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In this article we will discuss about:- 1. Meaning of Allele 2. Main Features of Alleles 3. Main Features of Multiple Alleles 4. Test for Allelism 5. Examples of Multiple Alleles.
Meaning of Allele:
Alternative form of a gene is known as allele. Alleles are of two types, viz., either dominant and recessive or wild type and mutant type. Mendel found only two forms of a gene for all the seven characters which he studied in garden pea.
Later on it was observed that in some cases a gene had more than two allelic forms. The existence of more than two alleles at a locus is referred to as multiple alleles. Presence of multiple alleles adds to the variability for a character having such alleles.
Main Features of Alleles:
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Alleles have some important characteristics which are briefly presented below:
1. Alleles are alternative forms of a gene. They occupy the same locus on a particular chromosome.
2. Alleles govern the same character of an individual.
3. A haploid cell has single copy of an allele, diploid two and polyploid more than two for a character.
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4. An individual may have identical alleles at the corresponding locus of homologous chromosomes (homozygous) or two different alleles (heterozygous).
5. The alleles may be dominant and recessive types or wild and mutant types.
Main Features of Multiple Alleles:
Important features of multiple alleles are given below:
1. Multiple alleles always belong to the same locus and one allele is present at a locus at a time in a chromosome.
2. Multiple alleles always control the same character of an individual. However, the expression of the character will differ depending on the allele present.
3. There is no crossing over in the multiple allelic series. If two alleles are involved in the cross, the same two alleles are recovered in F2 or test cross progeny. This is based on classical concept of gene, according to which crossing over takes place between gene but not within a gene.
4. In a series of multiple alleles, wild type is always dominant. Rest of the alleles in the series may exhibit dominance or intermediate phenotypic expression when two alleles are involved in a cross.
5. The cross between two mutant alleles will always produce mutant phenotype (intermediate). Such cross will never produce wild phenotype. To put in other way, multiple alleles do not show complementation.
Test for Allelism:
There are two types of tests that are used for allelism, viz., recombination test and complementation test.
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These are briefly discussed below:
1. Recombination Test:
Earlier it was believed that recombination can occur between two genes but not within a gene. Thus, if a cross between two mutants says m1m1 and m2m2 produces wild type in test cross or in F2 then m1 and m2 are considered as non-allelic because production of wild type is not possible without recombination.
If no wild type appears in test cross or F2 then m1 and m2 are considered as allelic forms. Now intragenic recombination has been reported in many organisms. Hence this concept is no more valid.
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2. Complementation Test:
Alleles may be arranged in two ways, viz., cis position and trans position (Fig. 13.1). When two wild alleles are located in one chromosome and their mutant alleles in homologous chromosomes (++/ab), it is known as cis-arrangement.
Thus, in cis position alleles are linked in coupling phase. On the other hand, when one wild and one mutant type alleles are located in each homologous chromosome (+a/+b), it is known as trans position or repulsion phase of alleles.
Complementation refers to appearance of wild phenotype when two mutants are crossed. Complementation test is used to determine whether two mutant alleles belong to same gene or two different genes. If there is complementation, the mutants are located in different genes, otherwise they are located in the same gene.
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Oliver in 1940 first demonstrated that intragenic recombination occurred in Lozenge gene of Drosophila. The two mutant alleles are considered to belong to the same gene if their cis heterozygotes produce wild type and trans-heterozygotes lead to mutant type.
If their both trans and cis heterozygotes lead to development of wild type, the mutant alleles are located in two different genes. Thus, cis-trans test is more reliable test of allelism.
Examples of Multiple Alleles:
Several cases of multiple alleles and pseudo alleles are known both in animals and plants.
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Some well-known cases of multiple alleles include:
(1) Fur colour in rabbits,
(2) Wing type in Drosophila,
(3) Eye colour in Drosophila,
(4) Self-incompatibility alleles in plants,
(5) ABO blood group in man, etc. Now it is believed that most of the genes have multiple alleles, if in depth investigations are made.
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1. Fur Colour in Rabbits:
The fur colour in rabbits is a well-known example of multiple alleles. In rabbits, the fur colour is of four types viz., agouti, chinchilla, Himalayan and albino.
