Question

I have an issue understanding maternally and paternally imprinted gene questions.

Here are some examples:

1. A phenotypically normal man named John is heterozygous for a deletion on chromosome 15 that contains the maternally imprinted gene SNPRN gene. Absence of SNPRN gene function results in Prader-Willi syndrome. The man marries a normal woman named Jane who does not carry the deletion. They have 2 children. Which of the following statements is FALSE?

A. John inherited the chromosome with the deletion from his mother

B. There is a 25% chance that both children will have Prader-Willi syndrome

C. Half of John's sperm contain a version of the SNPRN gene that would be expressed in his children

D. All of Jane's eggs contain a version of the SNRPN gene that would be expressed in her children

E. in Jane's somatic cells, there is one active copy of the SNRPN gene.

The answer is D, but how are choices B and E true? I'm in a genetics course at a university so being detailed would be helpful please.

2. The Igf2 gene in mice encodes a growth factor gene whose expression is necessary for normal size. The absence of the Igf2 gene products results in small mice. The Igf2 gene is a maternally imprinted gene, meaning it gets inactivated during oogenesis in females. A normal-sized female mouse heterozygous for a deletion of the Igf2 gene gives birth to a normal-sized male. This male is then crossed to a normal female (no deletion). Assuming that this male inherits the Igf2 deletion from his mother, what fraction of the progeny from this male will be small?

A. 1/4 B.0 C.1/2 D.1/3 E.1

The answer is C, but again I don't understand why.

Answer 1

1

B.-- is also true statement. Because the statement is for both children to be Prader willi syndrome , not probability of any child expressing the trait. When father is having the deletion allele, the deletion will express in offspring. In that case 50% of the probability that any child can suffer from the condition. But both children suffering from condition is the product of the probability. Which means 1/2 probability of first child affected by Prader-willi syndrome, 1/2 probability for even 2nd child. Both being Peter -willi is the product of both events which means, it is 1/4 or 25%.

E-- is a true statement because the Jane has received one copy of the gene from mother and one from the father. In maternal imprinting, mother's allele is silenced, so it does not express. Father's allele is expressed. Since Jane is normal, fathers allele must have got inherited and expressing, that is why there is one active copy of the gene in Jane.

2. Maternal imprinting is silencing allele of the maternal origin. The allele from maternal origin gets silenced in the oogenesis.

Igf2 gene encoding growth factor ( N) makes nice normal in size. A mutation of the gene (n) makes mice small.

Normal female mice heterozygous for deletion mutation (Nn), gives birth to normal sized male with deletion from mother. The mice is normal because mothers allele is imprinted , which means deletion allele (n) is imprinted. So the male born has deletion but it is imprinted which he got from mother. Another normal allele (N) he has got from father.

This male mice crossed to normal female. Chance of the progeny being small with deletion allele is---

So the probability of the offspring being small is 1/2.