Question

After genotyping alleles on chromosome 18 the following genotypes were determined: Father-
5/8 Mother 6/3 Child 3/3/5 (each number represents a different allele for the same gene on a
different chromosome). In which parent and at what stage of meiosis did nondisjunction most
likely occur. Explain your reasoning.

Answer 1

Answer:

The nondisjunction has occurred in the mother and in the anaphase stage.

Explanation:

During the anaphase of meiosis, nondisjunction can occur. Sister chromatids (or homologous chromosomes for meiosis I) split during anaphase and are dragged to opposing poles. Nondisjunction occurs when the separation between sister chromatids or homologous chromosomes fails, leading both sister chromatids or homologous chromosomes to be drawn to one pole of the cell.

Answer 2

Non-disjunction occurred during the stage of meiosis II in the mother.

• Non-disjunction occurs when homo-logous chromosomes and/or sister chromatids are not separated during mitosis/meiosis.

• Meiosis II Non-disjunction occurs as a consequence of the failure separation of sister chromatids in Anaphase II.

• In this case, the mother is able to pass alleles number 6 and 3 to the offspring.

In conclusion, non-disjunction occurred during the stage of meiosis II in the mother.

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