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Genetic Terms and Mendel's Experiments


 Genetic terms Phenotype-  this is the physical expression of genes in an organism. It is  denoted with words such as Tall, Dwarf or Short, Smooth, Wrinkled, Long wing, Vestigial wing, etc. 


    Genotype-  the genetic constitution of a cell or an organism, as distinguished from its physical and behavioral characteristics, i.e., its phenotype. It is denoted with letters such as TT or Tt for Tall, SS or Ss for Smooth, etc. The genotype of  an organism may be homozygous or heterozygous. 

  Allele or  Allelomorphic  pair- a pair of contrasting genes controlling a character. The pair could be identical- T&T or different-T&t                                                                                                                     Homozygous-individual with identical alleles(TT  , tt, SS,  ss) controlling same  character or trait. Such organisms always produce identical gametes  during meiosis and are thus said to be pure breeding or true breeding or breeds true.

   Heterozygous- individual with non-identical or dissimilar alleles(Tt, Ss,Rr) controlling same character or trait. Such organisms produce different or non-identical gametes during meiosis and are thus said not to be pure breeding  or true breeding or  does not breed true.

   Dominant trait or character  -this is controlled by dominant genes that  can express itself  in all generations in the presence of the contrasting gene. It  suppresses the effect  of the contrasting gene.

  It is represented with capital letters  TT , SS, ZZ which is found in birth, moths, fishes and some amphibians etc.

   Recessive trait or character  -this is controlled by  recessive genes that can not express itself in all generations, but only in certain  generations in the absence of  the dominant gene.

  It is denoted with small letters such as  tt, ss . 

   Filial generations  - these are the series of offspring produced from  genetic crossings. It is denoted with letter F and a subscript to  show the particular generation, e g F1, F2  , F3  for first  , second and third Filial generations respectively.

   Hybrid  - a product of crossing between  two contrasting parents. For example, Tall x Short to produce All Tall F1 offspring. That  is TT x tt to give Tt.   

   Monohybrid cross-  a cross between a pair of contrasting traits, example. Black × White, Smooth  x rough, African and American e.t.c

At F1  the dominant trait or character is expressed in all the offspring while the recessive trait is masked. .At F2  , the genotype ratio of the offspring  is one ratio two ratio one i.e  1: 2:1.

while the phenotype is three ratio one, i.e 3:1 in complete dominance condition. 

Dihybrid cross- a cross between two pairs of contrasting characters, e.g. Tall &Smooth x Short &Wrinkled. At F1  , the dominant character is expressed  in all the offspring, while the recessive character is masked. However , at F2, both the dominant and recessive characters are expressed in the offspring in varying genotypic ratios  and phenotypic ratio of 9:3:3:1.   

        Brief history of genetics 

   The biggest name  in the entire history of genetics beyond any doubt is that of Gregor Johann Mendel (1822-1884). It was Mendel, more than any other scientist, who  synthesized the basic principles of  heredity into a body of knowledge  that has formed the very core of modern genetics.

  It must be emphasized here that Mendel was not the original pioneer in the field of genetics. As with all other scientific achievements, many scientists before the period of Mendel had laid the foundation. 

However, it was Mendel  who combined the ideas put forth by other scientists into a definite set of  working principles that are acceptable  even today.

 Like so many other discoveries by famous scientists, Mendel's  ideas on the mechanism  of inheritance did not gain any importance and his principle temporality  died with him.

  Fortunately for science, three scientists, Hugo De Vries, Tschermarck and Correns rediscovered the ideas, when they obtained the same  results in the experiments conducted by  each one of  them  independently.

  The rediscovery of Mendelism brought new emphasis to the field of heredity and the modern science of genetics was born.

  The basic ideas and  the conclusions drawn by scientists  after this rediscovery came  to  be known as Mendelian Genetics. Gregor Johann Mendel  Mendel (1822-1884) was born in to the family  of a poor peasant in Moravia, Austria.

   He received his school education with utmost difficulty due to poverty in the family. In 1843 he joined a church as a monk where in 1847 he became  the abbot (head) of the monastery at Brunn, Austria (now  called Bruno in Czechoslovakia).

   In  1851 he studied natural history in University of Vienna and long after he also study mathematics, for 2 years.

   After his return from School,  he worked as a teacher in natural history and mathematics between the interval of 1856 & 1865. 

  It was during this period of his teaching that he developed curiosity over the pattern of inheritance of characters or traits from  parent organisms to offspring.

  After that he designed breeding experiments in the pea plants.

   He then analyzed the results, given a mathematical interpretation to the result and disclosed them in 1866.

 However, unfortunately for Mendel, his results and conclusions could not convince the contemporary biologists.

   Mendel died in 1884  without knowing that he had laid the foundation for modern genetics.

   It was only in 1900, that Mendel's work was rediscovered and its significance was made known to the scientific world.

  Nevertheless, Mendel is now regarded as the father of modern genetics'  for his significant and pioneering contributions to the field of genetics 

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