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It's normal in haploid organisms, that is in prokaryots, animals sex cells and gametophyte of plants (for flowering plants it's only the pollen and egg cell again).
Would happen, that if you got wrong version or your good version was damaged in some way during your life, you would have no correct enzyme to provide it's function
Cis or trans? That's what matters.
Only one allele is "expressed" during genomic imprinting: the specific parental allele. This happens during embryogenesis usually, and is needed for proper development of the organism.
Genetic diversity is why diploids enjoy having two alleles. And in case if one allele is not good, then hopefully the other can compensate for it.
only a small number of genes show imprinting, so don't generalize that phenomenon
Also, it's a very simple thing to realize that double the alleles means double the product. If you have two perfectly good alleles and you lose one, you are left with one good allele. Usually half of the protein is all you need, so that's not a such a huge problem. However, there is such a thing as halpoinsufficiency, meaning that half of it just isn't enough to do the job
"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter
I was only commenting on the last question asked:
When is it normal to have only one allele?
And genomic imprinting is when it is normal. And again that is normal for the animal kingdom, especially humans. I guess I should point out that there really is two alleles, it is just only one is being expressed, while the other is silenced.
There are other considerations that need to be made..
For X-chromosome alleles in humans and most other diploids, only one allele is generally necessary for expression; only one X-chromosome is active, the other becomes a barr body.
Both alleles are generally important for diversity of protein expression. Proteins may interact differently based upon an allele's composition, thus allowing for phenotypic diversity (this is important in, for example, immunity).
However, in a lot of cases (particularly in important proteins such as p53 -> a tumor suppressor protein) most alleles are preferred to be functioning. An allele that can produce an acceptable amount of protein product without both copies of the allele functioning is said to be a haplosufficient allele. Conversely, an allele that requires both alleles to be functioning is haploinsufficient. The "haplo" here refers to a single copy, as opposed to a "diplo," or two copies.
Any more ambiguities?
Experience: Cell Biology, Confocal Microscopy, Developmental Biology, Evolutionary Biology, Genetics, Genomics, Physiology
Well, I don't know if it is an ambiguity: but why do we still have the extra X chromosome? I know we need it for germ cell lineage, but why keep it in the somatic cells of the organism? The energy expenditure of having to duplicate it every replication seems we should have gotten rid of it. But I am going to guess that the Xi still has some gene activation that is important to the organism. Still, transpose those genes to some other chromosome, and be done with Xi.
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