PR PHILIPPE VAGO, ISM / SCIENCE PHOTO LIBRARY PR PHILIPPE VAGO, ISM / SCIENCE PHOTO LIBRARY
Female karyotype with trisomy 18. G-banded karyotype of a female patient with an extra chromosome 18, a condition known as trisomy 18, or Edwards syndrome. The karyotype is the complete set of chromosomes in an organism. Human beings have 23 Chromosomes and normally there are two copies of each chromosome in each of our cells, one set from the mother and the other from the father. However, this person has three copies of chromosome 18. The sex chromosomes (X and Y, bottom right) determine gender - here, the XX chromosome pair indicates that this person is female. Edwards syndrome occurs in around one in 6,000 live births and around 80 per cent of those affected are female. Foetuses with Edwards syndrome have a very low rate of survival, with the majority.
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