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Heredity Inheritance and Variation of Traits

Hello viewers,

Today we are going to discuss about another important chapter which is heredity inheritance and variations of traits in living organism.  So as we always do, we will start with the definition first and then we will go deeper in the details of it.  So let’s get started…..

Mendel’s work containing his discoveries was first published in the year 1866 in an ordinary journal, “Brunn Society on Natural Science” in the name “Experiment in plant hybridization”.  But his contribution remained unnoticed until 1900, when his laws were rediscovered by three European botanist – Hugo De Vries of Holland, Carl Correns of Germany, and Enrich Von Tschermak of Austria.


Heredity Inheritance and Variation

It is the mode of transference of characters from the parents to the offsprings or from one generation to another.


Genetics:  The branch of science concerned with heredity.

a. Heredity: The mechanism by which the physical, mental etc. characteristics of parents are transmitted from one generation to the successive generation of living organism.
b. Locus: The specific position of a specific gene on a particular chromosome is called the locus of that gene.
c.  Allele: The two genes situated on the same locus of a homologous chromosome pair are said to be allele to one another (one pair of allelic genes is generally found to be responsible for a character).
d.  Genotype: The genetic composition of a character is called genotype.
e.  Phenotype: The external manifestation of a character is called phenotype.
f.  Homozygous: When the allelic genes responsible for a character carry the similar traits it is called homozygous.
g.  Heterozygous: When the allelic genes responsible for a character carry the contrasting traits it is called heterozygous.
h.  Dominant character: In heterozygous condition, out of two contrasting traits the one that gets phenotypic expression is called dominant character.
i.  Recessive character: In heterozygous condition, out of two contrasting traits the one that does not get phenotypic expression is called recessive character.
j.  Hybridization: Cross between two unlike parents.

  1. Hemizygous: The condition of having only one allele of a pair, as seen for genes on the X chromosome in males.
  2. Backcross: The cross of F1 individual with either of the two parents is known as backcross.
  3. Testcross: When only the cross is done with recessive parent then it is called testcross.
  4. Reciprocal cross: When two same parents are crossed in such a way that if in one experiment ‘A’  is used as female parent and ‘B’ is used as the male parent and in another experiment ‘A’ is used as the male parent and ‘B’ is used as the female parent then it is called as reciprocal cross.
  5. Emasculation:  The process by means of which the anthers (male reproductive parts) are removed from the flowers is called emasculation.
  6. Monohybrid cross: When a cross is made keeping only one character in consideration, it is called a monohybrid cross.
  7. Dihybrid cross: When a cross is conducted considering two characters in consideration, it is called a dihybrid cross.
  8. First Law of Mendel: The characteristics of a diploid organism are controlled by alleles occurring in pairs,  of a pair of such alleles, only one can be carried in a single gamete. OR Genes do not blend in a heterozygous condition, during the gametogenesis they get segregated from one another and get distributed among the gametes.
  9. Second Law of Mendel: The characteristics of a diploid organism are controlled by alleles occurring in pairs, each of the two alleles of one gene may combine randomly with either of the alleles of another gene. OR

One pair of allelic genes is not influenced in its inheritance by the association of any other gene pair situated on different homologous chromosome pair.  At the time of gamete formation the genes are assorted independently to different gamete.



Mendel’s monohybrid cross

Mendel crossed between true breeding tall and dwarf pea plant.  In the F1 generation all the plants were tall.  When these tall plants were interbred out of all F2 progenies ¾ were tall and ¼ were dwarf.


Characters taken:-  Length of the pea plant 1. Tall and 2. Dwarf

Let the tall be represented by ‘T’

And dwarf be represented by ‘t’

Female         Male

TT       X     tt   —————- P1 generation

T                 t    —————- Gametes

Tt           —————- F1 generation

Tt        X      Tt

T,t               T,t  —————-  Gametes

F2 progenies

T t
t Tt Tt


  1. Explain Mendel’s monohybrid cross.


Ans.  First write the cross    


A homozygous tall plant (Tt)

is crossed with

a homozygous dwarf plant (tt)

In F1 generation: all appeared as tall plant (Tt)

After the interbreeding among these plant,

In F2 generation: Tall : Dwarf = 3 : 1


Looking at the F1 progenies, we do see that all are phenotypically tall, and hence it may appear that the genes responsible for tall and dwarf characters have got blended.

Had there been any blending of genes in these progenies, their interbreeding would have produced all the tall progenies in F2 generation.  But in actual practice 25% of the F2 progenies are dwarf.  This proves that genes do not blend in heterozygous condition, but get segregated from one another during gametogenesis and gets distributed to a single gamete.


Therefore in F2 generation ¾ tall and ¼ dwarf.

Thus the phenotypic ratio is Tall : Dwarf  = 3 : 1

The genotypic ratio is Homozygous tall:heterozygous tall:homozygous dwarf = 1:2:1

  1. What is phenotypic ratio or general ratio?

Q  What is genotypic ratio?

Incomplete/partial dominance

In certain living organism out of the two contrasting traits in hybrid characters none can dominate over the other and this phenomenon is called the incomplete dominance.  Here an intermediate character is phenotypically expressed in hybrid condition.  Thus in the plant Mirabilis the red colour of the flower is found to be incompletely dominant over the white colour of the flower.  In hybrid condition the flowers are  pink in colour.

Let ‘R’ is represented as red colour of the flower.

and ‘r’ is represented  as white colour of the flower


Female         Male


RR       X     rr   —————- P1 generation

R                 r    —————- Gametes

Rr           —————- F1 generation

Rr        X      Rr

R,r               R,r  —————-  Gametes

F2 progenies

R r
r Rr rr


Therefore the phenotypic ratio is R : P : W =  1:2:1

Therefore the genotypic ratio is R : P : W =  1:2:1

It is the only case where the phenotypic ratio and genotypic ratio are same.



