Chapter 13, Problem 1BYB

Summary Introduction

To explain: The factor that causes a change in the number of chromosomes.

Introduction:

A change in the number of chromosomes is called aneuploidy. If there is an extra chromosome, it is called a trisomy and if there is a chromosome less, then it is called a monosomy.

Explanation of Solution

Aneuploidy can lead to the development of various disorders, many of which may be fatal. The most common way for this to occur is by addition or deletion of a chromosome. This happens during nondisjunction of chromosomes during meiosis. A normal diploid cell will give rise to two cells with monosomy (addition or duplication) and two with trisomy (deletion).

Another way for this to occur is by chromosomal structural anomalies. Though these are harmless to the person in whom it developed, but if it is passed on, it will lead to manifestation of some disorders by causing aneuploidy.

Translocation is a structural anomaly in which a part of one chromosome gets attached to another chromosome. A specific type of translocation called Robertsonian translocation causes the trisomy of $21$ in case of Down syndrome. Inversion is also a structural anomaly in which a part of chromosome gets inverted. The inverting part may include the centromere (pericentric) or may not include the centromere (paracentric).

Conclusion

Many reasons can contribute to the increase in the number of chromosomes like addition, duplication, deletion, translocation or inversion and many more.