Neither of the above fish contain black pigmentation. However, when mated, many of their offspring express the Stripe-sided (Sr) gene and have black pigmentation along the lateral line (below left). The albino redcap female parent contains the Sr gene, but its expression is completely suppressed in an albino swordtail.

These albino swordtails contain black pigmentation because they express the Nigra (N) gene which is only partially suppressed in albino swordtails. Albino fish expressing Nigra may develop melanomas as they age.

Fish expressing the Sr gene contain small black spots along the lateral line that are composed of macro- melanophores (orange swordtail offspring on the left). If the non-albino fish also contains the Xmrk oncogene, the melanocytes proliferate rapidly resulting in multiple melanomas (orange swordtail offspring on the right).  

 

The albino redcap female and the orange redcap hifin male parents shown above are both offspring of an albino koi female swordtail that was purchased from a large pet store chain. This is an example of why it can be very difficult to remove a deleterious gene from the fish population. The store bought albino koi fish contains the Sr gene and the Xmrk oncogene, and when bred to an albino swordtail by the hobbyist, the melanoma-producing genes will be silently maintained in the population. If these albinos are bred to non-albino fish, Sr and Xmrk genes will no longer be suppressed and the offspring will develop melanomas.

Nigra Gene

Stripe-Sided Gene