Describe deficiency and Duplications in the structure of chromosomes


Deletion or deficiency represents the loss of segments of a chromosome which may include the loss of a single gene or part of a gene. Deletions become the first chromosome aberrations when in 1917 bridges demonstrated that a sex linked recessive gene (EO) for cosine eye colour came t expression in a Drosoplulla presumed to be heterozygous condition. He explained this unusual expression of EO gene by saying that a section of the homologue containing the dominant allele (+) of EO was missing due to deltion. Deficiencies may be classified of terminal type is produced when a single break occur near the end of a chromosome. Intercalary type of deletion on the other hand results by two breaks which occur some where in the length of the chromosome.

In either of the types of deficiencies the deleted chromosome is left without genes carried away in the delted portion unless the deleted part rejoins to the same or to another chromosome without centromer the deleted segment cannot move towards the pole of the spindle during cell division but lags the dividing cell and is excluded from the chromosome group when the nuclear membrane forms around the chromosomes of the daughter cell.

GENETIC CONSEQUENCES OF DELETION:
(1)        Recessive genes in heterozygous conditions due to deletion of its dominant allele express itself. Thus the expression of recessive allele in deleted heterozygous condition is called pseudo dominance. The wings with cut margins or notch is a sex linked recessive trait in Drosophilla. Notch express itself as a dominant in heterozygous deletion females. Another sex linked recessive trait Eosin eye colour express itself as pseudo dominant allele in deletion heterozygous condition.

(2)        Duplications:
The presence of a section of chromosome in excess of the normal amount in known as Duplication. The repeated section of the chromosomal material may be present in one pair of homologous chromosomes or may have been transposed to a non homologue or on occasion may even exist independently with its own centromere. Following are possible types.

(1)        TENDEM:
When any segment of chromosome exists twice in a normal order i.e. sequence de-de.

(2)        PEVERSE TENDEM:
In which the duplicated sequence is in reverse order i.e. sequence deed.

(3)        DISPLACED HOMOBRACHIAL TENDEM:
In this case the duplicated segment place itself in a new position at the same chromosome arm as shown by the sequence of ab-be.

(4)        DISPLACED HETEROBRACHIAL:
In this case the duplicated segment align itself on different arm of the same chromosome.

(5)        TRANSPOSITION TANDOM:
In this case the duplicate segment is shifted to different atm.

CONSEQUENCES DUPLICATION:
Duplication has proved a useful mechanism by which genetists can study dosage effect. It has also helped us to study the phemotypic effect of alleles.

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