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MULTIPLE ALLELES
Allele is a shorter term than allelomorph (another form) is the alternate form of gene. Many genes have two alternate forms but several other have more than two alternate forms. More than two alleles at the same locus give rise to a multiple allelic series. Multiple alleles can be defined as a series of forms of a gene situated at the same locus of homologous chromosomes. According to Mendel, each gene had two alternate forms or allele morphs are being dominant and the other being recessive. Dominant being the wild type from which recessive mutant was evolved through mutation. Likewise, a wild type can mutate in many ways and produce many mutant forms and a mutant can again undergo another mutation and give rise to a new mutant. Hence, a gene can exist in more than two allelomorphs. Usually wild type allele is dominant over its recessive allele. wild allele is represented as +. Multiple alleles can be defined as a series of forms of a gene situated at the same locus of homologous chromosomes affecting same character. Multiple alleles are different forms of the same gene that is the sequence of the bases is slightly different in the genes located on the same place of the chromosome.
Multiple alleles are alternative states at the same locus. Remember: each individual will only have two alleles for a trait but there are several alleles to choose from.) The classical example for multiple alleles is human blood group self incompatibility in tobacco, coat colour in rabbit, self incompatability genes in brassica. The number of possible genotypes in a series of multiple alleles is ½ n (n+1) n = no of alleles Di-allelic genes can generate 3 genotypes. Genes with 3 alleles can generate 6 genotypes. Genes with 4 alleles can generate 10 genotypes. Genes with 8 alleles can generate 36 genotypes Important features of multiple alleles
- Multiple alleles always belong to the same locus and one allele is present at a locus at a time in a chromosome
- Multiple alleles always control the same character of an individual
- Wild type allele is dominant over other alleles
- There is no crossing over in the multiple alleles
- In a series of mutiple alleles wild type is always dominant
- When two mutant types are crossed wild form cannot be recovered
- The cross between two mutant alleles will always produce mutant phenotype. Examples of multiple alleles are 1) fur colour in a rabbit, 2) ABO blood group in man 3) Wing type in drosophila 4) Eye colour in drosophila etc. Fur colour in Rabbit. In rabbit, three alternate forms of genes, which controls coat colour. C causes wild type and its alleles.
Skin colour in rabbit In rabbits, four kinds of skin colour are known.
CC, Ccch, Cch, Cca^ - Agouti (wild type) cch, cch, cchch, cchc - Chinchilla (salivary grey hair)
Blood Group
Genotype Antigen found
Antibody present
Compatible blood group A IAIA, IAIA^ A B A and O B IBIB, IbIb^ B A B and O AB IAIB^ AB None A,B, AB,O O ii None AB O
Recent studies shows that antigen is galactosamine and B is galactose Antibodies A, B, AB and None and are naturally present in the serum of individuals having A,B,AB, and O blood group respectively. The agglutination or coagulation of RBCs leads to clotting of blood due to interaction between antigen antibody. The blood group B cannot be transferred to an individual having blood group A because the recipient has antibody against antigen B which is present on the RBCs of blood group B. Similarly the reverse transfusion is not possible. The blood group AB does not have antibody A and B. Hence individuals with AB blood group can accept all types of blood, viz., A, B, AB and O. Such individuals are known as universal acceptors or recipients. The O blood group does not have any antigen and has antibody against antigen A and B, It cannot accept blood group other than O. Individuals with blood group O are known as universal donors , because transfusion of blood group O is possible with all the four blood types. The consideration of Rh (rhesus) type is important in blood transfusion. Each blood group has generally two types of Rh group, viz positive and negative. The same type of Rh is compatible for blood transfusion Opposite type lead to reaction resulting in death of the recipient. These are few examples of multiple alleles Now it is believed that multiple alleles are present almost for all genes.
Multiple alleles in plants The classical example of multiple alleles in plants is ‘self incompatability alleles’ which prevents self fertilization.
Multiple alleles in Maize Multiple allelic series affecting seed color is seen in Maize. A Mutation a Mutation a’ (purple) (white) light purple
Multiple allele in cotton L mutation l mutation lB^ ll^ mutant lI narrow leaf Broad leaf mutant broad lacinated mutant intemediat
For information About 30% of the genes in humans are di-allelic, that is they exist in two forms. About 70% are mono-allelic, they only exist in one form and they show no variation. A very few are poly-allelic having more than two forms.
Pleiotropism In general one gene affects a single character. But many genes are known to affect more than one character such genes are known as pleiotropic genes and the condition is termed as pleiotrophy. An example of a pleiotropic gene in human beingsis the recessive gene s which produces sickle cell anemia in the ss homozygotes. These gene causes changes in two or more parts of characters, which are not related, then the gene is said to be pleiotropic gene. E.g. In cotton the Punjab hairy lintless gene lic produces seeds without lint. This gene also causes incomplete lancinations of the leaf, reduction in boll size and fertility. In a plant a gene may produce red pigment in several organs, such as flowers stem, leaves but still it is not correct to say that the gene is pleiotropic because the gene has only one general effect, the production of pigment. A gene for wing may be vestigial gene can be called as bristle gene or a fecundity gene. A number of other recessive genes produce marked and often detrimental effect in human beings. They are referred as syndromes. Penetrance Most genes produce identical phenotypes in all the individuals in which they are present in the appropriate genotype. For example, all the seeds having the w gene governing the seed shape in pea, in the homozygous state (ww) have uniformly wrinkled shape. Similarly, those seeds that have either WW or Ww genotype are uniformly round. The ability of a gene to produce identical phenotypes in all the individuals carrying it in the appropriate genotype is known as complete expressivity. As opposed to this, many genes have incomplete expressivity in that they produce variable phenotypes in the individuals that have this gene in the appropriate genotype. Expressivity In general, genes express themselves in all the individuals in which theya re present in the appropriate genotype, this is known as complete penetrance. But many genes do not
