Definition - What does Mosaicism mean?

Mosaicism occurs when there is different genetic material within cells of the same embryo or fetus. When an egg is fertilized by sperm, the fertilized egg or zygote, is expected to further divide, equally sharing the genetic material during each division. In mosaicism, this does not occur, giving one or more cells more or less genetic material than others

Depending on the number of irregular cells and specific chromosome involved, it is possible that mosaicism can lead to embryos that do not properly implant, pregnancies that end in miscarriage, or disorders after birth.

FertilitySmarts explains Mosaicism

In many situations, a fetus with mosaicism may develop normally and grow to have a completely normal life, but in some situations, this genetic imbalance may lead to fetal abnormalities. For example, Turners Syndrome is a condition where a female, normally XX, is completely missing an X. This mosaicism leads to physical abnormalities such as short stature, learning disabilities, cardiovascular problems and infertility.

In Klinefelter syndrome, males, normally genetically XY, are born with an extra X chromosome, making them XXY. Due to the influence of the extra X chromosome, males with this syndrome have a more curvaceous and feminine body, breast growth, small genitals and sterility.

Preimplantation genetic screening (PGS) during in vitro fertilization (IVF) provides the opportunity to detect genetic errors like mosaicisim and decide against transferring so-called mosaic embryos. There is debate around the effectiveness of a single biopsy that occurs with PGS and if it can sufficiently determine if an embryo is chromosomally abnormal. Others perspectives see this viewpoint as limited to previous embryo selection methods that had a more limited ability to detect mosaic embryos.

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