Epistasis

 Epistasis

The term epistasis is Greek word mean “standing upon”. It is a type of intergenic interaction in which a gene masks or modifies the expression of another gene present at a different locus.

Genes

Epistatic Gene: The gene which modifies or mask the phenotypic expression of another gene called epistatic gene.

Hypostatic Gene: It is a gene whose phenotypic expression is affected by a epistatic gene.

Example:

Epistasis can be best demonstrated in Bombay phenotype and coat color of Labrador retrievers.

 

Bombay phenotype:

Bombay phenotype is a rare blood group phenotype in which individuals are genetically of type A, B, or AB blood group but phenotypically O type blood group. This blood phenotype was first discovered in Bombay (Mumbai) in India by Dr. Y.M. Bhende in 1952.

In 1952, A woman was found to be genetically type B but functionally type O. she was found to lack both the A and B antigens and was thus typed as O. Her mother was type AB while her father was type A. This woman was carrying a homozygous recessive mutation in a gene designated FUT1 (encoding an enzyme, fucosyl transferase), which prevented her from synthesizing the complete H substance. 

The enzymes produced by the IA and IB alleles are unable to recognize the incomplete H substance. Thus, neither the terminal galactose nor N-acetylgalactosamine can be added.

Genetic basis of Bombay Phenotype:

The ABO blood groups are determined by antigen A and antigen B present on the surface of RBC. The production of antigen A and antigen B antigen is controlled by gene I^A and I^B and is also dependent upon gene H (present on chromosome 19). The dominant H gene allele produces H substance, a precursor for the antigen A and antigen B. Antigen A and antigen B will only be produced if the H substance is present. The allele I^A and I^B modifies the H substance to antigen A and antigen B respectively.

 

Genotype		H Substance	Type of Antigen	Blood group
I gene	H gene
IA IA ,  IAi	HH, Hh	Produced	A	A
IB IB , IBi	HH, Hh	Produced	B	B
IA IB	HH, Hh	Produced	A and B	AB
IA IA ,  IAi	hh	Not produced	None	O (Bombay)
IB IB , IBi	hh	Not produced	None	O (Bombay)
IA IB	hh	Not produced	None	O (Bombay)

Antigen A and Antigen B

The A and B antigens are sugars that are bound to membrane lipid molecules (fatty acids) of the red blood cell. The specificity of the A and B antigens is based on the terminal sugar.

 Antigen A and B are produced by adding a specific terminal sugar to the H substance. The H substance itself contains three sugar molecules—galactose (Gal), N-acetylglucosamine (AcGluNH), and fucose.  

The IA allele is responsible for an enzyme that can add the terminal sugar N-acetylgalactosamine (AcGalNH) to the H substance. The IB allele is responsible for an enzyme that can add a terminal galactose. Heterozygotes (IAIB) add either one or the other sugar to the H substance. Persons of type O (ii) cannot add either terminal sugar; these persons have only the H substance protruding from the surface of their red blood cells.

Multiple Allele

Multiple Allele

An allele is the alternative form of a gene. When more than two forms of a gene exist on a single locus of a chromosome, the alleles are then called as multiple alleles.

Formation of multiple alleles:

Multiple alleles are formed by gene mutation. A slight change in the nucleotide sequence of a gene results in the formation of alleles.

Number of multiple alleles: 

The number of alleles controlling a character varies. The ABO blood group system is controlled by three alleles of gene I. Some genes may have as many as 300 alleles for a character. Multiples alleles exist in the individual of a population, but individuals have only two of those alleles. It is because most of organisms are diploid having two homologs of each chromosome.

ABO BLOOD GROUP:

History

ABO blood group system was discovered in 1901 by Karl Landsteiner of the University of Vienna. Later he was awarded a Nobel Prize. ABO blood groups are found in all humans and in many primates such as apes, chimpanzees, baboons and gorillas.

Genetic Basis

The ABO blood groups are controlled by Gene I located on the chromosome 9. Gene I has three allelic forms i.e. I^A ,I^B and i. The I^A and I^B alleles each encode a glycosyltransferase that catalyzes the synthesis of the A and B antigen, respectively. The O or i allele encodes an inactive glycosyltransferase that leaves the ABO antigen precursor (the H antigen) unmodified.

 

Blood type	Antigen on RBC	Allele for Antigen	Possible Genotypes	Dominance relation
Type A	Antigen A	IA	IA IA ,  IAi	IA Dominant to i
Type B	Antigen B	IB	IB IB , IBi	IB Dominant to i
Type AB	Antigen A & B	IA and IB	IA IB	IA and IB  Co-Dominant
Type O	None	i	ii	i is recessive

 

Blood group types

A person’s blood group may be one of four types: A, B, AB, or O. These blood group types are due to the presence of antigen A and antigen B on the surface of RBC. A person having antigen a on the surface of RBC will have blood group A, having antigen B on the surface of RBC will have blood group B, having both A and B antigens will have blood group AB. When no antigen is expressed (nor A neither B), then the blood group is said to be O blood group.

Ability to produce Antibodies against antigen A and Antigen B

Blood group is determined by antigens present on the surface of RBCs. The immune system has the ability to produce antibodies against the foreign antigens. In normal conditions, the body do not produce antibodies against body own cells but can produce against any foreign agent that enters into the body.

Blood group A: An individual with the blood groups A has antigen A on the RBC surface, therefore, it only produces Antibodies against the antigen B.

Blood group B: A person having blood group B produces antibodies against the antigen A.

Blood group O: A person having none of antigen on the surface of RBC (Blood group O) produces anti-A and Anti-B antibodies.

Blood group AB: Those have AB blood group do not produce antibodies.

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Codominance

 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.

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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.

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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. 

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Johann Mendel

 Johann Mendel was born in 1822 to a peasant family in the Central European village of Heinzendorf. He studied philosophy for several years. In 1843 he was admitted to the Augustinian Monastery of St. Thomas in Brno (now part of the Czech Republic). In 1851 he attended the University of Vienna, where he studied physics and botany. In 1854, he returned to Brno and started teaching physics and natural science for the next 16 years. 

In 1856, Mendel started his research and performed set of hybridization experiments with the garden pea. His experiments continued until 1868. In 1884, Mendel died of a kidney disorder.

Mendel first reported the results of some simple genetic crosses between certain strains of the garden pea in 1865.

In the early twentieth century,  Hugo de Vries, Carl Correns, and Erich Tschermak (botanists) performed hybridization experiments and reached conclusions similar to those of Mendel.

About the same time, Walter Sutton and Theodor Boveri (cytologists), independently published papers linking their discoveries of the behavior of chromosomes during meiosis to the Mendelian principles of segregation and independent assortment. 

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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.