|MadSci Network: Genetics|
In general, gene expression (in terms of mRNA production) should be proportional to the number of copies of the gene present in the genome. If multiple alleles of a gene are present, the expression of each should be similar. So for a gene with 2 alleles (call them A and a), the level of expression in an animal homozygous for A would be 2(A) (since it has 2 copies of the allele, assuming it got one from the father and one from the mother). A heterozygote would show levels of expression of 1(A) + 1(a), and a homozygote for a would be 2(a). This should hold true for genes introduced by transgenic methods as well, assuming that your transgene is present in similar numbers as the complementary endogenous gene, and that it did not insert into a genomic location that somehow inhibits it from being expressed.
Keep in mind that this is a rather idealized explanation and there are a number of things that are not taken into account. If, for example, the function of the product of gene A is to stimulate production of gene A mRNA, then an organism homozygous for A may end up making a lot more A mRNA than the 2(A) amount that we predicted.
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