Short Notes: Principles of Inheritance and Variation

Principles of Inheritance and Variation

Heredity is the transfer of character from parents to their offsprings. These hereditary characters are present on the chromosomes in the form of genes.  These gene combinations express characters which may be more similar to one of its two parents.

The differences in characters of offspring mainly depend upon unique process of crossing over that occurs during meiosis.  This is one of the main reasons of producing recombinations.

Gregor Johann Mendel was born in 1822 in Heinzendorf, which was a part of Czechoslovakia.  He began his genetic experiments on garden pea in 1856 in the garden at the monastery.

Selection of pea plant:  The main reasons for adopting garden pea (Pisum sativum) for experiments by Mendel were –

  • Pea has many distinct contrasting characters.
  • Life span of pea plant is short.
  • Flowers show self pollination, reproductive whorls being enclosed by corolla.
  • It is easy to artificially cross pollinate the pea flowers.  The hybrids thus produced were fertile.

Working method:
  Mendel’s success was also due to his meticulous planning and method of work –

  • He studied only one character at a time.
  • He used all available techniques to avoid cross pollination by undesirable pollen grains.
  • He applied mathematics and statistics to analyse the results obtained by him.

Mendel’s work and results:

The results obtained by Mendel were studied and on their basis he proposed certain laws known as “Laws of heredity”.  These laws are discussed below:

1)  Law of dominance:
This law states that when two contrasting genes for a character come together in an organism, only one is expressed externally and shows visible effect.  It is called dominant and the other gene of the pair which does not express and remains hidden is called recessive.
2)  Law of segregation or Purity of gametes:
This law states that both parental alleles (recessive and dominant) separate and are expressed phenotypically in F2 generation.  When F2 generation was produced by allowing F1 hybrid to self pollinate, to find out segregation or separation it was observed that both dominant and recessive plants appeared in 3:1 ratio.

3)  Law of Independent assortment:
The law of independent assortment states that when inheritance of two or more genes occur at one time, their distribution in the gametes and in the progeny of subsequent generations is independent of each other.  To prove this, he did a dihybrid cross.  He crossed homozygous dominant smooth and yellow seeded (YYRR) with homozygous recessive wrinkled and green seeded (yyrr) plants. The F1 hybrid was self pollinated and F2 generation was obtained with the phenotypic ratio of 9:3:3:1 and genotypic ratio of 1:2:1:2:4:2:1:2:1.

Test Cross:
A cross between F1 hybrid (Aa) and its homozygous recessive parent (aa) is called Test Cross. This cross is called test cross because it helps to find out whether the given dominant phenotype is homozygous or heterozygous.

Incomplete dominance:
When neither of the alleles of a character is completely dominant over the other and the F1 hybrid is intermediate between the two parents, the phenomenon is called incomplete dominance.

The most common example of incomplete dominance is that of flower colour in 4’O clock plant.  Homozygous red (RR) flowered variety was crossed with white (rr) flowered variety.  F1 offspring had pink flowers (Rr).  This is called incomplete dominance.  Incomplete dominance is also known to occur in snapdragon.  The phenotypic ratio and genotypic ratio in F2 generation in case of incomplete dominance is 1:2:1.

Multiple Allelism / Codominance:
When a gene exists in more than two allelic forms, it shows the phenomenon of multiple allelism.  A well known example is the inheritance of A, B and O blood groups in human being.  The gene for blood group occurs in three allelic forms  IA, IB and i.  A person carries any two of these alleles.  The gene IA produces glycoprotein (sugar) A and the blood group is A.  The gene IB produces glycoprotein B and the blood group is B.  The gene ‘i’ is unable to produce any glycoprotein and so the person homozygous for it , has O group blood. The genes IA and  IB are dominant over ‘i’.   When IA and  IB are present together, both are equally dominant and produce glycoproteins A and B and the blood group is AB.  They are called codominant alleles.

