1. Nondisjunction Disorders
- mistakes during meiosis

2. Diagram: Meiosis in the female egg (only represent the 23rd chromosome)
The female will only ever donate an X chromosome to a child XX
XXXX X
Germ cell (parent cell)
duplication
End of Meiosis I
End of Meiosis II
End of Meiosis II
Egg polar body
Polar body polar body

3. Diagram: Meiosis in the male sperm (only represent the 23rd chromosome)
the male may donate an X chromosome to a child or the male may donate a y chromosome to a child Xy
XXyy XX X yy y
Germ cell (parent cell)
duplication
End of Meiosis I
End of Meiosis II
End of Meiosis II
Sperm Sperm
Sperm Sperm

4. 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)

5. Fig. 17.9d 5

8. 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

9. 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

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

11. 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

12. 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