The wild gene is described first and is given the symbol of a capital letter (A,B,C etc) the next in the series has the same capital letter but is super scribed on top with a suitable small letter This small letter is the identifying mark for the allele.
The series of alleles have the same capital letter but with different small letters on top. We shall study some examples of multiple allelic series both in plants and animals. The following traits have been proved to be multiple alleles in different organisms:
1. Coat Color in Rabbits:
In rabbits a series of skin colors are seen. These are –
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Full Color: (Wild)
The skin color of the ordinary rabbit is referred to as full color with banded strips of yellow and grey with a brown or black tip.
Chinchilla:
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In certain rabbits the skin lacks the yellow pigment only the mixture of black and grey giving a silvery appearance.
Himalayan albino:
These rabbits have pink eyes and their fur is white except for the tips of feet, tail, nose, ear etc which are either dark or brown. Such a type of pigmentation only at the extremities is known as acromelanism (Serra 1965)
Albino:
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In these rabbits there is total absence of pigmentation. The eyes however are pink in color. The above series of skin color is known to represent a multiple allelic series as revealed by crosses.
An albino crossed to a wild (full color) rabbit produces in the F all wild rabbits. Self breeding the F, results in 3/4 wild and 1/4 albino in the F2 generations indicating the color gene and the albino genes are alleles.
When the Himalayan albinos are crossed with wild ones, the F, shows all wild, but in the F2 generation 3/4 wild and 1/4 Himalayan albinos are produced indicating again that color gene and Himalayan albino genes are alleles.
The most interesting cross now would be a cross between albino and Himalayan albino. If albino and Himalayan are due to two different genes there must be a dominant allele in each of the two resulting in the appearance of full color in the F, progeny.
In a cross between Himalayan and albino, the F, are all Himalayan and the F2 has Himalayan and albino in the ratio of 3:1. Reversion to full color does not occur.
This is due to the fact that a gamete never carries Himalayan and albino genes together. It has only one of the genes; similarly colored animal in the heterozygous condition will carry either Himalayan or albino but never both.
This clearly reveals that the Himalayan gene, color gene and the albino gene are located on the same locus on the homologous chromosome (any two in the any one only in the gamete); that is why they do not segregate.
It is obvious then that these are allelic representing the variants of the same gene. Himalayan and albino are allelic to each other and both are allelic to full color.
Full color is dominant over both Himalayan and albino, while Himalayan is recessive to full color and dominant over albino. Albino is recessive to both Himalayan and full color.
The other variant of skin color, Chinchilla is less intense in color. It is dominant to all others except to full color. The genes for all these represent a multiple allelic series starting with the wild and ending up with albino. The series will be as follows in the order of decreasing dominance.
These represent a series of mutations occurring in the same locus starting with wild gene. Genotypically each one could be either totally homozygous or heterozygous except for albino. In albino, since the allele is completely recessive it has to be homozygous to produce the trait.
In other cases each dominant allele can have the recessive alleles. For e.g. wild type may have in addition to a dominant allele an allele either of chinchilla, Himalayan albino or albino. Three types of heterozygous genotypes are possible for the wild type.
Chinchilla may have both dominant alleles of its own in the homozygous condition one of its dominant allele and the other either Himalayan or albino. Himalayan will have two genotypes; both alleles of Himalayan or one Himalayan and one albino.
The genes for various colors are – wild C, chinchilla C, Himalayan and albino C”. The .various genotypes are given below in a tabular form.
Albinism in other organisms:
Allelic series of genes bringing about albinism have been found in many other animals like mouse, rat, guinea pig, cat- etc., in the domesticated cat one of the alleles in the albino series gives a color called Siamese color favored by many.
In human beings also an allele of albinism is there leading to loss of pigmentation in skin and hair Albinos have all recessive alleles of the lowest order in the series and can exist only in the homozygous condition.
Sequential mutation from C to C has occurred in almost all mammals. But this trait has not been encountered quite frequently in the wild state as they cannot compete with the normal ones. They survive only in the domesticated state.
2. Eye Color in Drosophila:
T.H.Morgan (1909) a geneticist, discovered a mutant white eyed fly among a population of red eyed flies of Drosophila nelanogaster this color originated as a mutant in the X chromosome. Subsequently several mutations have been identified in the same locus all affecting the eye color.