 Phenotypic (Blood group)                        Genotype
A                                                               IAIA  /  IA IO
B                                                               IBIB  /  IB IO
AB                                                             IAIB
O                                                               IOIO  (ii)

Chromosome theory of Inheritance:
Chromosome theory of inheritance was proposed by Sutton and Boveri independently in 1902.  The two workers found a close similarity between the transmission of hereditary characters and behaviour of chromosomes while passing from the one generation to the next through agency of gametes.

Salient features of chromosome theory:

  • Both chromosomes as well as genes occur in pairs in the somatic or diploid cells.
  • A gamete contains only one chromosome of a type and only one of the two alleles of a character.
  • The paired condition of both chromosomes as well as Mendelian factors is restored during fertilization.

Parallelism of behaviour between chromosomes and Mendelian factors:

  • Both the chromosomes as well as Mendelian factors (whether dominant or recessive) are transmitted from generation to generation in an unaltered form.
  • A trait is represented by only one Mendelian factor inside a gamete.  A gamete similarly contains a single chromosome out of a pair of homologous chromosomes due to meiosis that occurs before the formation of gametes.
  • An offspring contains two chromosomes of each type, which are derived from the two parents through their gametes that are involved in fusion and formation of zygote.  It also contains two Mendelian factors for each character.  The factors come from two different parents through their gametes.

Linkage and Recombination:
Linkage is the phenomenon, where two or more linked genes are always inherited together and their recombination frequency in a test cross progeny is less than 50%.

A pair of genes may be identified as linked, if their recombination frequency in a test cross progeny is lower than 50 percent.  All the genes present on one chromosome form a linkage group and an organism possesses as many linkage groups as its haploid number of chromosomes.  If the two genes are fully linked, their recombination frequency will be 0%.

Sex Determination by chromosomes:
Those chromosomes which are involved in the determination of sex of an individual are called sex chromosomes while the other chromosomes are called autosomes.

1) XX – XY type:  In most insects including fruit fly Drosophila and mammals including human beings the females possess two homomorphic sex chromosomes, named XX.  The males contain two heteromorphic sex chromosomes, i.e., XY. Hence the males produce two types of gametes / sperms, either with X-chromosome or with Y-chromosome, so they are called Heterogamety.

2) ZZ – ZW type:  In birds and some reptiles, the males are represented as ZZ (homogamety) and females are ZW (heterogamety).

3)  XX – XO type:  In round worms and some insects, the females have two sex chromosomes, XX, while the males have only one sex chromosomes X. There is no second sex chromosome. Therefore, the males are designated as XO.  The females are homogametic because they produce only one type of eggs.  The males are heterogametic with half the male gametes carrying X-chromosome while the other half being devoid of it.

Sex determination in Humans:
Human beings have 22 pairs of autosomes and one pair of sex chromosomes.  All the ova formed by female are similar in their chromosome type (22+X).  Therefore, females are homogametic.  The male gametes or sperms produced by human males are of two types, (22+X) and (22+Y).  Human males are therefore, heterogametic. The two sexes produced in the progeny is 50:50 ratio.

It is a phenomenon which results in alteration of DNA sequences and consequently results in changes in the genotype and phenotype of an organism.

Gene / Point mutation:
Due to change in a single base pair of DNA. Ex. Sickle cell anemia (GAGàGUG).

Chromosomal mutation:
Due to change in structure or number of chromosomes. Ex. Down’s syndrome.

The chemical and physical factors that induce mutations are known as Mutagens. Ex. UV rays.

Genetic Disorders:
Pedigree analysis:  It is a system to analyse the distribution and movement of characters in the family tree.

Mendelian Disorders:
These are mainly determined by alteration or mutation in the single gene.  These disorders are transmitted to the offspring on the same line as the principle of inheritance.
Examples : Haemophilia, Cystic fibrosis, Sickle cell anemia, Colour blindness, Phenylketonuria, Thalesemia, etc.

