Complete Dominance , Incomplete Dominance, Co-Dominance, Multiple Alleles, Lethal Allele, Pleiotropy, Epistasis, Polygenic inheritance

 

Incomplete Dominance and Co-Dominance

Gregor Mendel used pea plants for experiments which have a number of easily observable traits that were determined by just two alleles.  For all the traits he studied, one allele was dominant for the trait & the other was recessive form.  Things aren't always so clear-cut and simple in the world of genetics, but luckily for Mendel & for the whole science world, the organism that he chose to work with had a genetic make-up which was fairly clear-cut and simple.

In Mendel's experiments, offspring always looked like one of their two parents due to the complete dominance of one allele over the other. This is not always the case because some genes display incomplete dominance; that is, individuals with heterozygous alleles exhibit a phenotype intermediate between those with homozygous alleles.

Incomplete Dominance

As per Mendelian Genetics, a certain percent offspring of a mommy black mouse & a daddy white mouse would be black & the others would be white.  There was no options that a white mouse & a black mouse could produce a GREY mouse.  According to Mendel, the phenotype of the offspring from parents with different phenotypes always resembled the phenotype of at least one of the parents and the phenomenon of Incomplete Dominance was not known at his time. 

With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits. It's like mixing paints, red + white will make pink.  Red doesn't totally block (dominate) the pink, instead there is incomplete dominance.

RED Flower x WHITE Flower ---> PINK Flower

We can use the Punnett Square to solve problems involving incomplete dominance

R – allele for red flower

r – allele for white flower

Homozygous red flower crossed with homozygous white flower

Red (RR)  X white (rr)    --->   pink (Rr)  ---> 100%

	R	R
r	Rr	Rr
r	Rr	Rr

 

Codominance

Complete dominance is the situation in which one allele is clearly dominant over the other, and in incomplete dominance neither allele dominates over the other. In a third type of dominance, codominance, both alleles are simultaneously expressed in the heterozygote. The human blood groups designated M, N, and MN are examples of codominance. These groups are distinguished by the presence of two specific proteins on the surface of red blood cells. Group M individuals have one protein, group N have the other protein, and group MN have both proteins.

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 a slightly different N marker.

Homozygotes (L^M L^M  and L^N L^N ) have only M or an N markers, respectively, on the surface of their red blood cells. However, heterozygotes (L^M L^N ) have both types of markers in equal numbers on the cell surface.

For codominance, we can use Mendel's rules to predict inheritance of alleles. For example, if two people with L^M L^N genotypes had children, the M, MN, and N blood types and L^M L^M, L^M L^N and  L^N L^N  genotypes are observed in their children in a 1:2:1 ratio.

The three types of dominance

Complete dominance - only one of the two alleles is expressed in heterozygote individuals

Codominance - both alleles are equally expressed in heterozygote individuals

Incomplete dominance – varying levels of expressions in heterozygote individuals and display an intermediate phenotype.

Multiple alleles

In Mendel's experiments using pea plants, two alleles existed for each gene. Although human beings and all diploid organisms have only two alleles for a given gene, multiple alleles exist in a population level.  As a result different individuals in the population may have different pairs of these alleles.

For example, CC gene specifies coat color in rabbits. The CC gene comes in four common alleles: CC, c^ch, c^h, and cc:

·       A CCCC rabbit has black or brown fur

·       A c^ch c^ch rabbit has chinchilla coloration (grayish fur).

·       A c^h c^h rabbit has Himalayan (color-point) patterning, with a white body and dark ears, face, feet, and tail

·       A cccc rabbit is albino, with a pure white coat.

Multiple alleles allows many possible dominance relationships. In this case, the black CC allele is completely dominant to all the others; the chinchilla c^ch allele is incompletely dominant to the Himalayan c^h and albino cc alleles; and the Himalayan c^h allele is completely dominant to the albino cc allele.

The human ABO blood groups are also example of multiple alleles. There are four possible phenotypic blood types for this particular gene: A, B, AB, and O. The letters refer to two specific carbohydrate molecules on the surface of red blood cells. Individuals can have the A carbohydrate (blood type A), the B carbohydrate (blood type B), both the A and B carbohydrates (blood type AB), or neither carbohydrate (blood type O).

The ABO blood groups are formed by various combinations of three different alleles; IA (codes for carbohydrate A), IB (codes for carbohydrate B), and i (codes for the lack of any carbohydrate). Individuals with one or two IA alleles will have blood type A, those with one or two IB alleles will have blood type B, those with both the IA and IB alleles will have blood type AB, and those with the genotype ii will have blood type O.

