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A DNA letter is either one of A,T,C or G. Human genome is supposed to have 3 billion base pairs coming from each parent. But these are pairs - a combination of A-T or C-G. So, doesn't that make the total number of DNA letters from a single parent, equal to 6 billion ?

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    $\begingroup$ It's more common to say there are ~3.6 Gbp (Giga-basepairs). What you're asking is equivalent. You only need to know one of the letters in a given strand to know what the other is too. $\endgroup$ – Joe Healey Jul 3 '17 at 19:01
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    $\begingroup$ A, T, G and C are abbreviations for the deoxynucleosides of DNA and often, more loosely, for corresponding bases. They are not letters. 'We' presumably refers to the scientific community, which includes me, and I certainly never talk about 'letters' in relation to DNA. This may appear technical pedantry, but it is important in science to refer to things precisely without adding unnecessary implications. The implication of letter is that it helps to spell a word — presumably in relation to a protein. In many cases that is not so. If you use the correct terminology, your question does not arise. $\endgroup$ – David Jul 3 '17 at 22:31
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    $\begingroup$ For all those who have voted to close this question as off-topic (or are willing to do so), please at least read the reason you are chosing. This is certainly not a homework question, and does not show lack of research either, IMO. This is just misunderstanding of the terminology, as @david says. This is what downvotes are for, there is no need to vote-to-close this question. $\endgroup$ – another 'Homo sapien' Jul 5 '17 at 2:43
  • $\begingroup$ In response to the comment from @another'Homosapien', I should explain that when questions are voted off-topic under the category ‘homework’ it is not necessarily because they are considered homework in a literal sense. As I pointed out in a post on meta, ‘homework’ includes “A question that addresses a basic biology concept that may seem trivial to biology professionals”. I argued there that it would be better if this were separate category because of the ambiguity. Any takers now? $\endgroup$ – David Jul 5 '17 at 8:37
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If a DNA letter is one of A,T,C and G, and there are 3 billion base pairs, why don't we say that there are 6 billion letters in the genome?

It is a convention, and stating the actual number of bases would not add any information, but would actually lead to more confusion. It's like saying

I have six pairs of socks

vs.

I have twelve socks.

It means bascially the same, but now one could assume that these twelve socks are not from actual pairs (e.g. twelve left socks). The same goes for DNA. Saying we have pairs of bases emphasizes the redundancy of the DNA double strand.

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    $\begingroup$ @JoeHealey I actually realized a notable distinction between these two statements, see my edit $\endgroup$ – Adrian Jul 3 '17 at 21:07
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If a DNA letter is one of A,T,C and G, and there are 3 billion base pairs, why don't we say that there are 6 billion letters in the genome?

We do.

Just like English has 26 letters while this post has 522 letters (not counting numbers and punctuation), the genetic alphabet has 4 "letters" while the human genome has 6 and a half billion. The thing is people tend to stop using the letter analogy after talking about the 4 or 5 naturally available nucleotides. After that, we tend to go back to calling them nucleotides and base-pairs.

Whenever you have a question about something that doesn't seem to make sense, the first thing you should ask is if your premise is even right.

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You can refer to the Human genome as haploid or diploid. The haploid genome is around 3Gb, the diploid one is 6Gb. Usually, the bare term "Human genome" refers to the haploid one. The first picture on this Wikipedia article makes it clear.

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