Q. 3 - Biology

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 

Step-by-Step Solution

Answer

The correct answer is option (b). 

Step 1: 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. 

Step 2: 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.

Step 3: 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.