He just didn't seem to care to make it obvious by having revealed channelers in Rand's lines just to drive the point home, knowing it wasn't necessary. For Rand's ancestors in the Rhuidean columns, it wouldn't have been meaningful as it's too far in time. Rand no doubt had many ancestors with two dominant genes, some of them married people who reintroduced one recessive. We already knew it was genetic at that point. It was an arbitrary choice from RJ in the end. He could have done it (and might have, but kept this hidden as the channeling ancestor was latent because it was a male), or he could decide an heterogenous pair was introduced recently in Rand's Aiel line.


For Tigraine's line that isn't so strange. The WT has probably weeded out the ability from most of the lines from Ishara (all those in the Great Houses anyway) by removing the women in the Great Houses with the ability and making them Aes Sedai. In the direct Royal line the fact they have not succeeded to produce a single learner or sparker since before Ishara's time suggests there's a lot of men and women with two dominant genes among the higher nobles of Andor and those with an hetoregenous pair are rare enough that two of those from one generation getting matched, both passing their recessive gene rather than one of them passing a dominant, and that they produce a daughter who lives to adulthood and whose ability will be found and not a boy in whom it will stay hidden is rather rare. That was Morgase's case, and she was lucky to also marry a man who had a least one recessive (perhaps even a pair, I don't recall if Jordan confirmed in a Q&A that Gawyn doesn't have the latent ability). Tigraine of course had an heterogenous pair, and Morgase has two recessive genes, but Taringail can be either, like his father for that matter. It's plausible the ability re-appears from time to time in the major lines of Andor mostly thanks to the fact it's not unknown for a high noble, even a powerful High Seat or even a Queen, to marry outside the lines to Ishara.

Obviously whatever produces the spark (another recessive gene?) is even rarer in the Andoran lines, as they were under AS scrutiny, and because for 3000 years the WT virtually removed all female sparkers from the gene pools (the rest they miss or cast out rarely marrying either) and the odds male sparkers anywhere lived long enough to produce offspring are not inexistant but relatively slim (and if the AS learned of any, they would have kept an eye on his offspring). Even in the TR it's rare (probably not as much as we think though. 75% of sparkers die, and the Wheel helping to alter the odds to keep the area under the WT radar, it sounds like most of those 75% even died without many if anyone realizing they could channel - as happened with the apprentice of Mistress Baran before Nynaeve).