Extra Mendelian inheritance - Incomplete and Co dominance, Multiple alleles, Lethal alleles Epistasis, Pleiotropy

 Extra Mendelian inheritance- Alternatives to Dominance and Recessiveness

 Mendel’s experiments with pea plants suggested that:

 (1) two “units” or copies exist for every gene; alleles

(2) alleles maintain their integrity in each generation (no blending); and

(3) in the presence of the dominant allele, the recessive allele is hidden and makes no contribution to the phenotype.

Therefore, recessive alleles can be “carried” and not expressed by individuals. Such heterozygous individuals are sometimes referred to as “carriers.”  Further genetic studies in other plants and animals have shown that much more complexity exists.

The alleles of the same gene interacts in such a way to produce a new character, which is known as allelic interactions.

 Incomplete Dominance 

• In incomplete dominance, neither allele is completely dominant over the other.
• The heterozygous phenotype is intermediate between the two homozygous phenotypes.
• Example: In snapdragon flowers, red (RR) and white (WW) alleles produce pink (RW) flowers. 

Co-dominance

• In co-dominance, both alleles in the heterozygote are fully expressed.
• The phenotype shows both traits simultaneously.
• Example: Human ABO blood group system, where I^A and I^B alleles are co-dominant producing AB blood group. 

Multiple Alleles

• More than two allelic forms exist for a gene in a population.
• An individual still carries only two alleles, but the gene has multiple variants.
• Example: ABO blood group gene has three alleles: I^A, I^B, and Iⁱ.

 

Lethal Alleles

• Alleles that cause death when present in homozygous condition.
• They can be dominant or recessive.
• Example: In mice, the yellow coat color allele (Y) is lethal when homozygous (YY). 

Epistasis

• Interaction between genes where one gene masks or modifies the expression of another gene.
• It affects phenotypic ratios in dihybrid crosses.
• Example: Coat color in Labrador retrievers where one gene controls pigment color and another controls pigment deposition.

 

Pleiotropy

• A single gene influences multiple, seemingly unrelated phenotypic traits.
• Example: The gene responsible for Marfan syndrome affects connective tissue, causing effects in skeleton, eyes, and cardiovascular system.

 

Incomplete dominance

 

Incomplete dominance is a form of gene interaction in which both alleles of a gene at a locus are partially expressed, often resulting in an intermediate or different phenotype.  It is also known as partial dominance.

• For eg, the pink color of flowers (such as snapdragons or four o'clock flowers (Mirabilis jalapa)

In incomplete dominance, the genes of an allelomorphic pair are not expressed as dominant and recessive but express themselves partially when present together in the hybrid. As a result F1 hybrids show characters intermediate to the effect of two genes of the parents. It occurs when neither of two alleles is fully dominant nor recessive towards each other. The alleles are both expressed and the phenotype, or physical trait, is a mixture of the two alleles.

In less technical terms, this means that the two possible traits are blended together.

F2 generation shows genotype ratio of 1RR:^:2Rr:1rr  and a phenotype ratio 1:2:1 red:pink:white.

 

  

    The phenotype of a heterozygous organism is a blend between the phenotypes of its homozygous parents. In the snapdragon, Antirrhinum majus, a cross between a homozygous white-flowered plant C^WC^W and a homozygous red-flowered plant C^RC^R will produce offspring with pink flowers C^RC^W. This type of relationship between alleles, with a heterozygote phenotype intermediate between the two homozygote phenotypes, is called incomplete dominance.

 

     

 

Self-fertilization of a pink plant would produce a genotype ratio of 1C^RC^R: 2 C^RC^W:1C^WC^W and a phenotype ratio 1:2:1 red:pink:white.

  

Examples of incomplete dominance:

-Incomplete Dominance in Animals-

1. Chickens with blue feathers are an example of incomplete dominance. When a black and a white chicken reproduce and neither allele is completely dominant, the result is a blue-feathered bird.

2. When a long-furred Angora rabbit and a short-furred Rex rabbit reproduce, the result can be a rabbit with fur longer than a Rex, but shorter than an Angora. That's a classic example of incomplete dominance producing a trait 1different from either of the parents. 

-Incomplete Dominance in Plants-

1. Wild four-o-clocks tend to have red flowers, while "pure" four-o-clocks with no coloration genes are white. Mixing the two results in pink flowers which are a result of incomplete dominance. However, mixing the pink flowers results in 1⁄4 red, 1⁄4 white and 1⁄2 pink. That 1:2:1 ratio - a quarter like one parent, a quarter like the other, and the remaining half different from either - is common in cases of incomplete dominance.

2. The fruit color of eggplants is another example of incomplete dominance. Crossing deep purple eggplants with white eggplants results in eggplants of a light violet color.

-Incomplete Dominance in Humans- 

 Incomplete dominance is rare in humans; we're genetically complex and most of our traits come from multiple genes. However, there are a few examples

1. When one parent with straight hair and one with curly hair have a child with wavy hair, that's an example of incomplete dominance.

2. Eye color is often cited as an example of incomplete dominance. In fact, it's a little more complicated than that, but hazel eyes are partially caused by incomplete dominance of multiple genes related to green and brown eye color.

3. The disease familial hypercholesterolemia (FH) is an example of incomplete dominance. One allele causes liver cells to be generated without cholesterol receptors, while another causes them to be generated normally. The incomplete dominance causes the generation of cells that do not have enough receptors to remove all dangerous cholesterol from the bloodstream.

Codominance 

A condition in which both alleles of a gene pair in a heterozygote are fully expressed. Thus, Codominance, is where both alleles are simultaneously expressed in the heterozygote. As a result, the phenotype of the offspring is a combination of the phenotype of the parents. Thus, the trait is neither dominant nor recessive.

 

1.   Individuals with type AB blood.

The best example of co-dominance is ABO blood group. ABO blood grouping is controlled by gene I which has three alleles A, B, and O and show co-dominance. An O allele is recessive to both A and B. The A and B alleles are co-dominant with each other. When a person has both A and B, they have type AB blood. Thus, a person  inheriting the alleles A and B alleles will have a type AB blood because A and B alleles  are co-dominant and therefore will be expressed together.

Other co-dominance examples are the white-spotted red flower in plants and the black-and-white- coated mammals.

    

 

In co-dominance, it does not matter whether the alleles in the homologous chromosomes are dominant or recessive. If the homologous chromosome consists of two alleles that can produce proteins, then both will be produced and forms a different phenotype or characteristics to that of a homozygote.

Co-dominance vs polygenic inheritance –Co-dominance is a different concept from polygenic inheritance. While both of them do not conform to the Mendelian inheritance, they differ in a way that in polygenic inheritance multiple genes are involved to produce cumulative effects. In codominance, different alleles of a single gene affect the resulting trait. Examples of polygenic traits in humans are height, weight, skin color, and eye color.

 

2.     MN blood groups of humans

A person's MN blood type is determined by his or her alleles of a certain gene. An L^M allele specifies production of an M marker displayed on the surface of red blood cells, while an L^N allele specifies production of N marker. Homozygotes L^ML^M and L^NL^N have only M or an N markers, respectively, on the surface of their red blood cells. However, heterozygotes L^ML^N have both types of markers in equal numbers on the cell surface. 

Mendel's rules can predict inheritance of codominant alleles. For example, if two people with L^ML^N genotypes had children, there would be M, MN, and N blood types and L^ML^M,  L^ML^N  and L^NL^N genotypes in their children in a 1:2:1 ratio.

 

Question