CHROMOSOMAL ABERRATIONS
Dr. Jainendra Kumar
Professor & Head
Department of Botany & Biotechnology
College of Commerce
Patna (Bihar), India 800 020
Introduction
Occasionally, chromosomes undergo structural obliterations either as an adaptation change or as sudden freaks. Such a change may occur in one or more chromosomes of the organism. During normal course of life cycle, structural rearrangements in chromosomes do occur but less frequently but, under the effect of mutagenic agents such as radiations and conditions of stress like very high temperature, chromosomes are more prone to these changes. Structural changes in chromosomes were previously referred as chromosomal mutations. However, now, they are described as chromosomal aberrations not categorized as true mutations. In strict sense of the term, mutation is taken as architectural changes in gene (DNA) only.
Types of chromosomal aberrations
Four types of structural alterations have been found to occur in chromosomes of plants and animals. They are:
1. Duplication
2. Deficiency (Deletion)
3. Inversion
4. Translocation
Duplication is addition of copies of one or more existing genes on the chromosome causing increase in its size. This addition may occur terminally or at some intercalary position. Order of duplicated genes may be in tandem or in reverse order. Sometimes, genes of a chromosome may be duplicated on some other chromosome. Deficiency or deletion is loss of genes from a chromosome causing shortening in its size. It is always deleterious and may cause lethal effects.
Click here to see the drawings of duplication and deficiecy in chromosomes
Inversion is a special rearrangement of genes when an intercalary segment of the chromosome breaks and rejoins by rotating at 18 degree. Inversion may be either paracentric or pericentric. Paracentric inversion does not involve the centromere of the chromosome while pericentric type includes the centromere in the inverted segment.
Click here for types of inversion
Translocation is the fourth type of alteration in chromosome. Here, a segment transfer occurs between chromosomes. It occurs in three ways. Simple translocation is transfer of a terminal segment from a chromosome to a telomeric end of another chromosome which may be homologous or non-homologous. Due to involvement of a telomere which is normally non-sticky, this translocation is rare. Shift translocation is transfer of an intercalary segment from one chromosome to another at some intercalary position. Reciprocal translocation is most common type of translocation specially found in plants. One common example is evening primrose (Oenothera lamarckiana). In this translocation, which is also called Interchange translocation, two non-homologous chromosomes interchange parts.
Click for Translocation types
Inversion heterozygote and meiosis
Inversion heterozygote is an individual plant or animal having one inverted chromosome in a homologous chromosome pair but the other being structurally normal. Meiosis in an inversion heterozygote (paracentric or pericentric type) occurs in a unique way. During zygotene, synapsis of the affected homologues occurs by forming a special loop like configuration called inversion loop. This loop formation causes perfect gene to gene pairing of the homologues. If single crossing over occurs anywhere within the loop region such as between genes E and F involving the inner non-sister chromatids, the resulting recombinant chromosomes would be defective and fail to produce gametes.
Understand how meiosis occurs in an inversion heterozygote by looping of chromosomes during synapsis
Inversion is called crossing over suppressor due to the fact that it does not produce recombinants for the affected linkage groups.
Meiosis in Interchange heterozygote
Interchange heterozygote is a plant or animal with one or more chromosomes involved in reciprocal translocation. One chromosome of the homologues would be of normal architecture while its corresponding chromosome would be involved in translocation. Simplest interchange heterozygote would have two such homologue pairs. During meiosis, affected chromosomes of an interchange heterozygote form multivalents. After chiasmata are terminalized, multivalents are configured as a ring of chromosomes in metaphase I. Complete ring would be observed if all arms of the multivalent had chiasmata terminalized together. Segregation of adjacent chromosomes together to the respective poles in anaphase I would result into defective gametes having deficiency and duplication of different genes. Only alternate segregation as shown above would form viable gametes.
Look here for meiotic consequences in an interchange heterozygote
Significance of chromosomal aberrations
Alterations in chromosome architecture cause rearrangement of genes which lead to phenotypic changes in many cases. Alterations induce some degree of sterility. Duplication of genes may induce polyploidy like condition such as accentuation in fruit size in plants. In many plants, such as in Oenothera lamarckiana, interchange translocation has occurred among multiple chromosomes combined with self pollination system to maintain heterozygosity of the structural change. It is a case of adaptation to keep some lethal genes in balance.