Usually, we get two copies of each chromosome. However, the finely tuned process of meiosis and fertilisation can go awry such that a new zygote ends up with three copies. Depending on the chromosome, this can have serious consequences...

Sources for this episode:

For an explanation of non-disjunction which I found helpful when I was at sixth form, see: Amoeba Sisters, YouTube (2017), Meiosis (Updated) (online) [Accessed 25/03/2021].
Xu, X., Zhang, X., Han, J.-W., Adamu, Y. and Zhang, B. (2020), Potential Increased Risk of Trisomy 18 Observed After a Fertilizer Warehouse Fire in Brazos County and TX. International Journal of Environmental Research and Public Health 17: 2561- 2571.
Author unknown, NHS (date unknown), Edwards' syndrome (trisomy 18) (online) [Accessed 25/03/2021].

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