1. When Meiosis Goes Wrong:
Non-Disjunction

2. Normal Meiosis
In Anaphase I, homologous chromosomes split up and each one goes to a pole.
In Anaphase II, the chromosome break in two and each chromatid goes to a pole.

3. Nondisjunction of homologous chromosomes in meiosis I
Sometimes, in Anaphase I, the chromosome pairs DON’T split…
2 cells with an extra chromosome, 2 missing a chromosome
In Anaphase II, if a chromosome doesn’t split in two…
2 normal cells, 1 with too many chromosomes, 1 with not enough.

4. So?
When non-disjunction happens, the egg or the sperm produced has the wrong number of chromosomes.
If this egg/sperm fertilizes, the baby will have the wrong number of chromosomes.
This child often suffers from health problems and learning disabilities.

5. To Diagnose Non-Disjunction
“Normal” Male:
Doctors use a karyotype chart, which shows all your chromosomes.
A “normal” karyotype has 46 chromosomes (23 pairs)
Males have an X and a Y, females have 2 X chromosomes.
“Normal” Female:

6. Examples of Disorders…
Down’s Syndrome
3 copies of chromosome number 21
Heart defects, poor mouth development, learning disabilities
Shortened life expectancy

7. Examples of Disorders…
Trisomy 13
3 copies of chromosome number 13.
Major learning disabilities, small head, mouth deformities
Few survive to adulthood

8. Examples of Disorders…
Turner’s Syndrome
Only 1 copy of the X chromosome.
Short, female, webbed neck, infertile, heart defects.
Normal life expectancy and intelligence.