The fact that pink TBs only occur in 4Ns puzzled me, so I did a bit of research. This article postulates that the defective "t" (tangerine) allele that produces pink was the result of a mutation during the conversion from 2N to 4N:
http://www.telp.com/irises/TB%...
It also seems that the defective allele for pink is completely recessive, as Lucy says; i.e. all the alleles must be the recessive pink version of the
t allele for it to work. This article also indicates that the effects are additive or multiplicative (i.e. the number of copies increases the amount of pink pigment). This might be why there are no pink diploid TBs; perhaps just two copies of the allele doesn't produce enough of the pigment.
http://www.winterberryirises.c...
Edited to clarify no pink diploid TBs.
