Non-disjunction in meiosis I versus meiosis II: outcomes in the zygotes
Non-disjunction (a failure of chromosomes to separate) in meiosis I makes all four gametes abnormal, whereas in meiosis II it makes only two abnormal and leaves two normal.
Remember
Non-disjunction in meiosis I gives only abnormal gametes, so after fertilisation by a normal gamete no zygote is normal, but in meiosis II half the products are normal.
Non-disjunction is a failure of chromosomes to separate properly in cell division. In meiosis, how many gametes are abnormal depends on whether non-disjunction happens in meiosis I or meiosis II. The patterns below use a simplified model: one cell goes through meiosis with a single error, and each of the four gametes is then fertilised by a normal gamete.
In meiosis I, the two members of a homologous pair (the maternal and paternal copy of a chromosome) should go to opposite cells. If one pair fails to separate, one cell receives both chromosomes and the other receives neither. Meiosis II then divides each cell in two. It only splits sister chromatids (the identical halves of a replicated chromosome), so it cannot correct the fault. All four gametes are therefore abnormal: two have an extra copy and two have none.
Fertilisation by a normal gamete then gives two trisomic zygotes (three copies of that chromosome) and two monosomic zygotes (one copy), so none is normal.
If the error is in meiosis II, the first division was normal, so only one of the two cells is affected. In that cell the sister chromatids of one chromosome fail to separate, so one gamete gets two copies and the other gets none. The other cell makes two normal gametes. After fertilisation by normal gametes, that gives two normal zygotes, one trisomic and one monosomic.
When each of the four gametes from one meiosis is fertilised by a normal gamete, a zygote count with no normal members points to meiosis I, but two normal zygotes, one trisomic and one monosomic, point to meiosis II. Both errors can cause trisomic or monosomic outcomes, but a meiosis I error leaves no normal gametes, whereas a meiosis II error leaves two.
Key facts
- Meiosis I error
- all four gametes are abnormal, so after fertilisation by a normal gamete every zygote is trisomic or monosomic and none is normal.
- Meiosis II error
- two gametes are normal, one has an extra chromosome and one has a missing chromosome.
- Why meiosis II cannot fix a meiosis I error
- it only splits sister chromatids, so it cannot correct the fault and cannot restore the balance.
- Common ground
- both errors can cause trisomic or monosomic outcomes, but a meiosis I error leaves no normal gametes, whereas a meiosis II error leaves two normal gametes.
- Aneuploidy
- an abnormal number of chromosomes in the zygote.
- Down’s Syndrome
- non-disjunction in meiosis causes 96% of cases.
Exam traps
- Half of the zygotes being normal is the pattern after a meiosis II error, not after a meiosis I error, because no gamete is normal after a meiosis I error.
- A three-to-one split of trisomic to monosomic zygotes does not follow a meiosis I error, because that error gives two gametes with an extra chromosome and two with no copy, so fertilisation gives two trisomic and two monosomic zygotes.
- Failure of sister chromatids to separate in meiosis I is not what makes all four zygotes aneuploid, because sister chromatids normally remain together during meiosis I, when only homologous chromosomes separate.
Read the full notes
Tested in 6 SimpleMed+ questions
Practice it in Genetics, with the answer and a full explanation after every question.
Practice these questions