Final answer:
The mutation is called nondisjunction, resulting in aneuploidy. It can occur during meiosis I or II, and offspring with 2n+1 chromosomes, like Trisomy 21, have a higher survival chance than those with 2n-1 chromosomes.
Step-by-step explanation:
When a cell has too many chromosomes and another cell has too few chromosomes at the end of meiosis, the name of the mutation that has occurred is nondisjunction. This process leads to aneuploidy, which is an abnormal number of chromosomes in a cell. If nondisjunction occurs during meiosis I, homologous chromosomes fail to separate, resulting in two gametes with an extra chromosome (n+1) and two with a missing chromosome (n-1). If nondisjunction occurs during meiosis II, it involves sister chromatids that fail to separate. Most organisms with an atypical number of chromosomes do not survive; however, some, like humans with Trisomy 21 (Down Syndrome), may live with varying degrees of disability.
With respect to viability, offspring with 2n+1 chromosomes have a higher chance of survival compared to offspring with 2n-1 chromosomes. This is because the presence of an extra chromosome in some cases, such as Trisomy 21, can still allow for development into a viable organism, whereas the loss of a chromosome more often results in developmental issues that are incompatible with survival.