Question

Jes is planning to get pregnant using sperm from a sperm bank. The sperm bank does genetic carrier testing on all donors as a policy. Her [anonymous] donor is a carrier for Maple Syrup Urine Disease (MSUD). MSUD is a severe recessive metabolic disorder, with a general population carrier frequency of 1/80.

A. What is the chance that Jes is a carrier for MSUD?

B. Assuming that she is not a carrier, what is the chance that her baby will have MSUD?

C. Assuming that she is a carrier, what is the chance that her baby will have MSUD?

D. Would you advise Jes to get genetic carrier testing (cost is $100 out of pocket) before she purchases this sperm? Why or why not?

Answer 1

MSUD is a severe recessive metabolic disorder and it has general population carrier frequency as 1/80.

A. Jes is also a part of the population therefore she has 1/80 chance of being a carrier for MSUD.

B. Let the normal allele be M and MSUD allele be m. Therefore, MM will be normal, Mm will carrier and mm will be MSUD affected genotype. If Jes is not a carrier then her genotype will be MM and we know that the sperm donor is the carrier therefore the male genotype is Mm. When we cross MM x Mm we get the following punnett square -

	M	M
M	MM (normal)	MM (normal)
m	Mm (carrier)	Mm (carrier)

Thus if Jes not a carrier (MM) then there is 0% chance that her baby will be affected but 50% chance that her baby will be a carrier.

C. If she is assumed to be a carrier the her genotype will be Mm and the male genotype is known to be Mm. When we cross Mm x Mm we get the following punnett square -

	M	m
M	MM (normal)	Mm (carrier)
m	Mm (carrier)	mm (affected)

Thus there is 25% chance that her baby will be affected by MSUD if she is a carrier.

D. There is 1/80 chance that she is a carrier for MSUD and there is 1/4 (25%) chance that her baby will be affected if she is carrier. Therefore, overall chance of her baby getting affected is 1/80 * 1/ 4 = 1 / 320 0.3 %
So there is still 0.3% chance of her baby being affected if she ignores the tests and goes for the sperm. Knowing that MSUD is a severe disease she should opt for genetic carrier testing as a precautionary measure. If she is found out to be a carrier then there is 25% chance of her baby being affected, hence it would be better she avoid purchasing the sperm if she is found to be a carrier.

Hope this helped!