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a) Polytene chromosomes arise in Drosophila due to a failure of separation after repeated replication of the parent chromosome, banding pattern of which is clearly visible under a microscope. During division, if there is a heterozygous deletion of a portion of a chromosome its pair with its homolog would result in a ring-like structure. (deletion loops) The position of the deleted portion indicated by the bands that are present in the ring, while missing from the homolog is clearly detected in cytogenetic analysis.
This can be seen in the following picture. Here the portion of the chromosome corresponding to the loci D, E and F are deleted from its homolog and hence there is no sequence available to pair, so it forms a loop.
The lethality of the homozygous deletion of a particular gene suggests that it is an essential gene for the viability of the organism and there is no other gene exists with a redundant function.
b) It means that most of these deletions are viable in the heterozygous state. So, if the gene is dominant, the organism can still survive due to the presence of a normal counterpart. Alternatively, if the gene is recessive, then the deletion will “expose” the function of that recessive allele because the fly still survives due to the expression of that recessive allele. In Drosophila many genes have been identified in that manner.
c) Deletions of some of the regions have not been found because, in the heterozygous conditions, these deletions might be lethal. Hence, we are not been able to find the genotype to phenotype in that case. Say, if the gene has a homozygous deletion that is a lethal but heterozygous deletion that is not lethal but shows a particular phenotype, then it can be mapped. If the heterozygous condition is also lethal then it cannot be found.
d) The above phenomenon is called “haplo-insufficiency” because in haploid organism there is only one allele. So, this one copy of the gene is unable to make the fly survive. Haploid status of this gene is lethal; hence it is called as haplo-insufficient. In other words, the deletion mutation is dominant....

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