Multiple alleles

Multiple alleles

A given phenotypic trait of an individual depends on a single pair of genes, each of which occupies a specific position called the locus on homologous chromosome. When any of the three or more allelic forms of a gene occupy the same locus in a given pair of homologous chromosomes, they are said to be called multiple alleles.

1.  Characteristics of multiple alleles

·         Multiple alleles of a series always occupy the same locus in the homologous chromosome. Therefore, no crossing over occurs within the alleles of a series.

·         Multiple alleles are always responsible for the same character.

·         The wild type alleles of a series exhibit dominant character whereas mutant type will influence dominance or an intermediate phenotypic effect.

·         When any two of the mutant multiple alleles are crossed the phenotype is always mutant type and not the wild type

2. Self-sterility in Nicotiana

In plants, multiple alleles have been reported in association with self-sterility or self-incompatibility. Self-sterility means that the pollen from a plant is unable to germinate on its own stigma and will not be able to bring about fertilization in the ovules of the same plant. East (1925) observed multiple alleles in Nicotiana which are responsible for self-incompatibility or self-sterility. The gene for self-incompatibility can be designated as S, which has allelic series S₁, S₂, S₃, S₄ and S₅ (Figure 3.17).

The cross-fertilizing tobacco plants were not always homozygous as S₁S₁ or S₂S₂, but all plants were heterozygous as S₁S₂, S₃S₄, S₅S₆. When crosses were made between different S₁S₂ plants, the pollen tube did not develop normally. But effective pollen tube development was observed when crossing was made with other than S₁S₂ for example S₃S₄.

When crosses were made between seed parents with S₁S₂ and pollen parents with S₂S₃, two kinds of pollen tubes were distinguished. Pollen grains carrying S₂ were not effective, but the pollen grains carrying S₃ were capable of fertilization. Thus, from the cross S₁S₂XS₃S₄, all the pollens were effective and four kinds of progeny resulted: S₁S₃, S₁S₄, S₂S₃ and S₂S₄. Some combinations are showed in the table-3.5.