EXTENSIONS AND MODIFICATIONS OF BASIC PRINCIPLES OF MENDEL LAW

EXTENSIONS AND MODIFICATIONS OF BASIC PRINCIPLES OF MENDEL LAW

6.  EXTENSIONS AND MODIFICATIONS OF BASIC PRINCIPLES OF MENDEL LAW

After the reestablishment (rediscovery) of the Mendelian principle of genetics by Hugo de Vries, Shenmark and coworkers, a new era of genetics were started. Various scientists started working on a different combination of crosses in different plants. Few crosses followed Mendelian principle of genetics but mostly not and various new principles of inheritance was given by scientist.
6.1.    Incomplete Dominance
When one allele of a gene can not cancel the effect of other alleles of the same gene this is called incomplete dominance. In incomplete dominance, the intermediate to phenotype to both the parents appear in F1 generation. Incomplete dominance is an intragenic (allelic) gene interaction, In incomplete dominance, there is no clear-cut dominant-recessive relationship between the two alleles of a gene. Both the alleles express partially, creating an intermediate phenotype incomplete dominance is exhibited when the heterozygote has a phenotype intermediate between the phenotypes of the two homozygotes parents. 
For example, the snapdragon flower colour, When the red homozygous flower is crossed with the white homozygous flower, the result yields a pink snapdragon flower. The pink snapdragon is the result of incomplete dominance. Thus intermediate phenotype is expressed by F1 generation. 
When plants of the F1 generation are self-pollinated(a self-cross between two heterozygotes produces ), the phenotypic and genotypic ratio of the F2 generation is 1:2:1 (Red:Pink:White).


6.2.    Codominance:-
When both the allele of a gene show its phenotype in the F1 generation. In codominance the phenotype of the heterozygote is not intermediate between the phenotypes of the homozygotes; rather, the heterozygote simultaneously expresses the phenotypes of both homozygotes. An example of codominance is seen in the MN blood types, ABO blood systems cystic fibrosis.
6.2.1.    ABO System: The ABO system is due to a gene that codes for molecules on the surface of red blood cells. In the ABO blood group, genes responsible for blood group has three alleles. Allele A, allele B and allele O. These three alleles form four type of phenotype of blood group:-
    (i)    Blood group type O,
    (ii)    Blood group type A,
    (iii)    Blood group type B
    (iv)    Blood group type AB
The genotype of A blood group may be IO IA, IA IA. This is an example of codominance. The genotype of B blood group is IB IO or IB IB. This is also an example of dominance. But the genotype of AB Blood group is IA IB. This is the example of codominance.


6.2.2.    Roan Character in Cattle: Blood Group (AB) which is the resulting phenotype of (A) and (B) alleles of blood group. ABO blood group system displays the three gene interactions - Complete Dominance, Codominance and Multiple Alleles.    
6.2.3.    MN blood group system in humans: The MN blood group system in under control of an autosomal locus found on chromosome 4 with 2 alleles designated as LM and LN.  MN antigen system is based upon two genes glycophorin A and glycophorin B. Thus, in codominance, the heterozygote jointly expresses the phenotypes of both homozygotes i.e different alleles express their phenotypic effect simultaneously. 
Special case:- Bombay phenotype :
In 1952, an unusual situation comes before the scientist known as Bombay phenotype which is seen in a female. The h/h blood group, also known as Oh or the Bombay blood group, is a rare blood type. This blood was discovered in Bombay in India by Dr. Y.M. Bhende in 1952. The serum contained antibodies related to all red blood cells. The RBC lack all of the A B O blood group antigens and has an additional antigen which was previously unknown. Later research revealed that the patient was homozygous for a rare recessive mutation in a gene designated FUT-1 (encoding an enzyme for fucosyltransferase), which prevented her from synthesizing the complete H substance FUT-1 gene encode for enzymes that add fucose to the terminal portion of the carbohydrate chain protruding from the red cell membrane. In the absence of fucose, the enzymes specified by the IA and IB alleles are unable to recognize the incomplete H substance as a substrate. Neither the terminal galactose nor N-acetylgalactosamine can be added later to the terminal portion of the carbohydrate chain protruding from the red cell membrane.


