Chapter 11, Problem 14STP

Summary Introduction

To write:

The steps for cell division to result in an organism with trisomy

Introduction:

Reproductive cells which pass on genetic traits from the parent to child are produced by the process of meiosis. Gametes are sex cells and have half the number of chromosomes.When a male gamete fuses with a female gamete, the process is called fertilization. Fertilization allows genetic variations in organisms. The result of fertilization is a diploid cell with 2n number of chromosomes.

Answer to Problem 14STP

Trisomy occurs due to nondisjunction either in meiosis I or meiosis II.

• Nondisjunction in meiosis I results in two daughter cells, one with ( n -1) number of chromosomes and the other with ( n +1) number of chromosomes. In meiosis II, four daughter cells or gametes are formed, 2 with ( n -1) number of chromosomes and 2 with ( n +1) number of chromosomes. Fertilization of ( n +1) gamete with male gamete will result in trisomy ( 2n +1).
• Nondisjunction in meiosis IIresults in four daughter cells, two normal haploid cells, one with ( n -1) number of chromosomes and the other with ( n +1) number of chromosomes. Fertilization of ( n +1) gamete with male gamete will result in trisomy ( 2n +1).

Explanation of Solution

Nondisjunction during meiosis may result in gametes with abnormal number of chromosomes. Cell division during which sister chromatids fail to separate properly is called nondisjunction. If an offspring has a set of three chromosomes of one kind, it is called trisomy. This can be due to nondisjunction either in meiosis I or meiosis II.

Meiosis I- During anaphase I, if nondisjunction occurs, it will result in failure of tetrads to separate. At the end of meiosis I there will be two daughter cells, one with ( n -1) number of chromosomes and the other with ( n +1) number of chromosomes. Now both these daughter cells will undergo meiosis II resulting in the formation of four daughter cells or gametes, 2 with ( n -1) number of chromosomes and 2 with ( n +1) number of chromosomes.

When ( n +1) gamete fuses with a male gamete during fertilization, it will result in trisomy with ( 2n +1) number of chromosomes. When ( n -1) gamete fuses with a male gamete during fertilization, it will result in monosomy with ( 2n -1) number of chromosomes.

Meiosis II- At the end of meiosis I there will be two daughter cells, each with n number of chromosomes. If nondisjunction occurs during meiosis II, it will result in failure of sister chromatids to separate during anaphase II. This will result in the formation of four daughter cells or gametes, 2 with n number of chromosomes, 1 with ( n +1) number of chromosomes and 1 with n-1) number of chromosomes.

When ( n +1) gamete fuses with a male gamete during fertilization, it will result in trisomy with ( 2n +1) number of chromosomes. When ( n -1) gamete fuses with a male gamete during fertilization, it will result in monosomy with ( 2n -1) number of chromosomes. Two gametes are normal haploid ( n ), so they will fertilize to form diploid zygotes.