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Nondisjunction Disorders

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Presentation on theme: "Nondisjunction Disorders"— Presentation transcript:

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

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