Only 3% at the best of times have the ability at all. It cannot be as simple as "both parents can channel, therefore the kids can" like blonde hair or blue eyes on those grade-school level explanations of heredity. Even for two channelers, the odds of a channeling child have to be slim. The uberchannelers would be unique, so who can explain what their rules were? Maybe the circumstances of their conception imparted the ability, rather than any hereditary factor. After all, if it was heredity, then Elayne's kids should have been even more likely than Aviendha's to get it, since their parents were equally as strong, and their maternal grandmother could channel too.

We also know that twins who can channel are very rare, which sinks the simple genetic theory completely. Vandene and Adeleas were identical twins with the same genetic code and yet Moiraine says that the occurence is rare. They both obviously have the genes and the appropriate souls.

The soul question implies that Rand and his wives might not have children who all have the ability to channel, even though they are both carrying the relevant recessives.

Besides that, even if the laws did follow simple genetics there are issues like non-penetration whereby the genes just dont express despite the parents having all the necessary genes. That's the most likely explanation for a squib in the harry potter universe.