Question

Figure $13.6$Which of the following statements about nondisjunction is true?

a. Nondisjunction only results in gametes with $n+1$or $n-1$chromosomes.

b. Nondisjunction occurring during meiosis II results in $50$percent normal gametes.

c. Nondisjunction during meiosis I results in $50$percent normal gametes.

d. Nondisjunction always results in four different kinds of gametes

Short answer

The correct answer is option (b).

Step-by-step solution

Introduction

Nondisjunction's are the cause of chromosome number disorders. They are the result of incomplete separation of homologous (similar) chromosome pairs, that is failure of sister chromatids separation during meiosis ( I or II ). This leads to chromosome number anomaly meaning the chromosome number either increases or decreases as there can be duplication or loss of entire chromosome respectively.

Explanation for the correct option

It is evident from the given figure that during Meiosis II, out of the four gamete formed, two have normal number of chromosome i.e. $n$, one gamete has one extra chromosome copy ($n+1$)and the other one is lacking the chromosome that is present in another one as extra copy ($n-1$).

So, out of four gamete, two are normal meaning that $50\%$of the gamete formed during Meiosis II are normal. Thus option (b) is the right choice.

Explanation for incorrect options 

During nondisjunction in meiosis II gamete with n number of chromosome are also formed.

Thus, option (a) is incorrect.

In meiosis I only gamete with $n+1$and $n-1$are formed i.e. no normal gamete is formed.

Thus, option (c) is incorrect.

In meiosis I we have two $n+1$and two $n-1$gametes formed. Similarly in meiosis II we have $2\left(n\right)$, $n+1$,$n-1$gametes formed. So no four gametes formed are of different kind.

Thus, option (d) is incorrect.