Hereditary characters are transmitted from parents to offspring through discreet units called genes which were called factors by Mendel.
Even though it may not be possible to see a gene, its presence as a discreet segregating particle has been proved both in the plant and animal world in a large majority of cases.
Segregation of characters is possible when large variations exist in a character. For instance if all pea plants had green seeds or only yellow seeds, it would not be possible to visualize the segregation much less speculate on the existence of different genes.
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According to Mendel, in all sexually reproducing diploid organisms there are a set of factors for every character; meaning that red flower color is controlled by two genes, white flower color by two genes, green seed color by two genes, yellow seed color by two genes etc.
Red flower color and white flower color are not two characters really; similarly the yellow and the green seed color also.
Flower colour is a character and the red and white is the two variants; similarly seed color is a character and green and yellow are two variants.
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Why it is red flower color and white flower color are not regarded as two different characters but variations of the same trait because the genes responsible for these two colors (red and white) can interact producing either red or white depending on whichever character is dominant.
Then what about the genes that produce these characters, are they same or are they different? The answer would be they are neither totally same nor are they totally different. They represent a pair of genes which are related to each other.
They have arisen from the same source, but due to mutation one gene has become slightly different from the other since they are basically same, they occupy the same locus on the chromosome and control the-same character.
In the case of pea plant, red flower color is supposed to be the original color as it is dominant. This means that originally all pea plants used to produce only red flowers and subsequently the red gene mutated and the mutated form became the white gene.
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The genes for white and red flower color are variants of the same gene that is why they can control the variants of a single character.
Such genes are said to be alleles. Alleles maybe defined as alternative state of existence of the same gene occupying same loci on homologous chromosomes.
According to Mendel, a character can exist only in two states -dominant and recessive and hence for every character or every gene there are only two alleles (allelomorphs).
All Mendelian characters have only two alternate expressions or have only two alleles. But this may not be true (having only two alleles) in all cases, for a gene.
Once having undergone mutation may mutate again to give one more altered form or allele and in their case, there will be three alleles. Theoretically a gene ‘a’ may mutate any number of times leading to the formation of many variants or multiple alleles.
A multiple allelic series of a gene represents a1 sense of gene mutations in the same locus leading to a variety of phenotypic expressions segregated in different organisms. Multiple alleles may be defined ass series of gene mutations from wild to a number of mutated forms all of them affecting the same trait.
Dawson and Whitehouse have used the term pan alleles to denote multiple alleles. Multiple alleles should not be mistaken with multiple factors where non allelic genes present in different loci affect a trait; further reify pairs of genes are involved in each individual to delimit a character.
In multiple alleles, the genes occupy the same locus on the chromosome in different organisms producing variants of a trait due to sequential mutations in multiple alleles only one pair of genes is involved in each individual to delimit a character.
Multiple alleles are different from the diallelic series of Mendel in that the former produce many variants, while the latter produce only two variants.