Tay-sachs disease is an autosomal recessive disorder that results in a degeneration of the nervous system, producing symptoms at about 6 months of age. . Affected children loose hearing, sight, and the ability to move. Children with Tay-Sachs disease normally die by age 3. The normal (dominant) allele is responsible for producing an enzyme that breaks down fatty deposits around nerve cells. If the enzyme is missing, the nervous system is bound up by excess fatty deposits.

A couple are interested in having a child, but have learned that they are both carriers for the disease. What is the chance that their first child will have Tay-Sachs disease? Show how your arrived at your answer.


Both people are carriers for Tay-Sachs disease. That means that they are both heterozygous (Tt). As a result of meiosis, the man can produce sperm with either a dominant allele (T) OR a recessive allele (t) in a 50/50 ratio.

The woman's eggs will have the same genetic make up in the same ratio.

The Woman's Eggs

T t
The Man's Sperm T TT Tt
t Tt tt

The genotypes of the possible children are represented by the red and blue letters in the boxes of the Punnett square.

Homozygous dominant individuals do not have the disease and can not pass it on to their children.

Heterozygous individuals do not have the disease, but can pass the recessive allele to their children.

Homozygous recessive individuals have the disease.

In this case, there is a one in four, or 25% chance the first child will have Tay-Sachs disease.

You can make the appropriate changes to generate a Punnett square for the other possible mating, TT x Tt, which you should find would not produce any diseased offspring.  Although crosses of tt genotypes are theoretically possible, they could not occur in practice since the homozygous recessive, diseased individuals do not survive to reproduce.

Good luck with your report,


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