Wood Lemming - Sex Determination

Sex Determination

Wood lemmings produce about three times as many female as male offspring. This is due to an unusual genetic system where they have two different types of X chromosomes, the normal X and a mutated X*. Females with that mutation on their X*-chromosome inhibit the male determining effect of the Y chromosome. This leads to three genetic types of females: XX, X*X and X*Y and one genetic type of males XY. The X*Y females are fertile, but only produce X* ova, which means they only produce female offspring. Its not fully known the extent to which nature balances out the differences in the adult population, but sampling studies seem to suggest males do make up as little as 25% of the population at equilibrium. Females with abnormal genotype (XO, XXY, X*YY) occur regularly. The high female sex ratio may be an adaptation against local mate competition and inbreeding during population low points.