You often hear of questions like “Are you son of Mr.x”, “Do you belong to so and so family” in the society. If the answer is “yes” and the person questions back “How do you guess or how do you know?” The answer simply would be “You resemble Mr.x”. Some children are easily identified as sons or daughters of some known persons due to their resemblance in colour of skin, height, nose, colour of eyes and other distinguishable features with  their parents. The  same way  we  can  also  identify  siblings   according totheir identical characters. At the same time there are a number of characters by which the parents and offsprings, or siblings differ.

 
No human is hundred percent similar to his or her parents or his or her siblings. How do these similarities and differences arise? This question is answered by “Genetics”. Genetics also addressed the questions like how an elephant gives rise to an elephant baby, how a human is born to a human, how a pea plant produces only pea plants?

What is genetics?
            Genetics is a branch of biology that deals with the inheritance and variations of characters.
Genetics deals with structure and synthesis of genes, inheritance, mutations and variations.
What is inheritance?
            Inheritance is the process by which characters pass from parent to offsprings (progeny). It is the basis of heredity.
What is variation?
             Variation is the degree by which progeny differ from their parents and among themselves.
            Therefore genetics is the study of causes and pattern of inheritance in organisms, variations and their causes.

 Gregor Mendel    
 The idea of inheritance pattern was first given by Johann Gregor Mendel. His work and findings paved the way for the study of inheritance, hence J.G Mendel is considered as the “father of genetics”. Later on many scientists like Watson, Crick, Nirenberg, Khorana, Kornberg, Benzer, Jacob, Monod, Brenner, T.H.Morgan, Hugo de Vries, Sutton, Boveri, Conrat and others made significant contributions in this field.

          Khorana

Some of the important contributions of scientists in genetics are given below: 

Unit - III, Chapter - 9, Principles of Inheritance and Variation.  >> Page - 4

 
        Without the knowledge of genetics one can understand that characters of an individual are inherited from their parents. This is a common phenomenon observed in plants, animals and microorganisms. It is also very common that individual differences in characters exist in plants or animals. According to Darwin’s theory of natural selection, nature selects those organisms with better characters which is called “Natural Selection”. In fact man started to select those organisms with better characters for his needs which is called “Artificial selection”. For example we breed cows and buffaloes which are high milk yielding and resistant. In plants also we select those varieties which have desirable characters like high yielding, drought resistant, pest and disease resistance etc. , and breed them. Sahiwal cows of Punjab are bred for high milk yield and Ongole bulls in Andhra Pradesh for their agricultural utility as drought breeds. Now let us discuss about the scientific basis of inheritance and variations.
Terminology used to understand Mendel’s experiments and genetics:
Artificial cross pollination: The artificial transfer of pollen grains from the flower of one known plant to stigma of flower of another plant of the same or other species is called artificial cross pollination.

True breeding: Continuous self-pollination in a plant to produce and express stable trait inheritance for several generations.
Monohybrid cross: A cross between two parents differing in one pair of contrasting characters. e.g.: A cross between a tall pea plant (TT) and a dwarf pea plant (tt).
Dihybrid cross: A cross between two plants which differ in two pairs of contrasting characters is called a dihybrid cross.
e.g.: A cross between yellow and round seeded pea plant (YYRR) with a green and wrinkled seeded pea plant (yyrr).
First Filial progeny(generation) (F₁): The generation of progeny obtained by crossing two true breeding lines ( two pure parents or homozygous parents) is called a F₁ progeny.
Second Filial generation (F₂): The generation produced by self-pollinating the F₁ progeny.
Factors: Something that was being stably passing down unchanged from parent to offspring though the gametes over successive generations were called as a ‘factor’ by Mendel. It is later replaced by “Gene”.
Gene: Basic unit of inheritance that contain information required to express a particular character (trait) in an organism.

Alleles: Genes which code of a pair of contrasting traits are known as alleles. Alleles are slightly different forms of the same gene. e.g.: Tall (T) and Dwarf (t) alleles are two forms of the gene related to height of the plant.
Homozygous condition: If a allelic pair of genes are identical (similar) this condition is said to be homozygous condition. e.g.: TT (Tall), tt (Dwarf).
Heterozyous condition: If a allelic pair of genes is non-identical or dissimilar this condition is called heterozygous condition. e.g.: Tt (Tall).
Dominant factor: In a pair of dissimilar factors only one appears in F₁ generation which is called dominant factor. e.g.: T for tallness appears in F₁ so it is dominant over ‘t’ for dwarfness which does not appear in F₁ but reappears in F₂.
Recessive factor: In a pair of dissimilar factors the factor that does not appear in F₁ but reappears in F₂ is called recessive factor. ‘t’ for dwarfness is recessive.
Phenotype: External characters of an organism. e.g.: Tall, Dwarf
Genotype: Genetic composition of a character. e.g.: Tall = (TT) or (Tt)
                                                                                   Dwarf = tt.
The phenotype tall has two genotypes TT and Tt, while the phenotype dwarf has only one genotype tt.