It is a sex linked recessive disease, which shows its transmission from unaffected carrier mother to some of the male progeny.  Haemophilia is a disorder in which a vital factor for clotting of blood is lacking.  So clotting of blood is abnormally delayed and it can be fatal.  Bleeding can be checked by transfusion of the entire volume of blood or the clotting factor in concentrated form.

Sickle cell anemia:
It is an autosome linked recessive trait.  It is due to a mutant allele on chromosome 11 (autosome), that causes change of glutamine (GAG) to valine (GUG) at the sixth position of  β-chain of haemoglobin.  The disease is controlled by a single pair of allele, HbA HbA (normal) ; HbA HbS (carrier)  and HbS HbS (diseased). The patient has sickle shaped RBCs with defective haemoglobin.  They are destroyed more rapidly than normal RBCs.

It is due to a recessive mutant allele on chromosome 12 (autosome).  The affected individual lacks an enzyme (phenylalanine hydroxylase) that converts the amino acid phenylalanine into tyrosine.  As a result, this phenylalanine and its derivatives accumulate in the cerebrospinal fluid leading to mental degeneration (retardation) and are excreted in the urine due to its poor absorption by kidney.

Chromosomal Disorders: Due to absence or excess or abnormal arrangement of one or more chromosomes.
A change in the number of chromosomes in an organism arises due to non-disjunction of chromosomes, during gamete formation.

Aneuploidy:  This arises due to loss or gain of one or more chromosomes during gamete formation. Example: Down’s syndrome (47) and Turner’s syndrome (45).

Polyploidy:  In this, the number of chromosomes is the multiple of the number of chromosomes in a single set (haploid).  Accordingly, these may be haploid, diploid and polyploid.

Down’s Syndrome:  It was first described by Langdon Down (1866). It is due to trisomy of 21st chromosome, arising from non-disjunction of chromosomes during gamete formation.  As the maternal age increases, the instances of non-disjuction increase.  When such an ovum containing two 21st chromosomes (24) is fertilized by a normal sperm (23), the zygote (47) comes to possess three copies of 21st chromosome.

Symptoms:  Short statured with small round mouth, palm is broad with characteristic palm crease, physical, psychomotor and mental development is retarded.

Klinefelter’s syndrome:  It arises due to non-disjunction of X-chromosomes during ova formation.  When an ovum containing two X-chromosomes is fertilized by a Y-carrying sperm, XXY individual (47) appears.

Symptoms:  A male with underdeveloped breasts (gynaecomastia), sparse body hair, mentally retarded and sterile.

Turner’s Syndrome:  It arises due to non-disjunction of X-chromosomes during ova formation. When an ovum carrying no X-chromosome is fertilized by a sperm carrying X- chromosome, a zygote with XO appears.

Symptoms: A female with rudimentary ovaries, short stature, lack of secondary sexual characters, they are sterile.


  1. Heredity: – It can be defined as the transmission of characters from one generation to successive generations of living organisms.
  2. Alleles: – The various forms of a gene are called alleles.
  3. Phenotype: – The external / observable characteristics of an organism constitute its phenotype.
  4. Genotype: – The genetic constitution of an organism is its genotype.
  5. Homozygote: – It is an individual organism in which the members of a pair of alleles for a character are similar.
  6. Heterozygote: – It is an individual organism in which the members of a pair of alleles of a character are different.
  7. Dominant character: – The form of the character which is expressed in the F1 hybrid is called dominant character.
  8. Recessive character: – The form of the character which is suppressed in the presence of the dominant character in a hybrid is called recessive character.
  9. Monohybrid cross: – It is a cross between individuals of the same species, in which the inheritance of contrasting pairs of a single trait is considered.
  10. Dihybrid cross: – It is a cross between two individuals of the same species, in which the inheritance of contrasting pairs of two traits is considered.


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