These are briefly described below:
i. Agouti:
This has full colour and is also known as wild type. This colour is dominant over all the remaining colours and produces agouti colour in F1 and 3 : 1 ratio in F2 when crossed with any of the other three colours in rabbits (Table 13.1). This colour is represented by C.
ii. Chinchilla:
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This is lighter than agouti. This colour is dominant over Himalayan and albino and produces chinchilla in F1 and 3 : 1 ratio in F2 when crossed with either Himalayan or albino. This is represented by cch.
iii. Himalayan:
The main body is white while the tips of ear, feet, tail and snout are coloured. This colour is dominant over albino and produces 3 : 1 ratio in F2 when crossed with albino. This is represented by ch.
iv. Albino:
This has pure white fur colour and is recessive to all other types. This is represented by c.
Thus the order of dominance for fur colour in rabbits can be represented as follows:
Thus variation in fur colour in rabbits is due to multiple alleles of a single gene c.
2. Wing Type in Drosophila:
Wide variation in wing type of Drosophila is observed. There are five types of wing in Drosophila, viz., normal, nicked, notched, strap and vestigial wings. The size of wing goes on decreasing in all types. This variation in wing size is considered due to multiple alleles of the same gene.
The wild type is dominant over all other types. Crosses between other wing types exhibit intermediate expression in F1 and 1:2:1 segregation in F2. The nicked wings are notched at the margin, strap wings are very narrow and vestigial wings are miniature in size.
The order of dominance and gene symbol for wing size in Drosophila are given below:
These wing sizes are observed when the individuals are in homozygous condition. In heterozygous condition the expression is of intermediate type. Wild type is dominant over all other types.
3. Eye Colour in Drosophila:
The wild eye colour in Drosophila is red. There is wide variation for eye colour in Drosophila. First white eyed mutant was discovered and later on several other colours of eye were reported. The main eye colours include, wild, white, cherry, blood, eosin, apricot, ivory and cream.
The wild-colour is dominant over all other eye colours and exhibit 3 : 1 ratio in F2 generation. The cross between individuals of other eye colour exhibits intermediate expression in F1 and true expression only in homozygous condition.
The gene symbol and dominance order of various eye colours are given below:
This variation in eye colour was initially considered to be due to multiple alleles of the same gene. However, later on it was found to be due to pseudo-alleles.
4. Self-Incompatibility Alleles in Plants:
The most common example of multiple alleles in plants is the series of self-incompatibility alleles. Such alleles were reported in Nicotiana and later on they were found in several other plant species like Brassica, radish, tomato, potato, etc. In these species, self-incompatibility is governed by a single gene S which has multiple alleles, viz., S1, S2, S3, S4 and so on.
Now cases of digenic and trigenic self-incompatibility have also been reported. In evening primrose 37 and in red clover 41 alleles of self-incompatibility have been reported.
Crosses between individuals having self-incompatibility alleles will lead to three types of situations as given below:
a. Fully Sterile:
When both male female have similar alleles, viz., S1S2 x S1S2 the cross will be incompatible and there will be no seed setting.
b. Partially Fertile:
Such crosses are obtained when male and female plants differ for one allele, viz., S1S2 x S1S3. This cross will produces, S1S3 and S2S3 progeny. In other words, half of the progeny will be fertile.
c. Fully Fertile:
The fully fertile crosses are obtained when male and female plants differ in respect of both alleles, viz., S1S2 x S3S4. This cross will produce four fertile genotypes, viz., S1S3, S1S4, S2S3 and S2S4. Thus, plants which have self-incompatibility alleles are always heterozygous for this gene.
5. ABO Blood Group in Man:
ABO blood group is a good example of multiple alleles in man. The ABO blood group in man was first discovered by Landsteiner in 1900. Before dealing with ABO blood group, it is essential to define antigen and antibodies.
Antibody:
It is a type of protein which is commonly referred to as immunoglobin. It is usually found in the serum or plasma. The presence of antibody can be demonstrated by its specific reaction with an antigen.
