The classical phenotypic ratio resulting from the mating of dihybrid genotypes is 9:3:3:1. This ratio appears whenever the alleles at both loci display dominant and recessive relationships. The classical dihybrid ratio may be modified if one or both loci have codominant alleles or lethal alleles. A summary of these modified phenotypic ratios in adult progeny is shown below:
The genetic systems described so far have been limited to a single pair of alleles. The maximum number of alleles at a gene locus that any individual possesses is 2, with 1 on each of the homologous chromosomes. But since, a gene can be changed to alternative forms by the process of mutation, a large number of alleles is theoritically possible in a population of individuals. This is because the nucleic acid structure of a gene consists of many nucleotides and variations can arise at each of the nucleotide positions along its length. Thus there ar more than only two possible kinds of alleles in a gene; hundreds or perhaps thousands of possibilities exist and called multile alleleic series. i.e., A1, A2, A3,..An
A capital letter is used to designate the allele that is dominant to all others in the series. The corresponding small letter designates the allele that is recessive to all others in the series. Other alleles, intermediate in their degree of dominance between these two extremes, are usually assigned the small letter with some suitable superscript.
The colour of drosophila eyes is governed by a series of alleles that cause the hue to vary from red or wild type (w+or W) through coral (wcc), blood (w bl), cherry (wch), apricot (wa), honey (wh), buff (wbf), tinged (wt ), pearl (wp), and ivory (wi) to white (w). Each allele in the system exvept w can be considered to produce pigment, but successfully less is produced by alleles as we proceed down the hirearchy: w+ > wcc >w bl> wch> wa> wh> wbf> wt > wp> wi> w. The wild type allele (w+) is completely dominant and w is copletely recessive to all other alleles in the series.
A classical example of multiple alleles is found in the ABO blood group system of humans, where the allele IA for the antigen is codominant with the allele IB for the antigen. Both IA and IB are completely dominant to te allele i, which fails to specify any detectable antigenic structure. The hirearchy of dominance relationships is symbolized as (IA = IB) >1. (I stands for isohaemagglutinatinogen). Thus when paired with either IA or IB, its effect is masked. Since, three alleles of the gene exists in a population, it is considered a multiple allele system. The genotypes that produce the four blood groups are represented in the following table:
If the blood groups of the parents are known, the blood groups of the children can be determined (or vice versa) by using the