Genomic Imprinting In Humans (and classical genetics)

[stevoevo22]

Hey everyone!

I'm a bit confused about something I'm studying at the moment: Genomic Imprinting.
I know it's parent-specific gene expression, but my question is, what is the difference between it and normal expression?

eg//ALBINISM
AA-Normal pigmentation
Aa-Carrier but no albino phenotype
aa-Albinism

In genomic imprinting, it says that one allele is masked by the other one, but isn't that the same as the "Aa" genotype for albinism? And isn't that the case for most genes (that the dominant one masks the recessive one?)

Thanks in advance for your replies!
Steven

[Jesse2504]

Steven, you are right that dominant alleles mask recessive ones in normal expression, but with genomic imprinting because it is parental specific it ignores or masks the other.

For example if you had a dominant allele 'A' from the mother and recessive allele 'a' from the father and genomic imprinting occurred to favor the paternal allele, the recessive allele would mask the dominant which does not occur in normal expression. In this case you do not need to be homozygous for a recessive gene to be able to express it, as the dominant allele is masked due to parental specific gene expression.

[kolean]

It is a form of epigenetics, but in a sexual way . . .well paternal or maternal actually.
Here is a great website to check out for genomic imprinting:
http://www.geneimprint.com/site/home
I also wanted to point out that there is alot of maternal mRNA that get expressed upon fertilization, that can have a profound impact on the epimorphology of the genome (phenotype expression). Especially during the beginnings of development.

[littlemonkey]

After looking Angelman and Prader-Willi syndrome up by Wikipedia, I have a question. The paternal gene is silent, which made by imprinting effects. When the offspring gets a abnormal chromosome from its father, why dose it express different phenotype ( normal one have silent allele, abnormal person doesn't have that allele). I 'am sory about my English, I have just practiced to use it.

[kolean]

When the offspring gets a abnormal chromosome from its father

It says siblings (sister or brother) may have a chance of getting it or not getting it, depending on why there is a deletion (sporadic mutation in one sperm, as oppose to translocation thru meiosis and all sperm will have it).

But it does not talk about offspring, as one of the symptoms is infertility as an adult, thus no offspring.

[littlemonkey]

I mean that what is different between a silent allele and not having that allele.

[Darby]

As I understand it, imprinting is a chemical change in the DNA in the allele - methylation of some sort - that is potentially reversible. It affects expression of the allele but not the coding itself.

[kolean]

Methylation of histones. The allele is wrapped around the histones, and when their tails are methylated, the histones stay closely condense and do not let any transcription factors bind to their elements for transcription. So you can have the allele, but it won't be expressed: a type of constant repression.
http://www.ncbi.nlm.nih.gov/pubmed/1922 ... d_RVDocSum