Y chromosome effect on adult testis size

DURING the development of mammals with X and Y chromosomes the Y chromosome brings about the differentiation of the gonad primordia into testes 1 . But apart from carrying the genetic determinants of maleness, little is known about its genetic constitution. In man, the gene for hairy tips to the ears shows Y linkage in at least some pedigrees 2 . In mice there is evidence for a Y-linked histocompatibility locus 3,4 , and for effects of the Y chromosome on the frequency of spermatozoa with abnormal heads 5 . Here we show that the size of the testes of adult mice is affected by factors on the Y chromosome.