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 co-dominant 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 Punnett-Square method.
Possible combinations of alleles that are associated with ABO blood typing
A slightly different kind of multiple allelic system is encountered in the coat colors of the rabbits: C allows full color to be produced(typical grey rabbit): cch, when homozygous, removes yellow pigment from the fur, making a silver-gray color called chinchilla; cch, when heterozygous ith alleles lower in the dominance hirearchy, produces light grey fur; ch produces a white rabbit with black extremities called Himalayan, c fails to produce pigment, resulting albino. The dominance hirearchy may be symbolized as follows: C > cch > ch > c
Rh factor, fist detected in the red blood cells of Rhesus monkeys, was initially thought to be caused by a gene with only two alleles, R and r. The Rh antigen is produced by a dominant gene R and persons homozygous for recessive allele r are Rh negative. Thus Rh negative parents can have children that follow the laws of simple Mendelian inheritance.
The events leading to erythroblastosis thus arose from the Rh negative genotype of the mother(rr) producing antiserum against the antigens of Rh positive offspring(Rr). SInce the R allele acted as a dominant to r, Rh positive males married to Rh negative females could have either all or half their offspring phenotypically Rh positive, depending on whether the paternal genetic constitution was respectively homozygous (RR) or heterozygous (Rr). Thus once an Rh negative mother had an Rh positive child, or once she had recieved transfusions of Rh positive blood, she became immunized against Rh positive antigens by producing anti-Rh serum. Successive pregnancies in which Rh positive children were conceived could then lead to serious haemolyitc anemia among these offsprings.
Currently, the destruction of fetal cells in the maternal circulation is deliberately undertaken by giving on Rh negative mother an injection of Rh antiserum (anti-Rh gammaglobulin-RhoGam) soon after the birth of her first (and succeeding) Rh positive child. By this, the Rh positive fetal cells that cross the placenta during parturition are destroyed before the mother can produce her own Rh antiserum. The injected antiserum then disappears, so that Rh antiserum is no longer present at te next pregnancy and the Rh hemolytic disease is avoided.
But later it was found that Rh factor followed the multiple allelic system. Wiener proposed that there were wight main alleles at the locus for this characteristic on the chromosome.
However, the English geneticist, R.A.Fisher believes that gee controlling the production of Rh antigen is an example of pseudoallelism. He hypothesizes that there are actually three genes responsible for the R antigens and these genes are so close together that they move together and act as a single gene. He proposed letter symbols for the three dominant positives, CDE and the recessive alleles cde. Rh -ve persons have cde/cde. If any one of the dominant gene is present, then the person is Rh +ve. The Fisher theory seems to be more in accord with recent discoveries in biochemical genetics.