How many chromosomes do people have

When egg cells have too many chromosomes

The human genome is distributed over 46 chromosomes. 23 of them are from his mother, 23 from his father. When the egg cells form in women and the sperm cells in men, the chromosomes first exchange material with one another. Then they are distributed in such a way that in the end each egg cell and each sperm contains exactly 23 chromosomes. If the egg and sperm unite with each other during fertilization, the result is a double and thus complete genetic makeup.

If, however, errors occur during the maturation of the egg or sperm cells, there may be deviations from these numbers. "In humans, around two percent of all fertilized egg cells do not have double the set of chromosomes, but a triple set, making a total of 69 chromosomes," says Professor Manfred Schartl from the Biozentrum. Such embryos are not viable and die in the first trimester of pregnancy.

What is problematic for humans, other living beings easily put away. Frogs, amphibians and fish, for example, can live well with different chromosome numbers. This also applies to the Amazon parrot, a freshwater inhabitant from Mexico: In nature there are specimens with double, but also with triple sets of chromosomes. Together with colleagues from human genetics, Schartl and his team from Physiological Chemistry wanted to find out how this irregularity occurs in the fish.

There are only females in the Amazon marshmallow. Your egg cells can develop directly into a fish without any additional genetic material from a sperm. Because with the Mexican fish, the virgin generation is the rule. Biologists speak of this when a female can have young without fertilization. In order for the egg cells to develop into embryos, sperm must still come into play. These “steal” the Amazon marshmallow from males of a closely related species by seducing them to copulate. The sperm, however, only give the egg a purely mechanical impetus for further development; the male genome does not enter the embryo. The resulting daughters have all their chromosomes from the mother, with whom they are therefore genetically identical.


The Amazon Parrot emerged in the course of evolution when two other species of Parrot crossed. The maiden generation was not yet involved. The Würzburg team repeated this event in the laboratory. The crossing resulted in fish with double numbers of chromosomes. But their offspring also included animals with a triple set of chromosomes. The researchers have now used molecular genetic methods to trace how this came about.

As you write in Current Biology, what is known as automixis occurs in the fish examined, which was previously only known from insects. The number of chromosomes is not halved in all the resulting egg cells. There are therefore egg cells with both a single and a double set of chromosomes. If these fuse with sperm, then at the end of the process there are also offspring with a triple set of chromosomes.

Because automixis has now also been demonstrated in vertebrates for the first time, the Würzburg scientists draw two conclusions. On the one hand, they suspect that this mechanism ensures that embryos with a triple set of chromosomes can also develop in higher animals, including humans. On the other hand, this also explains the spontaneous occurrence of virgin generations in some vertebrates. Such reproductions without the participation of males are known, for example, from hammerhead sharks and monitor lizards, which were kept isolated in zoos for a long time. "Automixis can also produce egg cells with a double set of chromosomes in vertebrates, which then develop into embryos on their own," says Schartl.

The professor worked on these results with his colleagues Kathrin P. Lampert, who is now working at the University of Bochum, Dunja K. Lamatsch, who is currently doing research at the University of Sheffield, and Petra Fischer, as well as with the Würzburg human geneticists Professor Michael Schmid and Indrajit Nanda . The Bochum human geneticist Jörg T. Epplen was also involved.

"Automictic Reproduction in Interspecific Hybrids of Poeciliid Fish", Kathrin P. Lampert, Dunja K. Lamatsch, Petra Fischer, Jörg T. Epplen, Indrajit Nanda, Michael Schmid and Manfred Schartl, Current Biology, published online on November 1, 2007, DOI : 10.1016 / j.cub.2007.09.064

By Robert Emmerich