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Skip to 0 minutes and 13 secondsWe learned in week one that during cell division most dividing cells go through a process called Mitosis. This is true for all cells other than the sperm and egg cells which go through Meiosis. During cell division, and prior to both Mitosis and Meiosis, cells go through a phase where the chromosomes replicate so that each chromosome consists of identical sister chromatids. Separation of these sister chromatids occurs in both Mitosis and Meiosis 2. Separation of homologous chromosomes occurs in Meiosis 1. The process of separating the chromatids, or pairs of homologous chromosomes, is known as segregation. When segregation goes wrong, such that either both chromosomes, or chromatids, go to one pole and none to the other. This is known as nondisjunction.

Skip to 0 minutes and 59 secondsIn nondisjunction the daughter cells will either have too many, or too few, chromosomes. When nondisjunction occurs in Meiosis, the resulting gametes, the sperm or the eggs, are chromosomally unbalanced. If those gametes go on to participate in fertilisation the resulting fetus will have too many, or too few, of one or all chromosomes. Errors of ploidy refers to a rare situation where there are the wrong number of whole sets of chromosomes. Such as triploid, where there are 69 chromosomes. However the main focus of this presentation is on the aneuploidies where there's a single extra or missing chromosome. For example, a female with Turner's Syndrome has only one X chromosome. Someone with Patau's Syndrome has an extra 13, or trisomy 13.

Skip to 1 minute and 47 secondsSomeone with Edward's Syndrome has trisomy 18, and someone with Down's syndrome has trisomy 21. Turner's Syndrome often presents in adolescents with failure to menstruate and short stature. It can also present in infancy, or even antenatally with characteristic features such as puffy feet and web neck. Babies with trisomy 13, or Patau's Syndrome have complex malformations of the heart, abdominal wall, and brain. And many are still born, or survive for just hours, or days, after birth. Similarly over 95% of fetuses with trisomy 18, or Edward's Syndrome, die in utero. They also have complex heart and brain abnormalities, and characteristic skeletal changes. The trisomy you are likely to be most familiar with is Down's Syndrome, or trisomy 21.

Skip to 2 minutes and 37 secondsMany people with Down's Syndrome have fulfilling and happy lives. However, there's also an increased chance that they will have certain health problems including congenital heart problems, disorders of the digestive tract, vision and hearing problems, and an increased risk of leukaemia. Nondisjunction can happen in both men and women. However, most cases of trisomy take place because of nondisjunction at Meiosis 1 in the mother. This is likely to be due to the fact that this stage is very long in women, starting before birth and ending at ovulation. For this reason the risk of most aneuploidies increases with maternal age. Which is also why the risk of miscarriage increases in older women.

Errors in segregation

During this video we will learn about how errors in the segregation of chromosomes can lead to genetic disorders such as Down syndrome, Patau syndrome and Edwards syndrome.

Talking point:

Why do you think we don’t see trisomy of chromosome 1 routinely and why is Down syndrome the commonest of the trisomies? Reflect on how long ago your mother’s egg started the journey towards making you.

Would you like to know more?

We have included PDF versions of the Mitosis and Meiosis diagrams and the ‘Down Syndrome Risk By Age’ graph in the downloads section below.

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This video is from the free online course:

The Genomics Era: the Future of Genetics in Medicine

St George's, University of London

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