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Mendelism refers to the theoretical study of the hereditary principles of the genes. It was first formulated by Austrian born botanist, teacher Gregor Johann Mendel. He formulated two crosses to establish the principles of inheritance by taking a single gene or two genes at a time for the cross. Two kinds of the cross are studied mainly to understand the inheritance pattern which affects genotype as well as phenotype. Monohybrid cross is about the study of the expression of one gene at a time and produces a genotypic ratio of 1:2:1 along with a Phenotypic ratio of 3:1 in the F2 generation. Dihybrid cross involves study of two genes at a time and gives a phenotypic ratio of 9:3:3:1 and a genotypic ratio of 1:2:1:2:4:2:1:2:1. The cross where a single gene was considered was known as the monohybrid cross and this helped establish the Law of Segregation and the Law of Dominance. The cross where two genes were taken into consideration was known as the dihybrid cross and it helped establish the Law of Independent Assortment. This is an overview of what is called Mendelism or the Mendel’s laws of inheritance. But there are some exceptions to this thought too.
Till date there have been five possible deviations to Mendelism-
1. Incomplete dominance:
It is a condition where the two different alleles of a gene are present together and when expressed, a new phenotype is formed which is somewhat intermediate to the two.Here, dominance of the dominant allele over the recessive allele is not observed.The best example for this is the flower colour of the 4 o’clock plant (Mirabilis jalapa). The two flower colours for this plant are red and white.Let’s take the dominant allele to be R and the recessive allele to be r.When RR is present together the flower colour observed as red, and when rr is present together the flower colour observed is white.But when the heterozygous Rr is present together, the dominating effect of R over r is not observed and therefore both the alleles give rise to a new flower colour which is the pink colour.
2. Codominance:
It is the condition when both the alleles of a gene express themselves simultaneously when present in this heterozygous condition.
The most commonly studied example for this is the ABO blood group in humans.
Allele | Allele | Genotype | Phenotype |
A | A | AA | A blood group |
A | O | AO | A blood group |
B | B | BB | B blood group |
B | O | BO | B blood group |
A | B | AB | AB blood group |
O | O | OO | O blood group. |
The blood group in humans is controlled by the gene I. The gene I has three alleles namely A, B and O.
A and B are the dominant alleles whereas O is the recessive allele.
When A is present with O or A the blood group expressed is A blood group.
When B is present with O or B the blood group expressed is B blood group.
O blood group is only expressed when both the alleles are present in a recessive condition.
But when the dominant alleles A and B are present in a heterozygote the blood group expressed is AB blood group because of codominance.
3. Multiple alleles:
It is a condition where a particular gene has more than two alternative alleles.
These multiple alleles can exhibit codominance, incomplete dominance and normal dominant- recessive relationship too.
The example for multiple alleles can again be the ABO blood grouping in humans.
The gene I which governs the blood group in humans has three alternative alleles such as the A, B and O.
4. Polygenic inheritance:
It is the phenomenon in which more than one gene controls a single character.
It is also known as quantitative inheritance because here, the number of alleles determine the severity of the character and the alleles have a cumulative effect.
The various examples of polygenic inheritance is white spotting in mice, green color in meat and height in humans.
5. Pleiotropy:
It is the condition where one gene controls multiple characters.
The suitable example for this condition is the disease phenylketonuria.
It is a genetic disorder which is generally caused by low metabolism rate of the amino acid (phenylalanine) in the body. The enzyme phenylalanine hydroxylase is required to break phenylalanine into tyrosine. In the disorder the gene that codes for the enzyme phenylalanine hydroxylase is mutated, due to which the functional enzyme is not formed and the amino acid (phenylalanine) is stored in the body. This amino acid has adverse effects such as lung disorder, pain, eczema, brain disorders, etc.Therefore, we can see that a mutation in the single gene led to so many conditions.
Conclusion:
The Mendelian Laws explain the transfer of traits from one generation to another and decide the genotype and phenotype of an individual.However, when studies were conducted on different traits of organisms, what surprised most geneticists was inconsistency with Mendel’s monohybrid and dihybrid cross results. After research, it was concluded that these were deviations from Mendel’s work due to different reasons and phenomena.
