So even the smallest gap will usually contain many genes. Such embryos will usually spontaneously abort. This is usually the result of a failure of the first zygotic division. It is also lethal to the embryo. Any other cell division may also fail to complete properly and in consequence a very small proportion of tetraploid cells can sometimes be found in normal individuals.
All autosomal monosomies are lethal in very early embryogenesis.
They do not even feature in the table above because they abort too early even to be recognised as a conception. The incidence of trisomy 21 rises sharply with increasing maternal age. A small proportion of cases are mosaic and these probably arise from a non disjunction event in an early zygotic division. The symptoms include characteristic facial dysmorphologies, and an IQ of less than 50. The incidence is about 1 in 5000 live births.