Trisomy, the presence of a third copy of one chromosome, is deleterious and results in inviable or defective progeny if passed through the germ line. Random segregation of an extra chromosome is predicted to result in a high frequency of trisomic offspring from a trisomic parent. Caenorhabditis elegans with trisomy of the X chromosome, however, have far fewer trisomic offspring than expected. We found that the extra X chromosome was preferentially eliminated during anaphase I of female meiosis. We utilized a mutant with a specific defect in pairing of the X chromosome as a model to investigate the apparent bias against univalent inheritance. First, univalents lagged during anaphase I and their movement was biased toward the cortex and future polar body. Second, late-lagging univalents were frequently captured by the ingressing polar body contractile ring. The asymmetry of female meiosis can thus partially correct pre-existing trisomy. DOI: http://dx.doi.org/10.7554/eLife.06056.001, eLife digest Inside cells, DNA is found packaged into structures called chromosomes. Most human and animal cells contain two sets of chromosomes, one inherited from each parent. Chromosomes from one set pair up with the equivalent chromosome from the other set. However, egg and sperm cells only contain one copy of each chromosome, so that when the egg is fertilized, the resulting cell again has two sets of chromosomes. If there are either more or fewer than two copies of a chromosome in the fertilized cell, this can cause birth defects and conditions such as Down syndrome. An egg cell develops from a cell called an oocyte via a process called meiosis. The oocyte first duplicates its DNA so that it contains four copies of each chromosome. The oocyte then divides, and the resulting cells divide again, to produce four cells that each contains one copy of each chromosome. Only one of these cells is an egg cell: the other three are called polar bodies, and these normally self-destruct. The tiny roundworm C. elegans is a model organism used to study meiosis. Worms can be hermaphrodites or males; the hermaphrodites normally have a pair of ‘X’ sex chromosomes. However, sometimes problems with meiosis can produce hermaphrodite worms with three X chromosomes in each of their cells. In these cells, two of the X chromosomes pair with each other as normal, and one X chromosome remains unpaired. Cortes et al. examined meiosis in mutant worms that had an extra copy of the X chromosome by marking all the chromosomes with a fluorescent tag. This allowed the movement of the chromosomes to be tracked through images taken using a microscope. This revealed that an unpaired X chromosome moves more slowly than a normal paired set. Furthermore, the unpaired chromosomes tend to move toward the region of the oocyte that will develop into a polar body. Thus, when the oocyte divides, the unpaired chromosomes are placed in the polar body and eliminated. This mechanism improves the chance that the correct number of chromosomes will end up in the egg cell. Women with three X chromosomes are often fertile and in most cases produce normal offspring. Further work is needed to see whether human oocytes remove extra chromosomes by a mechanism similar to that seen in the roundworms. DOI: http://dx.doi.org/10.7554/eLife.06056.002