A new allele of the gene BMP15 which increases the prolificacy of sheep has been recently detected and identified in the Rasa-aragonesa breed. This gene has been called FecXR and has been found as a result of research on the Rasa-aragonesa data bank. It is located in the chromosome X, so that the males are hemicygotic. The homocygotic females for the wild gene have normal prolificacy, those heterocygotic are prolific and those homocygotic for this allele are sterile. Its effect is estimated to be 0,32 lambs per sheep and lamb. From the study of genetic parameters, it is concluded that this allele explains a part of the genetic variance, leaving another part yet to be explained.