Factor responsible for spermatozoan abnormality located on theYchromosome in mice

Two inbred strains of mice differing in the mean percentage of spermatozoa with abnormal heads were used: KE (16·1%) and CBA (5·9%). TheF1resulting from the crosses exhibited a heterosis effect, while in the backcrosses an obvious segregation of genotypes was observed; both generations showed a reciprocal difference, depending on the source of theYchromosome. The character of sperm head abnormality seems to be polygenically determined, one of the genes being located on chromosomeY. Seven generations of backcrosses were performed in which theYchromosome from CBA was introduced to the genetical background of the KE strain. In the seventh generation 10·2% of abnormal spermatozoa were found, which is significantly lower than in the KE strain. The difference shows the net effect of theY-linked locus. A correlated difference was found in the fertilization rate, indicating that a factor influencing male fertility is located on chromosomeY. It does not seem to influence the shape of normal spermatozoan heads. Karyotype analysis did not reveal gross abnormalities in the KE strain.