ANALYSIS OF THE FIRST CLEAVAGE DIVISION TO DETERMINE THE SEX-RATIO AND INCIDENCE OF CHROMOSOME ANOMALIES AT CONCEPTION IN THE MOUSE

Abstract

Fertilized mouse eggs were examined between 27 and 34 hr after superovulation and mating. Out of 1334 eggs examined, 432 were already at the two-cell stage. Of the one-cell eggs, 135 were at metaphase of the first cleavage division with 40 clearly distinguishable chromosomes, and it was possible to sex 123 of these preparations. Sixty-two were found to be male and sixty-one female, corresponding to a sex-ratio at conception of approximately 1:1. A moderate incidence of triploidy (4·1%) and aneuploidy (8·3%) was also observed at this stage and one example of hyperdiploidy, an embryo with 41 chromosomes (mouse 2n = 40). A method for distinguishing between all the autosomes and the sex chromosomes at the first cleavage metaphase is described.