The relatively recent dodecaploid Xenopus (X.) ruwenzoriensis (108 chromosomes) permits to catch the phenomenon of gene silencing or modification in the act. This is interesting for genes related to the immune system, many of which can be selected as a consequence of their utility or eliminated as a consequence of their cost. Among receptor genes, a trend toward diploidization is seen: neither T cell receptor, Immunoglobulin, or Major Histocompatibility Complex Class I and II and linked loci are present on all paralogs. The fate of linked Class I and Class II loci can be independent from one another. Six Class II beta sequences can be detected in heterozygous X. laevis and X. ruwenzoriensis but they are distributed on two (disomic) loci in X. laevis versus 6 (polysomic) loci in X. ruwenzoriensis. In the same two species 2 Class I sequences can be detected in heterozygous X. laevis versus 4 in X. ruwenzoriensis. One interpretation is that natural selection acts more on the number of genes than on the mode of inheritance (polysomic versus disomic).