Friday, September 25, 2009

Multiple Alleles & their Functioning

If a gene has many modified forms and each modified form of the same gene produces different phenotype in the same general character them all of them are known as multiple alleles.

The term locus is used to represent the position of a gene on the chromosome. The strand of DNA of each gene may itself be of varying lengths. During the curse of occurrence of mutations, the modifications might occur at different places along the gene strand, thus producing many types of mutations in the genes which will either disturb or block the normal function of the gene or will modify the function of the gene. Therefore there may be many sites of mutations within the strand of DNA of a single gene, each leading to the development of specific phenotype in the same basic character to be a number of mutant alleles in the same gene due to the above mentioned fact.

Example (1): Four types of body colour are found in rabbit (1) wild type (brownish grey) (2) Albino (entirely colourless) (3) Chinchilla (body colour silver type and is fully coloured) (4) Himalayan, it is another mutant which is white except at extremities i.e. nose, tail and limbs are black where as the rest of the body is white.

Homozygous stains of all four mutants are maintained by the breeders. If pure chinchilla albino or Himayan are crossed with pure wild type, all the F1 individuals are of wild colour, showing thereby that the wild colour is dominant over all the mutant types. If one mutant is crossed with another type of mutant, different results are obtained. Such crosses have revealed that chinchilla is dominant over Himalayan and albino mutants. However Himalayan is dominant over albino.

Above explanation indicates that there may be more than one units or sites of mutation within a single gene. The mutants with different sites of mutation within single gene leads to the development of different specific alterations in the functioning of the gene.

Example (2): Another example of multiple alleles is of the gene controlling the blood group series A, B, AB and O. Four blood groups have been distinguished due to three modified forms of the same gene. Four blood groups are based on the fact, when the blood from different individuals is mixed together, a clumping of blood cell might occur or not. If blood stream of blood group A is mixed with blood of B group, clumping of blood cells occur. Mixing of serum and blood cells of same group do not show any clumping. Blood group ‘O’ does not contain any antigens on its RBC’s but does contain antibodies of A and B type in its serum. Blood group A is under control of a gene ‘A’. The gene controls the synthesis of A-type antigens blood group B is under the control of a gene ‘B’. It controls the synthesis of B-type antigens on the R.BC and A type antigens in the serum. Blood group O is under the control of gene ‘L’. It controls the synthesis of only A and B type antigens in the serum but no antigens are synthesized on the R.B.C.

Different alleles have been found at the locus which is very closely linked together. Presence of many alleles at the locus suggests that a mutation can modify the genetic information in different ways.

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