To date, between the wild red eye and the mutant white eye about 10 variants have been reported these are sequential mutations, with the individual gene becoming more and more recessive with every mutation.
Wing size in Drosophila:
This is another multiple allelic trait in Drosophila. The normal wings are long in Drosophila. But there are two more variations at this, in the form of vestigial wing and antlered wing. These two traits arise as a result of mutation in the same locus.
Having arisen in the same locus, the vestigial wing and antlered wing are alleles of each other and also of the normal long wing these three form a series in the order of decreasing dominance.
The last (3rd) cross mentioned above needs some clarification. While normal wing is dominant over both vestigial and antlered, the two mutant alleles do not show dominance or recessiveness between them, with the result a cross between them results in an intermediate wing in the Fr interbreeding of the F, produces in the F2 vestige, intermediate and antlered in the ratio of 1:2:1. When two alleles interact without showing dominance or recessiveness they are called allelic compound in this instance the intermediate wing is a compound.
3. Sexual Incompatibility in Nicotiana:
In bisexual flowers, self pollination is more likely than cross pollination as both male and female gametes are in the vicinity.
But in many instances, even though the gametes (male and female) produced by “the plants are both functional, fertilization failsi to take place either due to failure of pollen to germinate on the stigma or the pollen tube growth is extremely slow.
Such a condition where the gametes of the same flower fail to fuse is known as self incompatibility or self sterility.
Nicotiana, a member of the family Solanaceae is a typical example of self incompatibility. Two types of self incompatibility have been noticed in flowering plants.
These are Gametophytic self incompatibility (GSI) and Sporophytic self incompatibility (SSI). GSI has been reported in families like Liliaceae, Gramineae, Solanaceae etc., while SSI is found in families Cruciferae, Cempositae etc.
East observed that multiple alleles are responsible for self incompatibility. He and Manglesdorf (1925) explained the genetic basis of self incompatibility. According to them the wild gene for self incompatibility can be designated as S gene, which has undergone a series of mutations producing the allelic series S1, S2, S3, S4, S5 etc.
In GSI suppose a plant is of the S (S2 genotype its pollen will have either gene. Hence it would not germinate on a pistil with S1 or S2 because similarity of genes would bring about incompatibility.
Fertilization would take place if the pollen grain carrying either S1 or S2 gene falls on SD3 S4 pistil. In the cases of SSI the pollen should not carry even a single gene that is similar to the female parent.
If the male parent has the Genotype SS2, the pollen carrying this can only pollinate a stigma where both SS2 should be absent are self incompatible because of the repetition of the same gene.
If the female parent has S3S4, S3S5 etc. pollination is possible.” Plants with bisexual flowers are known to have an extremely large multiple allelic series in order to highly restrict mating and to have very few selective mates.
In some instances a multiple series is known to have as many as 40 alleles. The following table gives the possible mating chances and the genotypes of the Offspring in a multiple allelic series having S S2 and S3 in both male and female.
Multiple Allelism in Maize (Mosaic Dominance):
In some organisms as in maize, the multiple allelic series may influence different combinations of traits in different parts of the body. Stadler who worked on a multiple series of gene (R) affecting the purple coloration of the plant.
In seed as well as in the stem and leaves found that an allele may affect various parts variously for e.g., if a particular allele produces color in the seed, it may or may not produce color in the stem or leaves. The following four combinations of phenotypic traits have been shown by Stadler –
1. RR Purple seed, purple plant
2. rr White seed, Purple plant
3. Rg Purple seed, green plant
4. r white seed, green plant
In this instance the gene R acts as though it is a compound gene having two parts -R controlling the seed color and r controlling the plant color.
When mutation occurs it may occur in only one component resulting in the change of the components of the compound gene.
As a result, the two components of the gene may cause various combinations of traits like purple seed-green plant, white seed-purple plant etc. Here the purple seed color and purple plant are dominant over white seed and green plant.
As a result of the mutational change there may be mixture of dominant and recessive characters in a plant (controlled by the same allele).
This is called mosaic dominance. There may be a question as to why these two components of a gene should not be regarded as separate genes, if they can undergo mutation separately.
However since these two are located so close they cannot be separated by cross over hence they have to be regarded as components of the same gene.
Stadler further noticed in maize that the R’ gene in various strains distributes the pigment in various ways. These may be possible due to sub alleles, which may be defined as variations of the components of a compound gene.