Phenotype (Blood group)	Genotype
O	ii
A	IAIA or IAi
B	IBIB or IBi
AB	IAIB

Lethal allele

Allele that results in the death of an individual.  Some genes have alleles that prevent survival when homozygous or heterozygous.

Example of a lethal allele is the lethal yellow allele, a spontaneous mutation in mice that makes their coats yellow. Mice that are homozygous (A^YA^Y) genotype die early in development. Although this particular allele is dominant, lethal alleles can be dominant or recessive, and can be expressed in homozygous or heterozygous conditions.

Different types of lethal alleles are recessive alleles, dominant alleles and conditional alleles.

Recessive lethals

A pair of identical alleles that are present in an organism that ultimately results in death of that organism are referred to as recessive lethal alleles. They are only fatal in the homozygous condition. Example is homozygous achondroplasia.

Dominant lethals

These are alleles that need to be present in one copy to be fatal. These alleles are not commonly found in populations since it result in the death of an organism before it can transmit its lethal allele on to its offspring.

An example is Huntington's disease, a rare neurodegenerative disorder.

Conditional lethals

These are alleles that will be fatal in response to some environmental factors.  Example is favism, that causes the carrier to develop hemolytic anemia when they eat fava beans.

Pleiotropy

Never always one gene can have control over a single trait.  Sometimes one gene may have multiple effects (pleiotropy). For example, albino individuals lack pigment in their skin and hair, and also have crossed eyes at a higher frequency than pigmented individuals (These two traits are not always linked).

Another example is the symptoms associated with sickle-cell disease are due to pleiotropic effects. Individuals with sickle-cell disease are homozygous for the mutant allele, resulting in sickle-shaped red blood cells. Because the sickle-shaped red blood cells deliver less oxygen to the tissues, sickle-cell disease has many pleiotropic effects such as pain in the bones of the back, the long bones, and the chest. As the disease progresses, additional symptoms develop. These include fatigue, paleness, rapid heart rate, shortness of breath, and yellowing of the eyes and skin (jaundice). People heterozygous for the sickle cell mutation do not have the symptoms of sickle cell anemia.

A human genetic disorder called Marfan syndrome is caused by a mutation in one gene, yet it affects many aspects of growth and development, including height, vision, and heart function. The individuals will have Unusually tall height, Thin fingers and toes, Dislocation of the lens of the eye and Heart problems.

Epistasis

Sometimes a gene at one location on a chromosome can affect the expression of a gene at a second location (epistasis).  Epistasis takes place when the action of one gene is modified by one or several other genes. These genes are sometimes called modifier genes. The gene whose phenotype is expressed is said to be epistatic, while the phenotype that is altered is said to be hypostatic. Sometimes hypostatic phenotypes are completely suppressed.

Examples of epistasis can be seen at both the genomic level and the phenotypic level. At the genomic level, it is highly possible that under certain conditions one gene could code for a protein that prevents transcription of the other gene. At the phenotypic level, examples include the gene causing albinism hiding the gene controlling the color of a person's hair.

Another example of epistasis is the genetic interactions that produce coat color in horses and other mammals. In horses, brown coat color (B) is dominant over tan (b). Gene expression is dependent on a second gene that controls the deposition of pigment in hair. The dominant gene (C) codes for the presence of pigment in hair, whereas the recessive gene (c) codes for the absence of pigment. If a horse is homozygous recessive for the second gene (cc), it will have a white coat regardless of the genetically programmed coat color (B gene) because pigment is not deposited in the hair.

Polygenic Inheritance - Multiple genes

Polygenic inheritance occurs when one characteristic is controlled by two or more genes. Often the genes are large in quantity but small in effect. Human features like height, eye color, and hair color come in lots of slightly different forms because they are controlled by many genes, each of which contributes some amount to the overall phenotype.

For example in wheat kernels, there are three genes that make reddish pigment in wheat kernels, A, B, and C. Each comes in two alleles, one of which makes pigment (A, B, C) and one of which does not (a, b, c). These alleles have additive effects: the aa genotype would contribute no pigment, the Aa genotype would contribute some amount of pigment, and the AA genotype would contribute more pigment. The same pattern for the B and C genes.

Pleiotropy and polygenic inheritance. The major difference between the two is that pleiotropy is when one gene affects multiple characteristics (e.g. Marfan syndrome) and polygenic inheritance is when one trait is controlled by multiple genes (e.g. skin pigmentation).