Final answer:
The most common extra chromosomes among live births are due to Trisomy 21, also known as Down Syndrome, which results from a baby having an extra copy of chromosome 21.
Step-by-step explanation:
The most common extra chromosomes among live births are associated with Trisomy 21, also known as Down Syndrome. This condition is a result of nondisjunction, where a baby inherits an extra copy of chromosome 21, leading to a total of 47 chromosomes instead of the usual 46. Individuals with Down Syndrome exhibit distinct physical features, developmental delays, and varying levels of cognitive impairment. While most other autosomal trisomies lead to non-viable pregnancies, Trisomy 21 is unique as affected individuals can survive into adulthood. The incidence of Down Syndrome is correlated with maternal age, with older mothers having a higher likelihood of having a child with this condition.