2.  Mistakes made during meiosis can lead to abnormal numbers of chromosomes.  This occurs when a homologous pair of chromosomes fail to separate.  These mistakes can occur in either meiosis I or II.

4.  Occurs when there is nondisjunction during meiosis I. Half the gametes will carry an extra chromosome 21.  A person with an extra chromosome 21 is said to have down syndrome.

5.  Trisomy 21 affects about 1/700 children in the USA.  People with down syndrome generally have a shorter lifespan, and will exhibit varying degrees of developmental delays.

6.  Other physical characteristics include a round face, flattened nose bridge, small teeth, short stature, heart defects, and susceptibility to respiratory infections.  Most people with down syndrome are also sterile, and therefore unable to have children.

7.  The occurrence of trisomy 21 increases with the age of the mother.  Women who are over 30 have a 1% chance of having a child with down syndrome. This risk increases with older mothers.  Women over 35 can have genetic testing done to find out if their fetus has trisomy 21.

8.  Occurs when there is nondisjunction in the sex cells.  Klinefelter’s occurs in males who have an extra X chromosome.

9.  Males with klinefelter’s will have smaller testes, some breast development, lanky, and poor facial hair.  These males are also sterile. Therefore many men do not realize that they have an extra X chromosome until they wish to start a family of their own.

10.  Occurs in females because of the nondisjunction of sex cells resulting in females with only one X chromosome.  Women with turner syndrome have a webbed neck, poor development ovaries (sterile), short stature, broad chest, heart defects, and increased weight.

11.  Males with XYY generally have regular fertility and regular physical characteristics.

12.  Women who have three X chromosomes usually do not experience any physical or mental characteristics due to the disorder.  In females only 1 X chromosome will be active in each cell.