FAQ: what is the tangerine gene / locus in parrots?
To be honest, it does not exist. It was the Australian veterinary Dr. Terry Martin who used the term “tangerine locus”. He name the gene, from which he thought it was responsible for the production of the red psittacofulvins in the mask of Agapornis roseicollis, the tangerine locus [1, p. 230]. Jim Hayward used the name tangerine for the orange face mutation [1, p. 107].
Now we know that other genes (MuPKS) are involved for the production of psittacofulvins  and there is no “tangerine gene” in parrots. BTW the name tangerine was given by MacArthur in 1934 to an orange coloured tomato and he talked about the tangerine mutation . It had nothing to with birds. T. Martin, A guide to color mutations & genetics in parrots, 1ste dr. ABK Publications, 2002.  T. F. Cooke e.a., ‘Genetic Mapping and Biochemical Basis of Yellow Feather Pigmentation in Budgerigars’, Cell, vol. 171, nr. 2, pp. 427-439.e21, okt. 2017, doi: 10.1016/j.cell.2017.08.016.  J. A. Jenkins en G. Mackinney, ‘Inheritance of carotenoid differences in the tomato hybrid yellow x tangerine’, Genetics, vol. 38, nr. 2, p. 107, 1953.