Which letter is supposed to be in capitals when using scientific names where it's [first initial of genus].[species]?

The genus name is capitalized and the species name is not. Further they both should be italicized or underlined. SM

Just to add, underlining is generally only used if written by hand.

CharonY, I started out my career in science with a mechanical typewriter (old habits). SM

Just an interesting note to expand on what SMF and CharonY have said. The procedure for marking corrections while proofreading a document includes underlining text and adding notations in the margin on what to do with that text (bold, italicize, etc). So, to italicize, one would underline the text to be italicized and then write "ital" in the margin beside the line. I'm guessing that the advent of the typewriter left the typist without italic letters to type, so they would use underlining in lieu of using italics (which probably continued for a few generations before word processing computer apps). And, as CharonY said, it is mostly used when writing by hand (now that it's easy to generate italic text with word processors).

The genus name is capitalized and the species name is not. Further they both should be italicized or underlined. SM

 

They have to be underlined only when written by hand. When typed, they should be in italics only without underline.

While I understand why they should be capitalized, what's the reason for underlining/italics?

Right, I forgot about typewriters. Good thing they phased out during my undergrad times (well before thesis writing times, luckily).

While I understand why they should be capitalized, what's the reason for underlining/italics?

Overall, I would guess that, being Latin (or Latinized) words/abbreviations, binomial nomenclatures are underlined/italicized to denote them as foreign words. It partly assures the reader that a typographical error in English has not occurred. Foreign words fully adopted into English are no longer italicized.

It is more of a taxonomical convention. For instance, names above the genus level are usually not italicized. Same for genes (non-capital, italics) protein (capital, non-italic).

 

Edit, this is only the case for prokaryotic gene symbols. For eukaryotic gene labels it is customary to have it italicized but the first or sometimes all letter are upper case (for the WT).

Edited April 18, 201114 yr by CharonY