Sunday, 22 November 2020

Mendelian genetics & mandelian law of inheritance

Mendelian Genetics : 
 
  Father of Genetics, Gregor Johann Mendel was born in a farmer family near Brunn in Austria in 1822. Gregor Johann Mendel conducted his historic experiments with garden pea (Pisum sativum) in the monastery garden for about nine years i.e. 1856-1864 and published his results in a less known journal - The Annual  proceedings of the Natural History Society of Brunn in 1865.  

  He chose garden pea as an experimental organism because many varieties were available that bred true for qualitative traits and their pollination could be manipulated. Mendel investigated seven variable characteristics in pea plants were - 

• seed texture (round v/s 
  wrinkled) 
• seed color (yellow vis green) 
• flower color (white v/s  
  purple) 
• growth habit (tall v/s dwarf) 
• pod shape (pinched or        
  inflated)   
• pod color (green v/s yellow) 
• flower position (axial or    
  terminal)

  Peas are normally self-pollinated plant as the stamens and carpels are enclosed within the petals. By removing the stamens from unripe flowers. Mendel could brush pollen from another variety on the carpels when they ripened. 

 After experiments, Mendel discovered that, when he crossed purebred white flower and purple flower pea plants (the parental or P generation), the result was not a blend. Rather than being a mix of the two, the offspring which is known as the F1 generation, was purple-flowered. When Mendel self-fertilized the F1 generation plants, he obtained a purple flower to white flower ratio in the F2 generation of 3 to 1. Punnett square were drawn to explain results of this cross as showed below.
  He then comes up with the idea of heredity units, which he called "factors". Mendel found that there are alternative forms of factors called genes that account for variations in inherited characteristics. For example, the gene for flower colour in pea plants exists in two forms, one for purple and the other for white. The alternative forms are now called alleles. For each biological trait, an organism inherits two alleles, one from each parent. These alleles may be the same or different from other. 

  An organism that has two identical alleles for a gene is said to be homozygous organisms for that gene and is called a homozygote. An organism that has different alleles for a gene is said to be heterozygous organisms for that gene and is called a heterozygote.

  Mendel hypothesized that pairs of alleles separate randomly or segregate from each other during the production of gametes i.e. egg and sperm. Because allele pairs separate during gamete production, a sperm or egg carries only one allele for each inherited trait. When at the time of fertilization sperm and egg unite, each contributes its allele, restoring the paired condition in the offspring. This is called the Law of Segregation. Mendel also found that during gamete formation, each pair of alleles segregates independently of the other pairs of alleles. This is known as the Law of Independent Assortment. 

  The genotype of an individual is made up of the many alleles. Phenotype viz. Physical appearance of individual is determined by its alleles as well as by its environment. The presence of an allele does not mean that the trait will be expressed in the individual that possesses it. If the two alleles of an inherited pair differ i.e. the heterozygous condition, then one determines the organism's appearance and is called the dominant allele; the other has no noticeable efect on the organism's appearance and is called the recessive allele. Thus, in the example above the dominant purple flower allele will hide the phenotypic effects of the recessive white flower allele. This is known as the Law of Dominance but it is not a transmission law : it concerns the expression of the genotype. Dominant alleles represents with upper case letters whereas recessive alleles represents with lowercase letters are used to represent.


Mendelian law of inheritance :       
     After experiments on garden pea (Pisum sativum) over seven years during l856-1864, he gave important laws of heredity, viz.,    
  (1) Law of Segregation, 
  (2) Law of Independent 
        Assortment, 
  (3) Law of Dominance.
These are briefly presented below. 

(1) Law of Segregation :

   According to this law, alleles segregate or separate from each other during gamete formation and pass on to different gametes in equal number. In other words, when alleles for two contrasting characters come together in a hybrid, they do not blend, contaminate or affect cach other while together. The different genes separate from each other in a pure form, pass on to different gametes formed by a hybrid and then go to different individuals in the offspring of the hybrid.

Thus main features of this law re as follows

• When both a dominant allele and a recessive allele of a gene come together in a hybrid after crossing between two plants having contrasting characters, they do not mix or blend together. 

• Both the allele separate together in pure form without affecting each other. Due to this reason, law of segregation is also known as law of purity of gametes. 

• These alleles separate into different gametes in equal number. Each gamete has only one type of allele (either A or a). 

• Separation of two alleles of a gene during gamete formation takes place usually due to the separation of homologous chromosomes during anaphase-I of meiosis, because alleles are located in the chromosomes.

• With complete dominance, segregation leads to phenotypic ratio of 3:1 in F2 for characters governed by single gene, and 9:3:3:1 ratio for characters controlled by two genes. 

• If crossing over does not take place, segregation of genes takes place during anaphase-I. If crossing over occurs, segregation of genes will take place during anaphase-II.

* Example : 
   When we make a cross between red (RR) and white (rr) flowered plants, we get red colour of flower in F1. In the F1 both the alleles R and r remain together without blending or mixing with each other, though only the effect of dominant allele is visible. In F2, allele for red flower colour and white flower colour segregate during gamete formation and pass on to the gametes in equal number, Thus two types of gametes, viz., R and r are formed. Each gamete has either R or r allele. When the F1 is self-pollinated. individuals with three genotypes, viz., RR, Rr and rr are obtained in F2 Here RR and Rr are all red and only rr individuals are white. Thus a phenotypic ratio of 3 red: 1 white is obtained. The overall mechanism is represented below.
  When selfed  seeds of RR were grown in F3, they all produced all the true breding individuals for red flower colour. The Rr individuals showed segregation in F3 similar to segregation in F2 generation. Individuals with rr genotypes were found true breeding for white flower colour when their selfed seeds were raised in F3 generation. 

