Question

Inheritance patterns cannot always be explained by Mendel’s models of inheritance. If a pair of homologous chromosomes fails to separate during meiosis I, select the choice that shows the chromosome number of the four resulting gametes with respect to the normal haploid number (n)?a. N+1, n+1, n-1, n-1b. N+1, n-1, n, nc. N+1, n+1, n, nd. N+1, n-1, n-1, n-1

Answer 1

Answer:

A (n+1, n+1, n-1, n-1)

Explanation:

This disorder in chromosomal number is referred to as ANEUPLOIDY. Aneuploidy is a condition whereby one or more chromosomes are present in extra copies or deficient in number, but definitely not in a complete set i.e. neither diploid nor haploid. Aneuploidy, which is when an abnormal number of chromosomes occur in a cell, is caused by meiotic nondisjunction i.e. failure of chromosomes to separate during meiosis.

Nondisjunction occurs when pairs of homologous chromosomes fail to separate during meiosis I or sister chromatids fail to separate during meiosis II. The occurrence of nondisjunction in meiosis I or II leads to different results in the daughter cells/gametes.

Since meiosis I is being considered in this question, if homologous chromosomes fail to separate during the anaphase of meiosis I, the result is two gametes that lack particular gametes (n-1) and two gametes with an extra copy of the chromosome (n+1)

That is, after cytokinesis, the four gametes will have a chromosomal number of n+1, n+1, n-1, n-1.

N.B: The number of chromosomes in a normal gamete is haploid (n).

This case of aneuploidy can lead to lethal cases. If a normal gamete (n) is fertilized by the gamete with an extra copy of chromosome (n+1), the resulting zygote will possess a condition called TRISOMY, also, when the normal gamete (n) fuses with a gamete with no copy of the chromosome (n-1), the resulting zygote will have a monosomy condition.

Answer 2

The correct answer is: a. n+1, n+1, n-1, n-1

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during mitosis  or meiosis. There are three forms of nondisjunction:  

1. failure of a pair of homologous chromosomes to separate in meiosis I ,  

2. failure of sister chromatids to separate during meiosis II, and  

3. failure of sister chromatids to separate during mitosis.

As a consequence of nondisjunction abnormal chromosome numbers appears. If nondisjunction occurs during the meiosis I (anaphase I), this means that at least one pair of homologous chromosomes did not separate. The consequence is two cells that have an extra copy of one chromosome and two cells that lack that chromosome.