Non-Disjunction Can Occur in Anaphase I and II
Definition
Non-Disjunction
The failure of homologous chromosomes or sister chromatids to separate properly during meiosis.
- Non-disjunction can occur in two critical stages of meiosis:
- Anaphase I: When homologous chromosomes fail to separate.
- Anaphase II: When sister chromatids fail to separate.
To remember the stages of meiosis, think of the acronym PMAT: Prophase, Metaphase, Anaphase, Telophase.
Down Syndrome is Caused by Non-Disjunction
- Down syndrome is caused by trisomy 21, where an individual has three copies of chromosome 21 instead of the usual two.
- This extra chromosome arises from non-disjunction during meiosis in one of the parent’s gametes.
- When a gamete with an extra chromosome 21 fuses with a normal gamete during fertilization.
- This results in a zygote with 47 chromosomes instead of the typical 46.
- Imagine a cell in Anaphase I of meiosis.
- Instead of the homologous chromosomes separating evenly, both copies of chromosome 21 move to the same pole.
- This error results in one gamete with an extra chromosome 21.
- If this gamete participates in fertilization, the zygote will have three copies of chromosome 21, leading to Down syndrome.
Characteristics of Down Syndrome
- Physical Traits:
- Distinctive facial features (e.g., almond-shaped eyes, flat nasal bridge).
- Short stature and low muscle tone.
- Health Challenges:
