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1) human babies are born from meeting of an egg cell and a sperm cell.

egg cells and sperm cells are gametes.

2) gametes need to have half the normal number of chromosomes

in order for their fusion to create a normal amount of chromosomes in the baby.

this fusion of 2 haploid cells allow the zygote (the single cell created by combining an egg and sperm cell) to be diploid.

3) this zygote then undergoes mitotic division to create copies of itself,

ultimately creating an entire organism.

4) certain cells in this growing organism are designated to undergo meiosis and create sperm or egg cells

in order to be used later for the reproduction, in the future.

5) mutations can occur in any cell.

only mutations that occur in gametes are passed onto the next generation.

somatic cell mutations can be harmless or cause disease in this current organism,

but somatic cells are not used for reproduction, thus it's never passed onto the next generation.

6) nondisjunction is a type of chromosomal mutation.

if nondisjunction occurs during meiosis (creation of gametes)

it can cause chromosomal disorders in the next generation.