Antigen:
An antigen refers to a substance or agent which, when introduced into the system of a vertebrate animal like cow, goat, rabbit, man, etc. induces the production of specific antibody which binds specifically to this substance.
Antigens are located in the red blood corpuscles (RBC). If a person has an antigen in his RBCs, his serum has usually natural antibodies against the other antigen. In human RBC two types of antigens, viz., A and B are present.
Depending upon the presence and absence of antigen A and B, the blood group in human is of four types, viz., A, B, AB and O. A person with blood group A has antigen A on the surface of RBCs; persons with blood group B will have antigen B; those with blood group AB have antigens A and B; and those with blood group O have no antigen on the surface of their RBCs (Table 13.2).
Recent studies have demonstrated that antigen A and B are not proteins, they are rather special types of carbohydrates. Antigen A is galactosamine and B is galactose. Antibodies B, A, none and AB 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 and 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, reverse transfusion of blood is also not possible. The blood group AB does not have antibody against antigen 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 then 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 leads to reaction resulting in death of the recipient.
These are only few examples of multiple alleles. Now it is believed that multiple alleles are present almost for all genes, but they can be identified, if in depth studies are made.
Pseudoalleles:
Pseudoalleles refer to closely linked and functionally related genes. A cluster of pseudoalleles is known as pseudoallele series or a complex locus or a complex region.
Main characteristics of pseudoalleles are given below:
1. Pseudoalleles govern different expressions of the same character. In other words, they are functionally related.
2. Pseudoalleles are considered to occupy a complex locus which is divided into sub loci. Thus, they occupy different positions, but on the same complex locus.
3. They exhibit low frequency of genetic recombination by crossing over. In other words, crossing over occurs between pseudoalleles, but at a very low frequency.
4. They exhibit cis-trans position effect. In trans heterozygotes such mutants produce mutant phenotype, but in cis-heterozygotes they produce a wild phenotype.
Examples of Pseudoalleles:
There are several examples of pseudoalleles. The well-known examples are lozenge gene and star asteroid in Drosophila.
These are briefly described below:
1. Lozenge Eye in Drosophila:
Green and Green (1949) studied lozenge locus in Drosophila. The mutant gene produces eye with glossy smooth surface. Several alleles of lozenge gene were identified and all mapped at one locus. All heterozygotes carrying two different mutants were lozenge in phenotype.
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But progeny of such heterozygotes produced wild type recombinants at a frequency much higher than expected spontaneous mutation. This indicated that Iz1 and Iz2 were pseudoalleles.
Pseudoalleles are closely linked genes which have similar phenotypic effects but can still be recombined with each other. The recombination between pseudoalleles is very rare. Such alleles are considered to be occupying a complex locus divided into sub loci between which recombination can occur.
Iz1 +/ Iz2 + Trans-heterozygote (mutant phenotype)
Iz1Iz2 /+ + Cis-heterozygote (wild phenotype)
2. Star Asteroid Eye in Drosophila:
Star (S) is a dominant mutant which produces slightly smaller eye than wild type in S/+ flies. Asteroid (ast) is a recessive mutant which gives much smaller eye in homozygotes (ast/ast). The S/ast heterozygotes have still smaller eyes.
Thus the order of eye size is as given below:
+ / + > S / + >ast / ast / ast> s /ast
The S and ast map in the same region. From the cross between star and asteriod (Star x asteriod), 16 wild type flies were recovered in a population of 57,000. This indicated intragenic recombination between S and ast which could produce wild type. Hence, these S and ast alleles are called pseudoalleles.
Thus, studies of pseudoalleles provided strong evidences in favour of intragenic recombination. A pseudoallele complex locus has several units of function, mutation and recombination. It means that a gene can be divided into sub units.
Isoalleles:
An allele which is similar in its phenotypic expression to that of other independently occurring allele is known as isoallele. In other words, isoalleles are those alleles which act within the same phenotypic range of each other.
Isoalleles are of two type as given below:
1. Mutant Isoalleles. Such alleles act within the phenotypic range of a mutant character.
2. Normal Isoalleles. Such alleles act within the phenotypic range of a wild character.