(2). Law of Independen   
      Assortment :

 This is the second law of inheritance discovered by Mendel. This law states that when two pairs of gene enter in F1 combination, both of them have their independent dominant effect. At the time of gamete formation these genes are segregate, but the assortment occurs randomly and quite freely.

Main features of this law are given below : 

• This law explains simultaneous inheritance of two plant characters.
• In F1 when two genes controlling two different characters, come together, each gene exhibits independent dominant behaviour without affecting or modifying the effect of other gene. 
• During gamete formation, these gene pairs segregate independently.
• The alleles of one gene can freely combine with the alleles of another gene. Thus each allele of one gene has an equal chance to combine with each allele of another gene.
• Each of the two gene pairs when considered separately, exhibits typical 3:1 segregation ratio in F2 generation. This is a typical monohybrid segregation ratio. 
• New gene combinations are formed by random or free assortment of alleles of two genes.

* Example : 
  When plants of garden pea with yellow round seeds are crossed with plants having green wrinkled seeds, we get yellow round seeds in F1. Thus yellow colour of seed exhibits dominance over green and round seeds shape over wrinkled independently. The F1 produces four types of gametes, viz., yellow round (YR), yellow wrinkled (Yr), green round (yR), and green wrinkled (yr). Selfing of F1 gives rise to all above four types of individuals in 9:3:3:1 ratio.

  The behaviour of all these genotypes was studied in F3 generation. Out of nine yellow round individuals only one (YYRR) was found true breeding in F3 generation. The other eight individuals showed segregation of various types.
  Similarly, out of 3 yellow wrinkled individuals only one (YYrr) bred true and others segregated in 3 :1 ratio. Same thing happened with green round individuals. The green wrinkled individual was also true breeding as shown in following table.
(3). Law of Dominance :

  According to Law of Dominance, recessive alleles will always be masked by dominant alleles. Therefore, a cross between a homozygous dominant and a homozygous recessive will always express phenotype of the dominant allele, while still having a heterozygous genotype. 

  This Law can be explained easily with the help of a mono hybrid cross experiment. In a cross between two organisms pure for any pair (or pairs) of contrasting traits (characters), the character that appears in the F1 generation is called "dominant" and the one which is suppressed (not expressed) is called "recessive". Each character is controlled by a pair of dissimilar factors. Only one of the characters expresses. The one which expresses in the F, generation is called Dominant. However, the law of dominance is not universally applicable.

* Mono-hybrid cross or Mono-hybrid tests : 

  In a monohybrid cross, the two parents differ through a single character. In this cross Mendel took a tall pea plant and crossed with a dwarf plant. He transferred the pollen grains of tall pea plants and placed them on the stigma of the dwarf pea plant and vice versa. To prevent self pollination he earlier removed all stamens from the flowers of the dwarf plant. Mendel noticed that all the progenies of F1 or first filial generation were tall plants. This outcome gave him the clue to state the Law of dominance.
 
 
  According to this law, from the pair of contrasting characters i.e. tallness and dwarfness one character tallness appeared in the F1 generation and the other character (dwarfness) remained hidden or suppressed. The character which appeared in F1 generation is called dominant and the other character remained hidden is called recessive character. These two contrasting characters are known as allelomorphic characters or allelomorphs or alleles. 

  Mendel now allowed the plants to self pollinate and produce F2 or second filial generation of plants. Among F2 plants he observed tall as well as dwarf plants in the ratio 3: 1. This ratio is monohybrid ratio. The recessive character (dwarfness) which was hidden in the F1 generation appeared in F2 generation. 

  Among the F2 tall plants 1/3rd were pure tall and the remaining 2/3rd were hybrid tall behaving like the plants of F2 generation. The recessive dwarf plant bred true. Thus, the F2 ratio are classified into two categories : Phenotypic ratio (3 tall : 1 dwarf) and Genotypic ratio (1 pure tall : 2 hybrid tall : 1 pure dwarf).

  In the parental generation both tall (TT) and dwarf (tt) plants have similar alleles of gene and hence called homozygous plants.The F1 plants have different alleles of the same gene (Tt) and hence known as heterozygous plants. Mendel represented the dominant factor through the capital letter and recessive factor by means of small letter. Thus, pure tall plant will be TT and dwarf plant will be represented by tt.

* Di-hybrid Cross (OR) Di-hybrid Test : 

  In a di-hybrid cross the parents differ through two characters. Mendel conducted a cross between a true breeding Round Yellow plants (RRYY) with true breeding Wrinkled Green plant (rryy). Round and Wrinkled are the shapes of seed coat whereas Yellow and Green are the colours of the seed coat.

  Mendel observed the seeds of the F1 plants were all Round and Yellow. This showed Round and Yellow were dominant over Wrinkled, Green. In the F2 generation four types of combinations were observed such as Round Yellow, Wrinkled Yellow, Round Green and Wrinkled Green. Thus the above types of offspring's of F2 generation were produced in the ratio of 9: 3: 3: 1. This ratio is called Di-hybrid ratio.
The results can be represented as follows:
  Mendel represented round character of seed by R and wrinkled by r. Similarly, he represented the yellow character by Y and green by y

  In his dihybrid cross experiment Mendel observed round and yellow characters are dominant over wrinkled and green so that all the F1 offspring's are round and yellow. In the F2 generation, he noticed an allele (dominant or recessive) of a given character freely combines with either one (dominant or recessive) of the alleles of another character. Hence, a dominant allele of a character combines, not only with the dominant, but also with the recessive allele of another character.









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