1. Nondisjunction Disorders
- mistakes during meiosis

2. If sister chromatids do not separate during meiosis I or II, an egg or a sperm may end up with too many chromosomes of one type (3 instead of 2)

6. The extra copies of chromosomes result in abnormal embryonic development, ex. Down syndrome (trisomy 21)
Round, full face
Impaired intelligence
Hand, tongue, eyelid abnormalities
Short height
Cardiac abnormalities

7. Trisomies are very common in human zygotes: 10-30% of all conceptions
Most trisomies (other than for chromosome 21) are lethal to the embryo: at least one-fifth of all recognized pregnancies spontaneously terminate (miscarry) during the first two months of pregnancy
1 in 600 babies are born with Down syndrome

8. Statistics for Down Syndrome
The risk of having a baby with Down syndrome increases with the mother’s age

9. Other nondisjunction disorders:
1. Turner syndrome
Monosomic: a female with a single X chromosome
Individual appears female but does not develop secondary sexual characteristics
Short, have thick, widened necks
1 in 3000 female babies

10. 2. Klinefelter syndrome
Nondisjunction in either the sperm or egg, the child inherits two X chromosomes and a single Y chromosome
Child appears male at birth, but develops female secondary sexual characteristics