Final answer:
A baby born with an extra chromosome in each cell is typically due to a meiosis error called chromosomal nondisjunction. This leads to conditions such as Down syndrome, or trisomy 21, where there are three copies of chromosome 21.
Step-by-step explanation:
The condition where a baby is born with an extra chromosome in each of its cells is most commonly caused by a meiosis error known as chromosomal nondisjunction. Down syndrome, also known as trisomy 21, occurs when there are three copies of chromosome 21 instead of the normal two. This additional chromosome disrupts the normal development which can lead to physical and mental abnormalities. The likelihood of such nondisjunction events occurring increases with maternal age, particularly in women over the age of 36. The contribution of paternal age is less significant because nondisjunction events in sperm are much rarer in comparison to those in eggs.