Structural Changes in Chromosome

Structural Changes in Chromosome

Deletion

                When a chromosome somehow loses a part of itself, deficiency of genes contained in deleted part occurs. The loss may be at the end of chromosome causing terminal deficiency and it may be central due to two breaks in the chromosome and the reunion at wrong places causing intercalary deficiency. Loss of a chromosomal segment may be so small that it includes only a single gene or part of a gene. In this case the phenotypic effects may resemble those of a mutant allele at that locus. Deletion never back mutate to the normal condition, because a lost piece of chromosome cannot be replaced in this way, a deletion can be distinguished from a gene mutation. A loss of any considerable portion of a chromosome is usually lethal to a diploid organism because of genetic unbalance. When an organism heterozygous for a pair of alleles (Aa) loses a small portion of the chromosome bearing the dominant allele, the recessive allele on the other chromosome will become expressed phenotypically. This is called pseudo dominance and the condition is called hemizygous (when only one allele is present in diploid organism)
deletion of chromosome

A deletion heterozygote may be detected cytologically during meiotic prophase when the forces of pairing cause the normal chromosome segment to bulge away from the region in which the deletion occurs. Overlapping deletions have been extensively used to locate the physical position of genes in the chromosome (cytological mapping).
deletion of chromosome


Duplication

                It is also called addition. A chromosome gains a portion of a homologous chromosome and is doubled for that chromosomal material. Extra segment in a chromosome may arise in a variety of ways. Generally speaking, their presence is not as deleterious to the organism as a deficiency. It is assumed that some duplications are useful in the evolution of new genetic material. During meiotic pairing the chromosome bearing the duplicated segment forms a loop to maximize the union of homologous regions.
duplication of chromosomes

duplication of chromosomes

Inversion

                A segment of the chromosome may break at two places and the broken ends reunite on the wrong ends after making a loop, the gene order is altered and is called inversion. Assume that the normal order of segment within a chromosome is (1-2-3-4-5-6) and that breaks occur in regions 2-3 and 5-6, and that the broken pieces is reinserted in reverse order. The inverted chromosome now has segments (1-2-5-4-3-6). During meiosis the synaptic configuration attempts to maximize the pairing between homologous regions in the two chromosomes. This is usually accomplished by a loop in one of the chromosomes. Crossing over within the inverted segment gives rise to crossover gametes which are unviable because of duplications and deficiencies. Chromatids which are not involved in crossing over will be viable. Thus inversions produce semi sterility and altered linkage relationships.
inversion of chromosome

 

If the inverted region contains centromere it is called pericentric inversion and if the centromere lies outside the inverted segment it is called paracentric inversion. Crossing over within the inverted segment in case of paracentric inversion, a dicentric chromosome produces which forms a bridge from one pole to the other during first anaphase. The bridge will break somewhere along its length and the resulting fragments will contain duplications/deficiencies. And acentric fragment will also be formed; and since it usually fails to move either pole, it will not be include in the meiotic products.

inversion of chromosome

Translocation

                It is exchange of segments between non homologous chromosomes. Chromosomes occasionally undergo spontaneous break, or can be included to break in high frequency by ionizing radiations. The broken ends of such chromosome behave as ‘sticky’ and may rejoin into non-homologous chromosomes. A reciprocal translation involves the exchange of segments between two non-homologous chromosomes. During meiosis, an individual who is heterozygous for a reciprocal translation must form a cross shaped configuration in order to affect pairing of all homologous segments. Translocation heterozygotes have several distinctive manifestations.
Semi sterility is usually characteristic of translocation heterozygote because of adjacent disjunctions. Some genes which formerly were on non-homologous chromosome will no longer appear to be assorting independently. The phenotypic expression of a gene may be modified when it is translocated to a new position in the genome. Position effects are particularly evident when genes in euchromatin are shifted near heterochromatin regions.
translocation of chromosome

translocation of chromosome




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