Extensions of Mendelian Inheritance is actually an exceptions, and revisions to mendel’s laws. Mendel studied those characters that have only one mode of inheritance in pea plants and all followed the relatively simple pattern of dominant and recessive alleles for a single trait. After years, there are many other modes of inheritance, which were discovered after Mendel’s work, that do not follow the dominant and recessive, single-gene model and were later collectilvely termed as “Extensions of Mendelian Inheritance“. Overview of variations on Mendel’s laws, includes multiple alleles, incomplete dominance, co-dominance, pleiotropy, lethal alleles, sex linkage, genetic interactions, polygenic traits,etc.,

Table of Contents

• Interallelic or Intra-genic interaction
  • Incomplete dominance (1:2:1)
  • Co-dominance (1:2:1)
    • Examples
      • MN Blood groups
      • ABO Blood groups
      • Hemoglobin alleles (HbA and HbS)
  • Multiple Alleles
    • Example
      • Hair colour in mice
      • Human blood type
  • Lethal Alleles

Interallelic or Intra-genic interaction

Intragenic interaction, also called as interallelic interaction is a type of gene interaction in which the alleles of the same gene interact with each other to deviate from the mendelian inheritance patterns.

• Haplosufficient:- One copy of gene in a diploid organism is sufficient to give normal phenotype.
• Haploinsufficient:- Haploinsufficiency describes a model of dominant gene action in diploid organisms, in which a single copy of the wild type allele at a locus in heterozygous combination with a variant allele is insufficient to produce the wild-type phenotype.
• Blending Inheritance:- The theory is that the progeny inherits any characteristic as the average of the parents’ values of that characteristic.

Incomplete dominance (1:2:1)

• A cross between parents with contrasting traits may generate an intermediate phenotype
• F₁ hybrids were not related to either of the parents but exhibited a blending of characters.
• Neither allele is dominant so the phenotype ratio is identical to genotype ratio in F2 (1:2:1)
• So,we can say that Incomplete dominance occurs when the phenotype of the heterozygote is the hybrid of the homozygotes
• This is usually due to loss-of-function mutation that leads to a dosage effect of the protein. Less protein leads to less product.
• Also termed as partial dominance
• Examples:-
  1. Flowers color in Mirabilis Jalapa and Snapdragon (A^RA^R: Red, A^RA^w: Pink, A^wA^w: White)-below figure
  2. Foot feathers in pigeon (F^SF^S:Footfeather, F^SF^NS: Mild foot feather, F^SF^NS: No foot feather

• Incomplete dominance at cellular/cell shape level
• Example:-
• Shape of RBC-Normal red blood cells are disk-shaped, but in sickle-cell disease, the abnormal hemoglobin HbS (β⁶ Glu–> Val) polymerizes reversibly when in deoxygenated state & forms a fibrous polymer, this stiffens the RBC membrane, increases cell viscosity & cause dehydration due to potassium leakage & calcium influx. These changes produce the sickle shape which leads to the distortion of RBC & disturbances of oxygen transport.
  1. Hb^AHb^A: Norma RBC, No Sickling
  2. Hb^AHb^S: No Anemia, Sickling only under low oxygen concentration
  3. Hb^SHb^S: Severe anemia, Fatal, RBC sickle shaped

Co-dominance (1:2:1)

• In co-dominance, both the factors of an allelomorphic pair express themselves equally in F₁ hybrids.
• It means a heterozygous for codominant genes exhibits both the characters side by side.
• These follow the law of segregation and F₂ progeny exhibits 1:2:1 ratio both in genotype and phenotypes.

Examples

MN Blood groups

• MN locus codes for surface glycoprotein on red blood cells and located on chromosome number 4. Karl Landsteiner, the discoverer of MN blood-typing.
• The ability to produce the M and N antigens is determined by a gene with two alleles.
• One allele allows the M antigen to be produced; the other allows the N antigen to be produced. Homozygotes (L^ML^M) for the M allele produce only the M antigen, and homozygotes (L^NL^N) for the N allele produce only the N antigen.
• However, heterozygotes (L^ML^N) for these two alleles produce both kinds of antigens. Because the two alleles appear to contribute independently to the phenotype of the heterozygotes, they are said to be codominant.

