Heredity and Evolution Basics and Definition

  • Reproduction Give rise to new individuals that are similar in some ways as well as they too have some amount of difference.
  • You must have heard people saying the eyes or nose is similar to your brothers.
  • Even during Asexual reproduction there is some amount of variation is seen .

The Transmission of characters(or traits) from the parents to their off spring is called heredity.

  • If one bacterium divides, and then the resultant two bacteria divide again, the four individual bacteria generated would be very similar.
  • There would be only very minor differences between them, generated due to small inaccuracies in DNA copying.
  • However, if sexual reproduction is involved, even greater diversity will be generated as two parents are involved.
  • These variations help different type of organisms to survive in different environment conditions.

Mendel’s Experiment

He is also known as father of Genetics

Before Understanding the experiment Let us know few terms.

Chromosome- It is a thread like structure in the nucleus of a cell formed by Group of DNA Chains.

  • The DNA chains carry Genes which transfer traits .
  • Genes work in pairs . and they Control characters .
  • For example two control the height of an organism ( which is a trait) two genes are there one is for tallness and another for dwarfness.
  • We can represent this pair of genes which denotes the character (height) by using letters Tt.
  • The Capital T denotes the tallness while small t denotes dwarfness.

The genes for controlling a character can be of two types .

  • One is Dominant and another is recessive
  • The gene which decide the appearance of of an organism in the presence of an alternative gene is known as dominant gene.
  • The gene which can decide the appearance of another identical gene is called recessive gene.
  • for example in a pea plant. For tallness the dominant Gene is T and the recessive gene is t for Dwarfness.
  • Genotype is the description of the genes present in an organism.
  • They always appear in pairs. TT,Tt,or tt are the examples of genotypes.
  • Genotype of a tall plant can be TT or Tt and for a dwarf plant it will be tt.

The Character which is visible on the organism is called its Phenotype.

  • Being tall or dwarf are the phenotypes of the organism.

F1 Generation –  when two parents cross or (breed) among themselves to produce he progeny(baby) is called first fillial generation or f1 generation.

F2 Generation – When the progeny of first generation breed among them selves and new progeny is formed , then this is called second filial generation or f2 generation.

Gregor Mendel was the first scientist to make a systematic study of patterns of inheritance which involved the transfer of characteristics from parents to progeny

Reasons For choosing Pea Plant For his Experiment

  • Mendel chose pea plants for studying inheritance because pea plants had a number of clear cut differences which were easy to tell apart
  • some pea plants were ‘ tall’ (having long stem) whereas others were ‘dwarf (having short stem).
  • Some pea plants produced round-yellow seeds whereas others produced wrinkled-green seeds, etc.
  • Another reason for choosing pea plants was that they were self pollinating (which enabled them to produce next generation of plants easily)

Mendels Experiment


  • Mendel crossed (cross-bred) the pure-bred pea plants differing in these traits and noted their occurrence in the progeny of succeeding generations
  • Mendel first crossed pure-bred tall pea plants (TT) with pure-bred dwarf (tt) pea plants and found that only tall pea plants were produced in the first generation or F1 generation
  • No dwarf pea plant or( short pea plants) were obtained in the first generation of progeny
  • Mendel then crossed the tall pea plants of the first generation (F1 generation) and found that tall
    and dwarf plants were obtained in the second generation (or F2 generation) in the ratio of 3 : 1.
  • Mendel called the repressed trait of ‘dwarfness’ as ‘recessive trait’ and expressed trait of ‘tallness’ as the ‘dominant trait.
  • It was also seen that there were no plants could be found of having medium heights.


As the Image Suggests all the Plants are tall. Which shows Tallness(T) is the dominant Trait.
In the Second Cross 3 tall plants and 1 dwarf plants is seen. With Genotype Ratio 1:2:1 (TT:Tt,Tt:tt)

Di-hybrid inheritance


  • In the dihybrid cross,two characters are considered one is Colour of the seed(yellow,green) and another is shape of the seed(Round ,Wrinkled)
  • the parent plants having the phenotype round-yellow seeds have the factors of inheritance or gene combination RRYY (in which RR are the dominant genes for round shape whereas YY are the dominant genes for yellow colour).
  • On the other hand, the parent plants having the phenotype wrinkled-green seeds have the factors of inheritance or gene combination rryy (in which rr are the recessive genes for wrinkled shape and yy are the recessive genes for green colour).
In the first Cross All the seeds are found to be Rounded and Yellow Coloured. Showing the dominant Traits

  • In the second Filial Generation  other characters are also seen in the ratio as shown
  • F2 ratio: Round-yellow= 9; Round-green= 3 Wrinkled-yellow= 3; Wrinkled-green= 1



  • The characteristics or traits of parents are transmitted to their progeny (off springs) through genes present on their chromosomes during the process of sexual reproduction
  • Genes work in pairs. There is a pair of genes for each characteristic of an organism (one is dominant Gene and the other is recessive gene).
  • Each parent possesses a pair of genes for each characteristic on a pair of chromosomes.
  • However, each parent passes only one of the two genes to its progeny through the gamete.

Inheritance of Blood Groups

  • A person has a one of the blood groups A,B AB or O
  • The Genes have forms as IA, IB ,IO
  • When IA IA combine The Blood Group is A
  • When IA IO combine The Blood Group is A(As IO is Recessive)
  • When IB IB combine The Blood Group is B
  • When IB IO combine The Blood Group is B(As IO is Recessive)
  • When IAIB combine The Blood Group is AB
  • When IOIO combine The Blood Group is O


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