Exceptions to the Mendelian Inheritance

In Mendel’s classic pea crosses, the F1 offspring always looked like one of the two parental varieties because one allele in a pair showed complete dominance over the other. Some phenotypes do not follow Mendel’s law of dominance. 

Following are the few exceptions to Mendelian inheritance. 

Incomplete Dominance 

Co-dominance 

Over Dominance

INCOMPLETE DOMINANCE 

When a cross between parents with contrasting traits generates offspring with an intermediate phenotype, the dominance relation between the alleles is called incomplete or partial dominance.

For example, if a four-o’clock (Mirabilis jalapa) or a snapdragon plant with red flowers is crossed with a white-flowered plant, the offspring have pink flowers. In this type of allelic interaction neither the red nor white flower color is dominant. 

Because neither allele is dominant, The F2   phenotypic and genotypic ratios are identical.   because neither allele is recessive, the upper- and lowercase letters are not used as symbols. Instead,  the alleles responsible for red and white color are represented as R1 and R2. 

Cross

When a cross between true-breeding red-flowered (R1R1) and true-breeding white-flowered plants (R2R2) is performed, all the F1 plants have Pink flowers (R1R2). When the F1 plants are self-pollinated the F2 plants have red, pink, and white flowers in the ratio 1 (R1R1) Red: 2 (R1R2) Pink: 1 (R2R2) White. 

OVERDOMINANCE

Overdominance Occurs When Heterozygotes Have Superior Traits

The phenomenon in which a heterozygote has greater reproductive success compared with either of the corresponding homozygotes is called overdominance, or heterozygote advantage.

For some genes, the heterozygotes have characteristics that increase survival in a particular environment. A heterozygote may be larger, disease-resistant, or able to survive harsh conditions. 

Sickle cell disease

This disease is an autosomal recessive disorder in which the affected individual produces a mutant form of hemoglobin.  Most people carry the HbA allele and make hemoglobin A. 

Individuals affected with sickle cell disease are homozygous for the HbS allele and produce only hemoglobin S. This causes their red blood cells to deform into a sickle shape under conditions of low oxygen concentration. This reduces the life span to only a few weeks compared with a normal span of 4 months, and therefore, anemia results. The homozygous HbSHbS individual usually has a shorter life span than an individual producing hemoglobin A.

The red blood cells of heterozygotes, HbAHbS, rupture when infected by the malarial parasite plasmodium, thereby preventing the parasite from propagating. People who are heterozygous HBAHbS have better malaria resistance than HbAHbA homozygotes. Therefore, even though the HbS allele is harmful in homozygous conditions it confers more resistance in heterozygous HbA HbS than in HbAHbA. 

The above Figure illustrates the predicted outcome of two heterozygotes.  In this example, 1/4 of the offspring are HbAHbA (unaffected, not malaria-resistant), 1/2 are HbAHbS (unaffected, malaria-resistant), and 1/4 are HbSHbS (sickle cell disease). This 1:2:1 ratio deviates from a simple Mendelian 3:1 phenotypic ratio.

CO-DOMINANCE

In a do-dominance relation, the effect of both alleles is equally visible in the phenotype of the heterozygote without being diluted by the presence of the other allele (as in incomplete dominance) or being suppressed by a dominant allele (as in complete dominance).

ABO Blood Group system

The ABO blood group system provides an example. Three alleles, I^A,  I^B , and i determine a person’s blood type. The two of these alleles, I^A,  I^B are codominant to each other, producing an AB blood type in the heterozygote. 

The ABO blood group system is determined by the antigen A and antigen B on the surface of RBC. These surface antigens are groups of interconnected sugars—oligo saccharides. The synthesis of these surface antigens is controlled by two alleles, designated IA and IB, respectively. The i allele is recessive to both IA and IB. 

A person who is homozygous ii has type O blood and does not produce either antigen. A homozygous IAIA or heterozygous IAi individual has type A blood and contains the antigen A. Similarly, a homozygous IBIB or heterozygous IBi individual has type B blood and produces surface antigen B.  A person who is IAIB has the blood type AB and expresses both surface antigens A and B. The phenomenon in which two alleles are both expressed in the heterozygous individual is called codominance. In this case, the IA and IB alleles are codominant to each other.

 

The above cross shows the possible offspring between two parents who are IAi and BI i. The IAi parent makes IA and i gametes, and the IBi parent makes IB and i gametes. These combine to produce IAIB, IAi, IBi, and ii offspring in a 1:1:1:1 ratio. The resulting blood types are AB, A, B, and O, respectively.