For Mandy- Down's syndrome child from 2 normal parents

Ok this is how it works.... Meiosis occurs to produce sperm and egg cells. When sperm and egg cells are produced, they only contain half the chromosome number. A DS child is produced when the mother's 21st chromosome doesn't split, so the child gets 2 chromosome 21 from the mom and 1 chromosome 21 from the dad, resulting in 3 chromosome 21... The picture below( sorry, hand drawn on the iPad!) shows u what happens in the mother when the egg cells are produced

( I ONLY DREW CHROMOSOME 21)

From the picture above, you can see 4 egg cells, one of them has TWO chromosome 21. If that egg is fertilised, it grows into a DS child. If the second egg is fertilised, It will usually become non-viable (does not survive). The 3rd and4th egg, when fertilised, should result in normal children.

Therefore a DS child is caused by what we called, non- disjunction of the mother's chromosome21 during meiosis second stage... Btw, non- disjunction is an A-level term!

For you O level peeps, just say the child inherits 2 chromosome 21 from the mother instead of one. Reason? It's a mutation :)

Hope that helps... Sorry abt the retarded drawing :p