Limericks are known to be well entrenched in the variation due to oral transmission.

And I reckon MtheGM knows that a blonde is unlikely to be heterozygous for melanin production in the skin; discussion of the improbability of a blonde participating in a true F1 cross (producing the required 1:2:1 ratios of homozygous "white":heterzygous khaki:homozygous black) is what gave the limerick its supposedly educative value.

Cheers, Rowan