What would happen to a cell if, during the splitting of chromosomes, two did not separate- and one cell got a double set and the other cell was missing a chromosome?

Explanation:

The correct pairing of the homologous chromosomes and their correct segregation (a chromosome for each daughter cell) are key to the proper development of the new cells, and specially of the new individual if we are talking about the early stages of embryogenesis. If there is an error during the splitting of chromosomes, and one cell gets a double set and the other misses a chromosome, numerical chromosomal abnormalities occur. These numerical abnormalities can happen on autosomes or sexual chromosomes, and common examples are as Down Syndrome (trisomy 21), Patau Syndrome (trisomy 13), Turner Syndrome (45,X) and others. The number of the trisomy indicates the chromosome pair that suffered the adding or the subtraction of its chromosome.