# Chromosomal Mutation. Nondisjunction.?

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Suppose a diploid cell with three pairs of homologous chromosomes (2n = 6) enters meiosis. How many chromosomes will the resulting gametes have in each of the following cases?

Number of Chromosomes: 3 only; 0 or 3; 0 or 6; 2 or 4; 0, 3, or 6; 2, 3, 4

Situations:

1.) Meiosis occurs normally (no nondisjunctions)

2.) Nondisjunction of one chromosome pair in meiosis I

3.) Nondisjunction of all three chromosome pairs in meiosis

4.) Nondisjunction in one chromosome of one daughter cell in meiosis II

5.) Nondisjunction of all three chromosomes of one daughter cell in meiosis II

### 4 Answers

• Just draw out daughter cells in Meiosis I and II if you get confused.

1. Regular meiosis is 2n –> n, so each daughter cells has 3 chromsomes

2. After Meiosis I, 1 daughter cell will have 4 chromosomes, and the other 2. If disjunction occurs normally in Meiosis II, end result is 2 daughter cells with 4 chromsomes, and 2 daughter cells with 2 chromosomes. (4,4,2,2)

3. After Meiosis I, 1 daughter cell has 6 chromosomes, and the other 0. After Meiosis II, 2 daughter cells with 6, and 2 daugther cells with 0. (6,6,0,0).

4. After Meiosis I, both daughter cells has 3 chromsomes. After Meiosis II, 2 daughter cells will have 3 chromosomes, and 1 daugter cell will have 4, and 1 daughter cell will have 2. (3,3,4,2)

5. After Meiosis I, both daughter cells has 3 chromosomes. Non-disjuction occurs in both daughter cells in Meiosis II, so it would be 3,0,3,0

• 5. This should actually be 0, 3, or 6

• explanation from Mastering Gen:

If one chromosome pair undergoes nondisjunction in meiosis I, half the gametes will have an extra chromosome (n +1), and half will be missing a chromosome (n – 1).

If all chromosome pairs undergo nondisjunction in meiosis I, half the gametes will have twice the normal haploid number of chromosomes (2n), and half will have no chromosomes.

If one chromosome undergoes nondisjunction in meiosis II, half the gametes will have the normal haploid number of chromosomes (n), one-quarter will have an extra chromosome (n +1), and one-quarter will be missing a chromosome (n – 1).

If all chromosomes undergo nondisjunction in meiosis II, half the gametes will have the normal haploid number of chromosomes (n), one-quarter will have twice the haploid number (2n), and one-quarter will have no chromosomes.

• Part B Nondisjunction

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