Trisomy 21 is a condition that occurs during meiosis in sex cell division. This means the production of egg and sperm cells. This condition is when cells do not reproduce properly, also called nondisjunction. This causes one gamete to have 2 #21 chromosomes. When this is combined with the opposite gamete, it causes there to be 3 #21 chromosomes. In a "regular" human,m there are 46 chromosomes overall. In a person with Trisomy 21, there are 47.
This is what happens for persons who have what is diagnosed as "down syndrome". It is because the person has too MUCH genetic information.
Some of the symptoms of Down Syndrome, or Trisomy 21 are:
-wide range of mental abilities
-round, full face
-enlarged and creased tongue
Joe: "Did you hear about the new kid? He has down syndrome! Poor guy!"
Bob: "We learnt about that in Bio class!"
Joe: "Your mama is sooo fat..."
Bob: "Oh ya?! Well, you have Trisomy 21!"