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Why is the "gene" the smallest unit of heredity? - (May/19/2015 )

Genetics is the study of heredity. Whenever we talk about gene expression we only include the sequences that suffer the transcription process (genes itself). Genes are regulated by regulatory elements: promoters, enhancers, insulators, etc. These sequences are not always part of other coding sequences (like lnc-RNAs). This sequences are also inherited, and they also play an important role in gene expression, thus phenotype. Why is the gene the smallest unit of heredity? repetitive sequences or elements are also inherited, and also contribute to regulation of gene expression by it's more like structural role in the nucleus. The gene by itself will not transcribe without a promoter. Why aren't the DNA molecules considered the smallest units of heredity? Or otherwise, why isn't the gene with it's most simple (or perhaps their most needed) regulatory elements considered the smallest units of heredity?


Why are the regulatory DNA elements, structural DNA elements, and other forms of heredity excluded from the definition? 


Finally, is there a point on still talk about "smallest units of heredity" in textbooks and elsewhere nowadays with the incredibly sensitive and/or massive technologies - that increases our knowledge to the point in witch we realize that DNA molecules are comprised of a lot of interesting non-gene stuff - tell us that heredity is indeed a ver complex and integrated process to have actually have a "smallest unit"? 


Sorry about my english, I'm very young and stupid xD -as we all- ! I'd like to get some other and more complete answers that of a textbook. 


Thank you. 


I guess the term "smallest unit" has some historical reasons, because when it started with Mendelian (classical) genetics, nothing was known about DNA, DNA regulation and so on. Genes were comprehensible units with later a known region and function. I.e. at that time a gene was just the blueprint for a polypeptide and from that you could explain (at least easy) inheritance phenomena.


With the development of molecular genetics the gene definition continuously changed and was refined and this still continues (e.g. with a more functional definition, evolutionary definitions, with including the regulatory elements to the gene, etc) and even questioned (see e.g. here), since you can understand it also as a concept and not as a stretch of protein/RNA coding DNA. Anyway even with the changing definitions and the very complex molecular machinery (splicing, regulation, etc, etc) it is still a good working unit which helps to explain, understand and deal with many of the phenomena found (e.g. diseases, phenotypes such as blood groups). And if you look at evolutionary processes, the forces that drive evolution and influence heredity (e.g. mutation, selection) work especially (or verifiably) on functional genes, because differences here (this would be different alleles) alter probability to survive and reproduce (think of a mutation that makes a protein more heat tolerant compared to a non mutated one). Anyway also regulatory and repetitive DNA regions are inherited but here it's much more difficult to detect inheritance phenomena and to understand the processes. Repetitive sequences even should be evolutionary neutral because they have no function at all (though it's not really sure), and they are used in population genetics, also dealing with inheritance on the population level.

It's a very complex topic with lots of different views and lots of different fields of genetics, therefore for sure it's difficult to understand.