The old dogma “one gene—one protein” is not always true for eucaryotic genes. For
example, a single a-tropomyosin gene can produces multiple distinct mRNAs which then
give rise to variant (but related) proteins. How does this occur?
In Eukayotes most of the genes are interrupted. Information in the genes are encrypted in sequences that will ultimately become a part of the polypeptide chain are called exons. But these exons are interrupted by long sequences which do not become a part of the polypeptide chain and they are called introns. Usually these introns are removed in a process called splicing inside the nucleus before the mRNA is sent out into the cytoplasm for translation.
Humans have less than 25,000 genes. But more proteins. So what ever genes that are there have to produce more proteins. So they go for alternative splicing. If we assume that a gene contains 5 exons and four introns and in one tissue or in one ocassion splicing happens in such a way that exon 4 (just an example) is also spliced out along with the introns, while in other occassions or in other tissues it is not. So it becomes a part of the mature mRNA which when translated yields a different protein which has different function.
Thus an eukaryotic cell can have multiple proteins from a single gene by alternative modes of splicing.