Mendel’s dihybrid cross

Characters taken :-

Colour of  seed 1. Yellow (Y) and 2. Green (y)

Contour of seed 1. Round (R) and 2. Wrinkled (r)

Female                   Male

YYRR       X        yyrr    —————- P1 generation

YR                       yr       —————- Gametes

YyRr                 —————- F1 generation

YyRr                    X       YyRr

YR, Yr, yR, yr               YR, Yr, yR, yr —————-  Gametes


F2 progenies

Yr YYRr YYrr YyRr Yyrr
Yr YYRr YYrr YyRr Yyrr


Hence the phenotypic ratio of the dihybrid cross:

Yellow round: yellow wrinkled: green round: green wrinkled = 9: 3: 3: 1


  1. Explain dihybrid cross from an example of animal. Show the phenotypic and genotypic ratio from this cross by a checkerboard.

Ans. First write the cross


Let a homozygous black Guineapig with rough hairs (BBSS)

is crossed with

a homozygous white Guineapig with smooth hairs (bbss)


In F1 generation all will be black rough (BbSs)

When these black rough of F1 are intercrossed,


In F2 generation we got the following ratio

black rough:black smooth:white rough:white smooth  = 9:3:3:1


Had there been any influence of Bb on Ss (Yy on Rr) in F1 progenies, during gametogenesis only two types of gametes would have been produced (cross over type and non-cross over type) resulting in the formation of only two types of progenies (cross over type and non-cross over type) in the F2 generation.  But here we find that four types of gametes have been formed (all non-cross over type in 1:1:1:1 ratio) with consequent formation of four different types of F2 progenies, which are in the 9:3:3:1 ratio.  Hence one pair of allelic genes does not influenced by any other gene pair, if situated on different homologous chromosomes.  Each gamete is independently distributed during gametogenesis.


Structure of chromosome

A chromosome is a long thread like structure composed of two finer threads chromatids.  Each chromatids in its turn is again composed of two to four very fine delicate thread called chromonemata that remain highly coiled with one another.  Each chromonema carries numerous chromomeres, which get deeply stained.  There are two constrictions on the chromosomes the primary constriction is called centromere, which does not absorb any dye. The secondary constriction is supposed to have some role in the formation of nucleolus and hence this region is called as nucleolar organizer.  The two ends of chromosomes are called telomeres.  In some chromosomes a kind of knob like structure is present called satellite body is found to be attached to an end of chromosome instead of one telomere.  Such a chromosome is called sat chromosome.

N.B. 13, 14, 15, 21, 22 No. of chromosomes having nucleolar organizer.


Sex determination

The chromosomes in human cells are of two types the autosomes and allosomes or sex chromosomes.  The sex chromosomes are those that are responsible for the determination of the sex and inheritance of sex-linked characters in man are haemophilia, colour blindness etc.


Sex chromosomes are two types ‘X’ type and ‘Y’ type.  In human body cells (somatic cell) there is one pair of sex chromosome.  Thus in man the composition of sex chromosome are XY and in a woman it is XX.  A man therefore produces two types of gametes one type having X chromosome and other having Y chromosome.  While the woman produces one type of gamete having only X chromosome.  It is the type of the male gamete which fertilizes a female gamete that determine the sex of a child.

XX                            XY

(X) (X)                      (X) (Y)

Structure of DNA molecule


The deoxyribonucleic acid is a micromolecule.  It is composed of a number of nucleotides, the units, joined with one another. A DNA molecule looks like double helix it is composed of two threads that get twisted one another to form a double helix.  Each nucleotide is composed of a deoxyribose pentose sugar, a phosphate group and a nitrogenous base.  The bases are four types namely Adenine, Thymine, Cytosine, and Guanine.

DNA                                                                           RNA

  1. The pentose sugar is deoxyribose                          1. The pentose sugar is ribose
  2. The bases are A, T, C, G                                           2. The bases are A, U, C, G
  3. It is a double helix                                                     3. It is a single helix


Gene:  Functionally a gene is a unit of heredity and structurally it is a part of DNA molecule that can provide the genetic code necessary for the synthesis of a particular protein controlling a part of a character.


Mutation:  Any sudden relatively permanent change in the germplasm or in the gene is called mutation.  Mutated characters are generally recessive in nature hence it is phenotipically expressed only in homozygous.


Write about the practical application of Mendelism. (4)


Ans. Mendelian concepts are applied in various practical fields for human welfare. New varieties of organisms are produced with desired characters through the process of hybridisation.
In the field of agriculture: Through selective breeding different high yielding varieties of paddy (Jaya, Padma and Ratna), wheat (Kalyan Sona, Sonalika etc) have been produced.
In the field of farming: By hybridisation very high yielding varieties of cow with very high milk producing capacity (Jersey, Holstein), hen with very high laying capacity (Leghorn), fishes with high egg laying capacity and fast rate of growth (Silver carp) have been produced.
In the field of industry: In the silk industry hybrid silk moths with high yielding capacity and better quality of silk have been produced.
In the field of pharmaceutical industry: New strains of fungi with high yielding capacity of antibiotic drugs have been produced.

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Hi viewers My name is Partha Pratim Goswami, (PpG) I am a teacher by profession. I have been teaching biology, chemistry and physics, sometimes mathematics also for last sixteen years. I would like to share my entire experience with all the viewers across the world through YouTube, which I think is an amazing platform to share all my experiences.

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