Co-dominant in Zebrafish:- In zebrafish, blue scales (BB) and red scales (RR) are co-dominant. When a fish has the genotype BR, it shows blue and red scales and when selfing was performed in F1 generation on it gives genotype and phenotype ratio of 1 : 2 : 1.  


6.3.    Lethal Alleles
Lethal alleles also referred to as lethal genes which lead to the death of an individual. Death occurs because of mutations in genes that are essential to growth or development. Lethal alleles may be recessive, dominant, or conditional depending on the gene or genes involved. Lethal alleles can cause the death of an organism permanently or any time after birth, though they commonly manifest early in development.


Example: Sickle cell anaemia, a disease in which the haemoglobin protein is produced incorrectly and the red blood cells have a sickle shape phenotype. A person that is homozygous recessive for the sickle cell trait will have red blood cells that all have incorrect haemoglobin. A person who is homozygous dominant will have normal red blood cells. Because this trait has an incomplete dominance pattern of expression, a person who is heterozygous for the sickle cell trait will have some misshapen cells and some normal cells. These heterozygous individuals have a fitness advantage; they are resistant to severe malaria. Both the dominant and recessive alleles are expressed, so the result is a phenotype that is a combination of the recessive and dominant traits.


6.3.1.    Recessive lethal mutations  
An allele is lethal only in homozygous form and when it is in heterozygous condition, it is normal.
Examples:-
1.  The recessive lethal curly allele in Drosophila affects wing shape in the heterozygote form but is lethal in the homozygotic condition.
2.  The recessive lethal plum eye allele in Drosophila affects eyes in the heterozygote form, but is lethal in the homozygotic condition.
3.   The recessive lethal stubble bristle allele in Drosophila affects bristle development in the heterozygote form, but is lethal in the homozygote.
4.   Tay Sachs disease of human- The heterozygous produces only half of the enzyme as one copy of the gene is normal. The heterozygote individual is phenotypicaly normal. 
6.3.2.    Dominant Lethal mutations 
The dominant lethal inheritance pattern is one in which an allele is lethal both in the homozygote and the heterozygote condition. Dominant lethal alleles are very rare because the allele only lasts for one generation and is, therefore, not usually transmitted. This allele can only be transmitted if the lethality phenotype occurs after reproductive age. 
Example:- Huntington’s Disease 
Huntington's disease (HD) is a neurodegenerative genetic disorder that affects muscle coordination and leads to mental retardation.    
6.3.3.    Conditional lethal alleles may kill an organism only when certain environmental conditions prevail - Temperature-sensitive (ts) lethals. A developing Drosophila larva may be killed at 30° C But it will survive if grown at 22° C.
6.4.    Multiple Alleles
Mutations are carried out very often resulting in the formation of many different forms of genes called multiple alleles and when more than one alternate form of the gene exist then the pattern of inheritance becomes different. The pattern of inheritance remains the same but a greater number of phenotypes and genotypes are observed. Examples of multiple alleles are as follows.
6.4.1.    Duck-Feather Patterns 
An example of multiple alleles is seen at a locus that determines the feather pattern of mallard ducks. One allele, M, produces the wild-type mallard pattern. A second allele, MR  produces a different pattern called restricted, and a third allele, md, produces a pattern termed dusky. The six genotypes possible with these three alleles and their resulting phenotypes
In this example, three alleles determine feather pattern but each individual can have two alleles. The F2 generation ratio is 1 : 2 : 1. The F2 generation ratio is one mallard two restricted and one dusky. The ratio indicates that MR gene is a dominant gene over M and Md gene. The order of dominancy of genes are :-

        MR    >    M    >    Md

The pattern produces by genes are one mallard, two restricted, one dusky or in a ratio 1 : 2 ; 1.


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