Nondisjunction Disorders - mistakes during meiosis

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

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

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)

Fig. 17.9d 5

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

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

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

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

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