A geneticist observes that four gametes donated by an individual contain the fol

Question

A geneticist observes that four gametes donated by an individual contain the following. numbers of chromosomes: cell one has 23 chromosomes, cell two has 23 chromosomes cell three has 22 chromosomes, and cell four has 24 chromosomes. Consider the following hypotheses and clearly state why each hypothesis does or does not support this nondisjunction event. a. The sister chromatids did not separate in meiosis I. b. The homologous chromosomes did not separate in meiosis II. The homologous chromosomes did not separate properly in meiosis I. c. d. The sister chromatids did not separate properly in meiosis II. e. The sister chromatids did separate properly in meiosis II.

Explanation / Answer

Meiosis is the type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell, as in the production of gametes and plant spores. Disjunction is the phenomenon of pulling apart of either homologous chromosomes or sister chromatids during anaphase of meiosis I or meiosis II, normally producing four haploid cells.

When nondisjunction occurs during Meiosis 1, if the homologous chromosomes fail to separate, two cells are produced, of which one has both homologues, whereas the other has no copies of chromosomes. During Meiosis II, centromeres of two separate and they separate into two progeny cells each (two of the cells have both homologues whereas two of them are devoid of copies of chromosomes). This activity is unusual and the cells contain both copies of the chromosome and not one. Upon fusion with a normal haploid gamete, half of the cells that are produced are abnormal, and the cell will contain three copies of the chromosome, instead of the usual two, and is termed as trisomic. The other half of the cells have just one copy and is termed as monosomic.

When nondisjunction occurs during Meiosis 2, homologous chromosomes separate usually in the first phase, with two cells each having a pair of sister chromatids but here both normal and abnormal segregation occurs in the second phase.

These phenomenons produces an abnormal number of chromosomes in the diploid cell.

Thus, by the given problem, and the above clarifications

(a) The sister chromatids did not separate in meiosis 1, this does not support nondisjunction it cannot be that they do not separate, it might happen that they don’t separate properly.

(b) The homologous chromosomes did not separate in meiosis 2, so this also does not support nondisjunction as cannot be that they do not separate, it might happen that they don’t separate properly.

(c) This holds true and supports for nondisjunction, as stated in (a).

(d) This also holds true and supports for nondisjunction, as stated in (b).

(e) The sister chromatids did not separate in meiosis 2, this does not support nondisjunction it cannot be that they do not separate, it might happen that they don’t separate properly.

The stages of the cell cycle according to the figure is :

(A) Interphase

(B) Interphase Prophase Transition

(C) Prophase

(D) Metaphase

(E) Early Anaphase

(F) Late Anaphase

(G) Telophase

(H) Cytokinesis

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