Hybridisation experiments conducted on pea plants, snapdragon plants, on birds like pigeons and poultry and on several animals like mice, rabbits, cats, guinea pigs and even man. However, it was discovered that the Law of Dominance didn’t hold true in all cases, that is Mendel’s laws were not universal in occurrence.
When a pure line red-flowered snapdragon, crossed with a true-breeding, white-flowered snapdragon, the flowers of the F1 hybrid were pink. That is, the F1 hybrid flowers had a phenotype that resembled neither of the parents but was in-between the two. Thus, when a dominant allele does not completely mask the phenotypic expression of the recessive allele in a heterozygote, a blending of both dominant and recessive traits takes place in the F1 and F2 heterozygotes. This phenomenon is called incomplete dominance.
The flowers of the F1 hybrid were self-pollinated and the genotypic ratios obtained in the F2 hybrid were the same result obtained by Mendel in his experiments on peas. This is because in the case of snapdragon plants, neither of the alleles, is dominant, but each allele has its influence on the colour and the resultant F1 hybrid is pink – intermediate in colour. It is the gene for flower colour that controls the amount of pigment in petals. Each allele is a code for a trait, that is, specific amount of pigment. When both alleles for pigment are present, the petals are dark red due to heavy pigment production. On the other hand, if none of the alleles for pigment exist, the flower is white. However, when only one of the alleles is present, only half the pigment is produced, creating a pink shade. The alleles are therefore exhibiting incomplete dominance, which is a form of intermediate inheritance, where one allele for a specific trait is not completely dominant over the other allele.
A gene is the unit of heredity that occupies a fixed position on a chromosome called the locus.
A gene generally has two forms called alleles. In homozygotes, the alleles are the same while in heterozygotes, they are not identical.
An allele may be different due to certain changes it has undergone, which modify the information contained by that particular allele. A gene that contains information for producing an enzyme. This gene contains two copies, which are the two allelic forms. Normal allele produces the normal enzyme, the modified allele could be responsible for producing a normal enzyme, a less-efficient enzyme, a non-functional enzyme or no enzyme at all. Now, if the modified allele produces a normal or a less-efficient enzyme we can say that the modified allele is equivalent to the unmodified or normal allele.