The paper says 1.5% of pairs were indeed between the same morphs. No mention of infertility, but white/white pairs produce 1/4 of chicks with both 2nd chromosom

The paper says 1.5% of pairs were indeed between the same morphs. No mention of infertility, but white/white pairs produce 1/4 of chicks with both 2nd chromosomes mutated and the only male chick of this kind they found was 50% underweight runt, though two such females grew up as normal white adults.