Nondisjunction results in monosomy or trisomy.Explain how these terms apply to this type of abnormal meiosis?

Meiosis creates four daughter cells that are haploid (n, or having only one set of chromosomes) from a single diploid cell (2n = having 2 sets, which is normal in human somatic/body cells).





During meiosis I, the chromosomes are doubled, then they are separated into two daughter cells having 2n each. In meiosis II, these chromosomes are split without being doubled, so each daughter cells gets one chromosome set.





Nondisjunction is when a chromosome is not separated during telophase of either meiosis I or II, causing one daughter cell to get twice the normal number of copies and the other to get none. (Note: wording gets confusing. In meiosis I, every chromosome has 4 sister chromatids, meaning that the chromosome is really doubled and connected at the centromere - but scientists still call this one chromosome. Thus, for every cs in the genome - say, for cs #16 - there are two doubled chromosomes, each with 4 sister chromatids. Eventually, each of the 4 haploid daughter cells gets two of these chromatids from each type of cs, making one cs for each daughter cell.)





Monosomy = having only one cs in the normal genome, when you should have two in the diploid condition.


Trisomy = having 3 cs in the diploid condition.





So, when abnormal meiosis occurs, you get one daughter cell with 2 cs of a particular cs (this usually only happens in one cs, and not the entire cs set) and one with no cs of that particular type. Both of these should have only one cs of the type. When you get fertilization, the other gamete (sperm or egg) unites with these daughter cells, bringing with it one copy of the cs type. For the daughter cell with an extra cs, you get 3 cs for that type. The other daughter cells gets only one cs.





I hope you understood that!

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