Problem with green-tailed guppies

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: Hi everyone,

: I'm having a problem with the green leopard mosaic guppies. I'm trying to create a strain of them but it seems to be impossible. I always have some yellow mosaics and/or blacks in every generation they produce. I tried to narrow it down to green and yellow. I'm always using green males of course, and I seem to have found a female which doesn't produce blacks. Whenever I use her for back-crossing with one of her (green male) offsprings there are no black-tailed males. However, when I mate the siblings of the next generation, the blacks will come back no matter how often I have used the "green-yellow" female before. I'm already in F30 or something like that and I still haven't had any positive results... It looks like the green tail can't be homozygous -- does anybody know the genetics for this? Is it dominant or recessive to black/yellow? Maybe sex-linked? Maybe incomplete dominance? It looks very much like the Siamese-Burmese-Tonkinese example in cat genetics, but I guess no one of you is familiar with cat genetics. When I think about it, this theory seems to be the most logical one. 
: Can anybody help me?

: TIA, Karin


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