When Meiosis Goes Wrong: Non-Disjunction

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.

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.

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.

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:

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

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

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.