Lecture Notes: Nondisjunction and Aneuploidy in Meiosis

Introduction to Nondisjunction

Nondisjunction: An error during meiotic division in germ cells.
- Occurs when homologous chromosomes don't separate during meiosis I or sister chromatids don't separate during meiosis II.
- Results in some cells with extra chromosomes and others missing chromosomes.

Aneuploidy: An abnormal number of chromosomes in a cell.
- Occurs due to nondisjunction.
- Examples include cells having additional or missing copies of chromosomes.

Consider a diploid germ cell with 2 pairs of homologs (2n = 4).
Nondisjunction in Meiosis I:
- During anaphase I, homologs typically separate.
- Nondisjunction results in homologs moving to the same spindle pole.
- Resulting cells have 3 chromosomes in one cell and 1 in the other.
Meiosis II and Gamete Formation:
- Normal separation during anaphase II.
- Resulting cells still vary in chromosome number (two with 3, two with 1).
- Gametes thus vary (n + 1 or n - 1).

Fertilization involves fusion of gametes:
- Trisomy (2n + 1): Resulting from n + 1 gamete fusing with a normal gamete.
- Monosomy (2n - 1): Resulting from n - 1 gamete fusing with a normal gamete.
Trisomy Example: Down syndrome due to trisomy of chromosome 21.

Assumes meiosis I occurs normally.
Nondisjunction in Meiosis II:
- Occurs when sister chromatids don't separate.
- Resulting cells: one with 3 chromosomes, one with 1, two with 2.
Gamete Variation and Fertilization:
- Gametes: one n + 1, one n - 1, two n.
- Fertilization outcomes vary in chromosome number.
- Zygotes can be trisomic, monosomic, or diploid.

Nondisjunction can lead to aneuploid zygotes.
Many human cells resulting from nondisjunction are naturally eliminated but can survive, leading to genetic disorders.

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