- A heterozygous organism contains one dominant gene and one recessive gene
- A tay-sachs disease carrier have one dominant and one recessive tay-sachs gene
- So, a tay-sachs disease carrier is heterozygous

- A tay-sachs disease carrier is heterozygous
- Crossing two heterozygous dominant organisms causes their offspring to be homozygous recessive of 25% probability
- Therefore, the offspring of two tay-sachs carriers will have 25% probability to be homozygous recessive in tay-sachs disease

- The offspring of two tay-sachs carriers will have 25% probability to be homozygous recessive in tay-sachs disease
- A homozygous recessive organism contains only recessive genes
- Hence, the offspring of two tay-sachs carriers will have 25% probability to contain only recessive genes for tay-sachs disease

- The offspring of two tay-sachs carriers will have 25% probability to contain only recessive genes for tay-sachs disease
- If only recessive genes are present , then the recessive trait will be visible / expressed
- Therefore, the offspring of two tay-sachs carriers will have 25% probability to have the tay-sachs disease

So, the offspring of two tay-sachs carriers will have 25% probability to have the tay-sachs disease
Question
Tay-Sachs disease is a recessive genetic disorder. If only one gene is present, an individual is a carrier. What is the probability of two healthy Tay-Sachs carriers having a child that has the disease?

Context
- A homozygous recessive organism contains only recessive genes 
- Crossing two heterozygous dominant organisms causes their offspring to be homozygous recessive of 25% probability 
- A heterozygous organism contains one dominant gene and one recessive gene 
- If only recessive genes are present , then the recessive trait will be visible / expressed 
- A tay-sachs disease carrier have one dominant and one recessive tay-sachs gene