enter image description here

The child has gotten 2 "sick" chromosomes from his dad and 1 healthy from his mom, leaving it with trisomy 13.

I can see how this can happen in the second meiotic division of the father but I can ALSO see how this can happen in the FIRST meiotic division by non-disjunction.

Let me illustrate both ways which I see it and you can tell me where I go wrong as my book only illustrates that is the second division:

enter image description here

The black is the healthy chromosome and the red is the "sick" one. As you can see, we should end up with a double sick chromosome in both cases??

  • $\begingroup$ I'm starting to believe that my professors/whoever made the material has misunderstood nondisjunction as everything I read seems to indicate that it can happen in both 1. and 2.? Example: genetics.thetech.org/sites/default/files/NonDisjunction12.gif If the "small" chromosome is the blue one you can notice that it can happen in both. Anyone else agree? $\endgroup$
    – Paze
    Jun 7, 2015 at 17:46
  • $\begingroup$ A) you are combining trisomy with translocation B) your final question is a bit unclear— are you asking why non-disjunction cannot happen in first division? $\endgroup$
    Jun 7, 2015 at 17:54
  • $\begingroup$ How am I combining trisomy with translocation? Nobody mentioned translocation. The small allele is the product of translocation, yes but it has nothing to do with the way the question is formulated. The final question asks if you can get trisomi from a nondisjunction in 1st meiotic division. $\endgroup$
    – Paze
    Jun 7, 2015 at 17:55
  • 1
    $\begingroup$ So you can remove the reference to the "sick" chromosome. It is unnecessary and confusing. $\endgroup$
    Jun 7, 2015 at 17:57
  • $\begingroup$ It's there to illustrate the fact that these are 2 different variations of the same chromosomes. Else people may think it was a different chromosome adding even more confusion. I don't know why this confuses you. I answered my own question anyhow. $\endgroup$
    – Paze
    Jun 7, 2015 at 18:12

2 Answers 2


Non-disjunction and trisomy 13.

Why can't this be 1st meiotic division non-disjunction as well?

It absolutely can be 1st meiotic division non-disjunction as well. Patau syndrome can occur as a result of either meiotic non-disjunction event, and furthermore a mosaic form of the disorder can occur after mitotic non-disjunction.

Non-disjunction generally.

Non-disjunction can occur in meiosis I, but it's not necessarily the same result as non-disjunction event in meiosis II. This image helps describe the formation of the gametes from non-disjunction events.

This image
(source: bioninja.com.au)

The most obvious point is that there are more aneuploidy cells born from a meiosis I non-disjunction event than a meiosis II non-disjunction event.

Furthermore non-disjunction occurs between homologous chromosomes in meiosis I or between sister chromatids in meiosis II. Was this subtlety perhaps what your lecturers/teachers were trying to describe?

Confusion somewhere in the question perhaps.

I think the problem comes from your first diagram, which appears to be an inaccurate sketch of what trisomy 13 looks like in a karyotype. This is what trisomy 13 actually looks like:

trisomy 13 chromosome microcope
(source: iupui.edu)

How non-disjunction leads to a disease karyotype.

Lets pretend that the 'long' chromosomes in the first diagram of this answer are chromatids of chromosome 13. Thinking about the fusion event between gametes - the aneuploidy (n+1) gamete formed from either meiotic non-disjunction event and a normal (n) gamete could fuse to make the trisomy 13 result shown in the karyotype above; there would be three 'long' chromatids rather than two.


Your illustration is wrong. During second meiosis the sister chromosomes go to each their cell, not the same cell.

This would correspond to switching the colors up in your illustration like this:

enter image description here

  • 2
    $\begingroup$ What textbook are they using for your genetics course in the First year of medical school? $\endgroup$
    – mdperry
    Jun 8, 2015 at 1:37

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