Consider this my day 16/17/18: There are three types of Down syndrome; trisomy 21 (nondisjunction), mosaic, and translocation. Down syndrome is caused by an extra 21st chromosome, but the way the chromosome arrives and presents itself determines the type of Down syndrome a person has.
L has trisomy 21. It is the most common and accounts for around 95% of people with Down syndrome.
It is set in motion long before a baby is conceived and is a random occurrence, kinda like winning the lottery 😉.
For a more in depth explanation of the three types of Down syndrome, click here: http://www.ndss.org/down-syndrome/what-is-down-syndrome/