Genetics question on genetic dwarfism: Seven months pregnant, an expectant mothe

Question

Genetics question on genetic dwarfism:

Seven months pregnant, an expectant mother was undergoing a routine ultrasound. While prior tests had been normal, this one showed that the limbs of the fetus were unusually short. The doctor suspected that the baby might have a genetic form of dwarfism called achondroplasia. He told her that the disorder was due to an autosomal dominant mutation and occurred with a frequency of about 1 in 25,000 births. The expectant mother had studied genetics in college and immediately raised several questions. How would you answer them?

1. How could her baby have a dominantly inherited disorder if there was no history of this condition on either side of the family?

2. Is the mutation more likely to have come from the mother or the father?

3. If this child has achondroplasia, is there an increased chance that their next child would also have this disorder?

4. Could this disorder have been caused by X rays or ultrasounds she had earlier in pregnancy?

Thank you!

Explanation / Answer

1. The spontaneous mutation in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene is responsible for causing achondroplasia. In normal development FGFR3 has a negative regulatory effect on bone growth. In case of the disease, the mutated form of the gene is constitutively active and leads to severally shortened bones. The effect is genetically dominant, with one mutated copy of the gene is sufficient to cause achondroplasia. By this, her baby have a dominantly inherited disorder if there was no history of this condition on either side of the family.

2. Studies have demonstrated that, new gene mutations for achondroplasia are exclusively inherited from the father, which occurs during sparmatogenesis. Oogenesis has some regulatory mechanism which prevents the mutation to pass through the ovum. So, the mutation is more likely to have come from the father.

3. Yes, if the child has achondroplasia, there is an increased risk that their next child would also have this disorder. A person with achondroplasia has 50% chance of passing the mutated allele of dwarfism to each of their offspring.

4. As the mutation, responsible for the disorder is spontaneous and is not due to the mutation in ovum genes so we can say that this disorder could have not been caused by X rays or ultrasounds the mother had earlier in pregnancy.

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