The XO and OY chromosome constitutions in the mouse

Abstract

A study was made of reproductive performance and embryonic mortality inXOandXXfemales. In the stock used, the mean litter size ofXOfemales (4·46) was greatly below that ofXXones (8·17). One series of pregnant females of both karyotypes was dissected after 15 days' gestation, and another series after 3½days' gestation. In the former, there was a significantly greater amount of embryonic mortality inXOfemales both before implantation and at the small and large mole stages. By far the greater amount occurred before implantation. The data from dissections after 3½ days' gestation concerned pre-implantation embryos, since normal embryos at this point are at the late morula or early blastocyst stage. The embryos fromXOfemales contained a large group of obviously and characteristically abnormal ones; they comprised 60/280 of the embryos fromXOfemales, compared with 4/189 of theXXones. They appeared to have developed abnormally from a very early stage, probably the two-cell stage, and were considered to represent the missingOYclass of zygotes. In addition, it was concluded that there was probably an abnormally low segregation of nullo-Xgametes fromXOfemales.