Back cross: A cross between F₁ offspring with any of its parents (dominant or recessive) is called a back cross. e.g.: Tt × TT or Tt × tt
Test cross: A cross between F₁ with its recessive parent to know the genotype composition is called a test cross. e.g.: Tt × tt.
Punnet Square: A diagram which represents the types of gametes produced by the parents, the formation of zygotes and the progeny is called a Punnet square. It was developed by Reginald C. Punnet. It is commonly called checker board.
Mendel’s Experiments
 Gregor Johann Mendel conducted hybridization experiments on garden pea (Pisum sativum) for seven years and proposed laws of inheritance.

      
He selected plants with contrasting characters regarding
Height                     -         tall /dwarf
Colour of seed        -         green/ yellow
Shape of seed         -         round/wrinkled

Colour of flower     -         violet/white
Position of flower   -         axial/terminal
Shape of pods         -         inflated/constricted
Colour of pods        -          green/yellow

    Advantages of selecting garden pea plant by Mendel for his hybridization experiment:
 Pea plant is an annual with well-defined characters.
 It can be grown and crossed easily.
 It has bisexual flowers with androecium and gynoecium.
 It is suitable for self-fertilization and cross fertilization.
 It has a short life cycle and produces large number of offsprings.
 It shows number of contrasting characters which can be studied easily.    
 

Mendel’s Monohybrid cross:
Monohybrid cross is across between two true breeding lines which differ in a pair of contrasting characters. Mendel crossed a homozygous tall plant (TT) with a homozygous dwarf plant (tt) and produced heterozygous tall plants (Tt) which are called monohybrids. The details of the monohybrid cross are given hereunder: 

The phenotypic ratio of Mendel’s monohybrid cross is 3 : 1 (Tall:Dwarf) while the genotypic ratio is 1 : 2 : 1 (homozygous tall : heterozygous tall : homozygous dwarf). The phenotypic probability of tall plants in F₂ is 0.75 and dwarf plants if 0.25. The genotypic probalility of tall plants and dwarf plants is 0.25 homozygous tall, 0.50 heterozygous tall and 0.25 homozygous dwarf plants.
Mendel’s first law – Law of Dominance

From Mendel’s monohybrid cross it is inferred that
 Characters are controlled by factors.
 Factors occur in pairs.
 “When a cross is made between two parents differing in a pair of contrasting characters, one of the characters appears in F₁ generation which is called dominant character; the other character which does not express in F₁ but appears in F₂ is called recessive character.”
 The law of dominance explains about the expression of one parental character in F₁.
 It also explains 3 : 1 ratio in F₂.
Mendel’s Second law - Law of Segregation or Law of Purity of Gametes
 The law of segregation states that “the two alleles of a gene when present together in a heterozygous state do not fuse or blend together in any way but remain distinct and segregate (separate) during meiosis or in the formation of gametes so that each gamete will carry only one of them”.
 Homozygous individual produces only one type of gametes, while heterozygous individual produces two types of gametes each with one type of allele.
 Segregation of genes is universal in all organisms reproducing by normal sexual method.

Incomplete dominance
 Sometimes F₁ phenotypes do not resemble either of the parental characters but show an intermediate character which is known as incomplete dominance.
 e.g.: Flower colour in Snapdragon (Antirrhinum species)

 Both phenotypic ratio and genotypic ratio of incomplete dominance are 1 : 2 : 1 (Red flowers : Pink flowers : White flowers)
Co - dominance:
This is a type of inheritance where the F₁ individual resembles both parents.
e.g.: 1. Blood group patterns in human beings (A, AB, B and O)
         2. Seed coat pattern and size in lentil (Lens culinaris) plants.
                        Seed coat pattern in Lentil (Lens culinaris)

                                                       
             Phenotypic and genotypic ratio = 1 : 2 : 1 (Spotted : Spotted and Dotted : Dotted)
 In the above example when a pure bred spotted lentil is crossed with pure bred dotted lentil the F1 heterozygotes show both spotted and dotted seed coat pattern.

 This shows that neither spotted nor dotted allele is dominant or recessive to each other.
 As the alleles express equally in heterozygote they are termed ‘codominant’.
 Self-pollination of spotted and dotted F₁ progeny produces F₂ progeny in the ratio 1 : 2 : 1 (spotted : spotted and dotted : dotted).
Pleiotropy
 Pleiotropy is a phenomenon where a single gene may produce more than one effect so that it may be related to more than one character.
 e.g.: Starch synthesis in pea seeds is controlled by a gene with B and b alleles.
 The genotype BB produces large starch grains and bb homozygotes produce small starch grains.
 BB seeds are round whereas bb seeds are wrinkled.
 Bb heterozygotes produce round seeds but produce intermediate size starch grains.
 So in case of starch grains the alleles show incomplete dominance and in case of shape of the seed the alleles follow law of dominance.
 Therefore more than one phenotype may be influenced by the same gene which depends on the gene product that produces a particular phenotype.