ABO Blood groups

• ABO is one of the 36 blood group systems in humans. Discovered by Landsteiner. Locus for ABO blood group is at chromosome 9.
• Characterized by the presence of specific antigens on the surface of RBCs.
• Different blood groups inherited due to presence of three alleles (I^A, I^B, I^O or i) in different combinations. The genes from each A and B loci are inherited in pairs as Co-dominant.
• I stands for Isoagglutinogen, I^A is responsible for production of A antigen, I^B is responsible for production of B antigen, I^O or I does not produce A or B antigen.

• People with AB blood types have both antigens naturally in their systems, so their immune system will not attack those blood cells.
• This makes people with the AB blood type “universal recipients” due to the co-dominance displayed by their AB blood type.
• The A type does not mask the B type and vice -versa. Therefore, both the A antigen and B antigen are equally expressed in a display of co-dominance.

Hemoglobin alleles (HbA and HbS)

• In regard to hemoglobin itself, there is codominance.
• The alleles HbA and HbS encode two different forms of hemoglobin that differ by a single amino acid, and both forms are synthesized in the heterozygote.
• Hb^AHb^S: No Anemia, Sickling only under low oxygen concentration.
• The A and S forms of hemoglobin can be separated by electrophoresis because it happens that they have different charges.
• Homozygous Hb^A/Hb^A people have one type of hemoglobin (A), and anemics have another (type S), which moves more slowly in the electric field.
• The heterozygotes have both types, A and S. In other words, there is codominance at the molecular level.

• Phenotypic  ratios  for simple genetic crosses (crosses for a single locus) with Codominance is given in table below.

Other examples of codominance in animals include speckled chickens, which have alleles for both black and white feathers, and roan cattle, which express alleles for both red hair and white hair.

Multiple Alleles

• Multiple alleles are more than two forms of the same gene in the population.
• For the sake of simplicity, we usually use examples of genes with only two possible alleles (A and a). But, a single gene can actually have many possible alleles (A, a, A1, A2, A’, etc.).

Example

Hair colour in mice

• This is determined by a single gene with a series of alleles, each resulting in different colouration. There are alleles for black, brown, agouti, gray, albino, and others.
• The twist here is that the same allele can be dominant or recessive depending on context.
• For instance the allelic series for coat colour in mice may be written as agouti > black > albino. This means that agouti is dominant to black, and black is dominant to albino; agouti is necessarily also dominant to albino.
• If the black allele is in the presence of an agouti allele, the mouse will be agouti because black is recessive to agouti. If that same black allele is paired with an albino allele, the mouse will be black since black is dominant to albino.

Human blood type

• There may be multiple alleles within the population, but individuals have only two of those alleles. This is because individuals have only two biological parents, and only one allele is contributed by each parent. An excellent example of multiple allele inheritance is human blood type.
• Blood type exists as four possible phenotypes: A, B, AB and O. There are three alleles for the gene that determines blood type, I^A, I^B and i.
•  The I^A allele codes for A molecules on the red blood cells, the I^B allele codes for B molecules on the surface of red blood cells, and the i allele codes for no molecules on the red blood cells.
• In this case, the I^A and I^B alleles are codominant (already discussed) with each other and are both dominant over the i allele. Although there are three alleles present in a population, each individual only gets two of the alleles from their parents.
• With three alleles we have several possible combinations of genotypes and resulting phenotypes, as shown in the following table.

•  Notice that instead of three genotypes, there are six different genotypes when there are three alleles.
• The number of possible phenotypes depends on the dominance relationships between the three alleles.

Number of alleles in a series decide number of genotype

• As ABO blood types has three alleles – A,B and O therefore number of genotypes can be calculated using n(n+1)/2 and it comes the value of 6. So, 6 genotypes (I^AI^A, I^AI^o, I^AI^B, I^BI^B, I^BI^O, I^OI^O).

Lethal Alleles

• Alleles that are lethal, or deadly, in a homozygous recessive individual can remain in a population’s gene pool.
• NO Intermediate condition. Both expresses themselves and are equally dominant.
• In F₂ generation both Phenotypic & Genotypic ratios are 2:1.
• The four types of lethal alleles are
  1. Dominant Lethal alleles:-•Lethal both in Homo/heterozygous condition. •Mutant genes get eliminated from population.
  2. Recessive Lethal alleles:- •Lethal only in homozygous condition. •No distinct phenotype of heterozygotes (some exception).
  3. Complete Lethal genes:- •Death at zygotic, embryonic or after birth.
  4. Sublethal / Semilethal genes:- •Lethality in sexually mature individuals.