Chromosomal Aberrations PDF

Structural Chromosome Aberration Dr. Pratibha Bisen Dept. Plant Breeding & Genetics College of Agriculture, Balaghat JNKVV Jabalpur (M.P.)  Introduction Chromosomes may undergo changes. This is called structural variations in chromosome...

Structural Chromosome Aberration Dr. Pratibha Bisen Dept. Plant Breeding & Genetics College of Agriculture, Balaghat JNKVV Jabalpur (M.P.)  Introduction Chromosomes may undergo changes. This is called structural variations in chromosomes or chromosomal aberration. The change may occur either in structure of the chromosomes or in the number of chromosomes. Structural chromosome rearrangements are changes in the physical structure of chromosomes that may result in birth defects, mental retardation and increased risk for infertility. Allelic variations are due to mutations in particular genes Chromosomal aberrations are substantial changes in chromosome structure These typically affect more than one gene They are also called chromosomal mutations  Alterations in Chromosome Structure There are two primary ways in which the structure of chromosomes can be altered The total amount of genetic The genetic material may information in the chromosome can remain the same, but is change rearranged Duplication Deletions Inversion Translocation  History Based on these, the chromosomal aberrations are grouped into two major kinds- variation in structure and variation in number. 1926 A.H. Sturtevant Discovered inversions in chromosomes. 1923 C.B. Bridges Discovered duplications, deletions and translocations in chromosomes  Deletion A deficiency means deletion of a small portion of a chromosome resulting in loss of one or more genes. A deficiency originates from breakage occurring at random in both chromatids of a chromosome (called chromosome break), or only in one chromatid (chromatid break). The breakage may be caused by various agents such as radiation, chemicals, drugs or viruses at any time during the cell cycle, either in somatic or in germ cells. Depending upon its location, a deletion may be terminal when a single break occurs near the end of the chromosome; or interstitial when two breaks occur in a middle portion of the chromosome. The effect of a deletion depends on what was deleted. A deletion in one allele of a homozygous wild- type organism may give a normal phenotype, while the same deletion in the wild-type allele of a heterozygote would produce a mutant phenotype. Deletion of the centromere results in an acentric chromosome that is lost, usually with serious or lethal consequences. Examples of human disorders caused by large chromosomal deletions: Cri-du-chat (“cry of the cat”) syndrome, resulting from deletion of part of the short arm of chromosome 5 The deletion results in severe mental retardation and physical abnormalities.  Duplication A duplication involves attachment of a chromosomal fragment resulting in addition of one or more genes to a chromosome. Whenever there is a duplication in a chromosome, there is a corresponding deletion in another chromosome. The origin of duplications can be traced to unequal crossing over during meiosis. Normally homologous chromosomes are paired in a perfect manner so that identical loci lie exactly opposite each other. The mechanism ensures that after crossing over between non-sister chromatids, equal exchange products are formed. If paired chromosomes are misaligned, it is not possible for exchange to take place between exactly opposite locations on two chromatids. Duplications do not produce any drastic consequences as like deletion in terms of phenotype and survival. Different type of Duplication Example is the Drosophila eye shape allele, Bar, that reduces the number of eye facets, giving the eye a slit-like rather than oval appearance Duplication loops form when chromosomes pair in duplication heterozygotes In prophase I, the duplication loop can assume different configurations that maximize the pairing of related regions  Inversion Inversion results when a chromosome segment excises and reintegrates oriented 180° from the original orientation. – There are two types  Linked genes are often inverted together.  The meiotic consequence depends on whether the inversion occurs in a homozygote or a heterozygote.  A homozygote will have normal meiosis.  The effect in a heterozygote depends on whether crossing-over occurs.  If there is no crossing-over, no meiotic problems occur.  If crossing-over occurs in the inversion, unequal crossover may produce serious genetic consequences.  Inversion loop in heterozygote forms tightest possible alignment of homologous regions Gametes produced from pericentric and paracentric inversions are imbalanced  Translocation Integration of chromosome segment into a non-homologous chromosome is known as translocation. It involves the transfer of a segment of chromosome to a different part of the same chromosome or a different chromosome.  Types of Translocation Simple Reciprocal Shift an attachment of a small terminal segment of a It requires at least three chromosome to the end of breaks in the a homologous the breakage takes place in two non-homologous chromosome. Among chromosome or to a non- these, Reciprocal, and homologous chromosome. chromosomes, and is followed by the reunion of shift are the most broken segment to the common types. wrong partners.