Concept of dominance
 Every gene contains information in the form of DNA codes to express a particular trait.
 A diploid organism shows two copies of the same gene called alleles.
 When these two alleles are not identical they are in heterozygous condition.
 One of them may produce an enzyme needed for transformation of a substrate.
 The modified allele may produce normal or less efficient enzyme,  a non-functional enzyme or no enzyme.
 If a normal enzyme is produced the same phenotype or trait is produced.
 If a less functional enzyme or non-functional enzyme is produced the phenotype is affected.
 The unmodified allele represents the original phenotype dominant allele.
 The modified allele represents recessive allele.
Inheritance of two genes (Dihybrid cross)
 A cross between two plants which differed in two pairs of contrasting characters is called a dihybrid cross.
 e.g.: Cross between a pea plant with yellow coloured and round seeds (YYRR) and another pea plant with green coloured and wrinkled seeds (yyrr).
 F1 hybrid of such cross was found to show yellow and round seeds. Thus the yellow and round characters are dominant over green and wrinkled ones.

Law of Independent Assortment
 Based on dihybrid crosses Mendel proposed his Law of Independent Assortment as
“When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters”.
The law of independent assortment can be illustrated as follows:

  Probability of F₂ generation    

Phenotypic ratio of Mendel’s dihybrid cross: 9 : 3 : 3 : 1 (Yellow Round : Yellow Wrinkled : Green Round : Green Wrinkled)
The probability of Yellow round         =    9/16
                                 Yellow Wrinkled   =    3/16
                                 Green Round         =    3/16
                                 Green Wrinkled    =    1/16

Now let us work out the genotypic ratio at the F₂ stage using the above Punnett square data 

By the above table we may derive the genotypic ratio of a dihybrid cross as
1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1

The phenotype Yellow and Round shows 4 types of genotypes YYRR, YYRr, YyRR and YyRr.
The phenotype Yellow Wrinkled shows 2 types of genotypes YYrr and Yyrr.
The phenotype Green Round shows 2 types of genotypes yyRR and yyRr.
The phenotype Green and Wrinkled shows 1 type of genotype yyrr.
Dihybrid test cross
A dihybrid test cross is made between a F1 dihybrid (Yellow and Round YyRr) and its recessive parent (yyrr).

Therefore the test cross ratio of a dihybrid cross would be 1 : 1 : 1 : 1.

Chromosomal Theory of Inheritance
 Sutton and Boveri proposed the “Chromosomal Theory of Inheritance” to explain the Mendel’s laws.
 This theory explains that the behaviour of chromosomes was parallel to the behaviour of genes.
 Both chromosomes and genes occur in pairs.
The two alleles of a gene are located on the homologous sites of homologous chromosomes.
 Thus the pairing and separation of chromosomes leads to segregation of factors or genes.
 This synthesis of ideas of chromosomal segregation and Mendelian Principles is called as “Chromosomal Theory of Inheritance”.    

T.H. Morgan provided experimental verification to this theory while working on fruit flies (Drosophila melanogaster).
Linkage and Recombination
When two genes in a dihybrid cross were situated on the same chromosome the proportion of parental gene combinations was higher than non-parental type.
This is due to physical association or linkage of the two genes on a chromosome.
T.H. Morgan defined Linkage as the physical association of two genes on the same chromosome.
The non-parental gene combinations that would result in a dihybrid cross are called Recombinations.
 When the genes are tightly linked on the chromosome the frequency of recombinations is very low.
e.g.: White eyed and yellow body genes of Drosophila were tightly linked hence show only 1.3% recombination.
 When the genes are loosely linked the frequency of recombinations is high.
e.g.: White eye and miniature wing genes of Drosophila were loosely linked hence show 37.2% of recombination.
 Alfred Sturtevant used the frequency of recombination between genes as a measure of distance between genes and prepared gene maps.
 Genetic maps are used in sequencing of genomes (Human Genome Sequencing).

Mutations
 The alteration of genes which results in changes in the genotype or phenotype of an organism is called a ‘Mutation’.
 Mutation leads to variation in DNA.
 Mutation was first observed by Hugo de Vries in Oenothera lamarckiana (Evening primrose).
 Mutations may occur as deletions or insertions in a segment of DNA.
 Chromosomal abnormalities or aberrations are also mutations that are commonly observed in cancer cells.
 The mutations that arise due to change in a single base pair of DNA are called ‘Point Mutations’. e.g.: Sickle cell anemia
 Mutations that occur due to deletions and insertions of base pairs of DNA are called frame-shift mutations.
 The chemical and physical factors that induce mutations are called ‘Mutagens’.
e.g.: UV radiation.
 Mutations cause great variability in a population that may produce desirable traits. Such crop plants are improved by selection and hybridisation.