Trisomy 21

Occurrence	1 in 650 to 1 in 1,000
Description	combination of birth defects including some mental retardation and characteristic facial features
Chromosome	an extra copy of the 21st chromosome-- generally through non-disjunction but occasionally the extra copy of critical chromosome material can be as a result of translocation
Cause of DS	95% of trisomies are of maternal origin and the risk of trisomy 21 increases as the age of the mother increases
Testing for DS	prenatal testing is available through amniocentesis

Mitosis and Meiosis

Students must understand cell division before studying Trisomy 21. One can teach mitosis in the very way suggested below for meiosis.

One of the best ways students can learn is through hands-on experiences where they can visualize what is going on. A useful activity utilizes Pull 'N' Peel Licorice. The licorice comes in ropes made of nine smaller ropes. Each student will need a large rope of licorice. Since we will be dealing with even numbers of chromosomes, have students pull apart the nine pieces and eat one of them. Each student should now have eight pieces of licorice. If you have an overhead projector you can project your licorice (actually their shadow) on the board so that students can follow your example.

(1) Take the eight pieces of licorice and pair them so you will have four pairs. It is helpful to have students make X's with each pair to represent two sister chromatids held together in the center by a centromere.

(2) Now walk your students step by step (moving the licorice to the appropriate place) through mitosis and/or meiosis.

(3) Students should easily begin to see how important it is that the chromosomes line up appropriately during metaphase in order for accurate division to occur. You can have them depict what would happen in anaphase if the chromosomes did not line up correctly.

(4) Once your classes are experts at all these phases you can show them how Trisomy 21 occurs. It can happen in either meiosis I or meiosis II. The chromosomes line up accurately in metaphase but do not pull apart accurately as they move into anaphase. In these cases the final gamete ends up with an extra copy of one of the chromosomes. In humans if the extra chromosome is #21 and that gamete is fertilized there will be three copies of #21 or trisomy 21. The resulting symptoms are often referred to as Down Syndrome.