The unit of translocation may be the chromosome (chromosome translocation) the single chromatid (chromatid translocation) or a segment of chromatid. There are four types of chromosomal aberrations. A metacentric chromosome may become submetacentric and vice versa (Fig. However, the whole arm of the chromosomes may be duplicated, thus giving rise to isochromosome. This is primary structural change of chromosome. The broken ends are sticky and the replication of the broken pieces may result in two sister chromatids which may be joined together due to their sticky ends. Adjacent-1 and adjacent-2 segregation produce unbalanced gametes. Deficiency can be detected by its two characteristics, namely, genetic effects and cytological effects. These are changes involving chromosome morphology, which results in changes in number and sequence of genes without altering ploidy or gene structure. Duplication 3. Long V-shaped chromosome has all the vital genetic material and the small chromosome is little more than a centromere plus some chromatic material. Duplication 4. Types of Inversions: 1. 22.17). This is simple translocation. 5.1. Three-strand double crossing over will yield one non-cross over chromatid, one cross-over chromatid and two acentric fragments. If the sequence of genes in the translocated segment is the same as that in the original segment with respect to centromere, it is referred to as encentric translocation. DNA damage and repair lead to chromosomal aberrations With the exception of polytene chromosomes, eu- karyotic chromosomes are uninemic. It is, therefore, evident that paracentric inversions have a drastic effect on the recovery of chromatids involved in crossing over. Image Guidelines 5. These aberrations may be of two kinds – Spontaneous aberration and Induced aberration. Chromosomes contain a number of genes on them. In Vitro Chromosomal Aberration Test The purpose of the in vitro chromosome aberration test is to identify agents that cause structural chromosome aberrations in cultured mammalian somatic cells. There are three special cases of whole arm translocation: Translocation occurring between two sub-telocentric chromosomes, each with sub-terminal centromere and single long arm, may exchange parts in such a way that major part of long arm of one is translocated to the short arm of the other producing one V-shaped long metacentric chromosome (i.e., with median centromere) and one small chromosome with two minute arms. Reunion of the broken ends takes place in a new combination, and inverts. There are two common patterns of distribution, adjacent and alternate (Fig. 43.2). Where all linkage groups or chromosomes in the haploid chromosome set are doubled, this is referred to as genome mutation. The modification brings about chromosomal mutations. (Fig. Contrary to centric fusion, in dissociation one metacentric chromosome with long arms and the second metacentric chromosome with short arms after reciprocal translocation give rise to two acrocentric chromosomes [Fig. When a chromosome contains more than one inverted segment, it is called complex inversion. Similar situation has been reported in several other plants exposed to X-rays, gamma-rays and mutagenic chemicals. Like deficiency and duplication, the translocations may also be homozygous or heterozygous provided the aberrations in question are not associated with lethality. This can be frequently observed in maize. Numerical chromosomal aberration. One of the most common methods is unequal crossing over, a process which produces one chromosome with duplication and another with deficiency. Adjacent centromeres again go to the same pole but these are now homologous as well as containing both translocated and non translocated chromosomes (1, 1′; 2, 2′). 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