· Heredity (Heres- Heir) or inheritance is the process of transmission of characters from one generation to another generation. Such transmissible characters are called hereditary characters.
· Genetics is the way or mechanism of heredity or cause of variation of living beings.
· Term genetics was coined by William Batson.
· Variation – Dissimilarities among the offspring of the same parents.
· It is more pronounced in sexually reproducing organisms.
· Father of genetics – G. J. Mendel
· Experimented plant – Pisum sativum (pea). May be self or cross-pollinated. (2n = 14)
· Mendel presented his work in "Barun Natural History Society"
· His paper was entitled "Experiments on plant hybridization"

Mendel's Laws

The conclusion from monohybrid crosses.
1. Law of dominance
2. Law of segregation of purity of gamete conclusion of Dihybrid crosses
3. Law of independent assortment.

· Mendel studied the inheritance of seven different pairs of contrasting characters in pea plants.
· Gene is a small DNA segment that determines a biological character of an organism. It is the functional unit of hereditary material. (Sometimes RNA also acts as genetic material in some viruses).
· Allele or Allelomorph: It is one of a number of alternative forms of the same gene in a chromosome that is responsible for determining contrasting characters. E.g. 'T' and 't' are two allelomorphs of the gene for plant height.
· Homozygous: Individuals having identical alleles. E.g. TT, (tall), tt (dwarf)
· Heterozygous: Individuals having different alleles. E.g. Tt, (tall as well as a dwarf)
· Genotype: Genetic expression of an organism. The genotypic ratio of F2 generation in monohybrid cross is 1:2:1 It is not an observable Phenomenon.
· Phenotype: It is the physical or observable expression of an organism. The Phenotypic expression of an organism. The phenotypic ratio of F2 generations in monohybrid cross is 3:1.
· Parental generation – Plants used as parents in a cross.
· Filial generation 
– Progeny obtained as a result of crossing.
· Hybridisation – Cross between dissimilar parents.

1. Law of Dominance
· When two pure lines with contrasting characters are crossed only one (dominant) form of the character appears in their form remains unexpressed in the F1 generation. E.g. Tall (TT) x Dwarf (tt)
Tall (Tt) (all are tall in F1 generation)
So, Tall – dominant, tt – recessive

2. Law of segregation (Law of purity of gamete)
· Factors for each character separate during gamete formation. So each gamete receives only one character.
· It's the first law of Mendelian inheritance.
E.g. Heterozygous tall (Tt) produces two gametes.

3. Law of Independent Assortment
· The genes of different characters are located in different pairs of chromosomes are independent of one another in their separation during gamete formation.
· This law can't be explained if Linkage is taken into consideration.

4. Monohybrid cross
In monohybrid cross single character is taken into consideration.
i. Phenotypic ratio is = 3:1
ii. Phenotypes are (no. of phenotype) = 2
iii. Genotypic ratio is = 1:2:1
iv. No. of genotypes is = 3
(Homozygous dominant, heterozygous, homozygous recessive)

5. Dihybrid cross
Two different characters are taken into consideration.
In dihybrid cross
i. Phenotypic ratio is = 9:3:3:1
ii. Number of Phenotypes is = 4
iii. Genotypic ratio = 1:2:2:4:1:2:1:2:1
iv. No. of genotypes is = 9


· Pure homozygous tall red offspring in a dihybrid cross in F2 generation = 1/16
· Pure homozygous dwarf white in F2 generation of a dihybrid cross = 1/16
· Pure homozygous offspring in a dihybrid cross in F2 generation will be.
= 1/16 + 1/16 = 1/8

6. Back Cross
· Cross between F1 individual and the parents whether dominant or recessive.

7. Test cross
· Cross between F1 individual and the homozygous recessive parents. E.g. Tt (F1 hybrid) x tt (Homozygous recessive parents)
· In test cross organism of unknown genotype is crossed with the homozygous recessive organism of the same trait.

8. Outcross
· Cross between F1 individuals and the homozygous dominant plant.

9. Test cross ratio
Possible types of gamates
· No. of possible types of gamate = 2n where n = no. of contrasting characters.
a. TT = 20 = 1 gamate 
b. Tt = 20 = 1 gamate
c. TT Rr = 21 = 2 gamates
d. TT RR = 21 = 2 gamates
e. Tt RR = 22 = 4 gamates 
f. TT rr yy = 20 = 1 gamate
g. Tt Rr yy = 22 = 4 gamates
h. Tt Rr Yy = 23 = 8 gamates

Non-Mendelian inheritance (Gene interaction)
Two types:
1. Allelic gene interaction or Intragenic gene interaction.
2. Non-Allelic gene interaction or Intergenic gene interaction.

· Allelic gene interaction modifies the phenotypic ratio of the F2 generation of monohybrid cross of Mendel.
· Non-allelic gene interaction modifies the phenotypic ratio of the F2 generation of Dihybrid cross of Mendel.

1. Allelic Gene Interaction:
(a) Incomplete (= partial or semi or intermediate) dominance.
· Characteristic of F1 hybrid is in between the two parents. For e.g. cross between Red and white flower produces pink flowers instead of Red in the F1 generation.
· Genotypic and phenotypic ratio is the same in incomplete dominance.
E.g. Inheritance of flower colour in Mirabilis Jalpa (Japanese four O'clock plants and Antirrhinum (Snapdragon)
In Incomplete dominance:
i. Phenotypic ratio is = 1 : 2 : 1
ii. No. of phenotypes is = 3 (Red, Pink, White)
iii. Genotypic ratio = 1:2:1
iv. No. of genotypes = 3

(b) Codominance
· Genes of allelomorphic pairs or alleles express themselves equally in F1 hybrids.
E.g. AB blood group

(c) Lethal genes
· Genes that produce drastic phenotypes sufficient to kill the bearer are called lethal genes.
· Every lethal gene is mutant and defective.
· Cueonot 1st Discovered a lethal gene in animals and named it as yellow lethal gene of a mouse.
· E. Bauer discovered the 1st lethal gene of plant in Antirrhinum majnus.
· Phenotypic ratio= Genotypic ratio= 2:1 in case of the lethal gene. i.e 3:1 modified into 2:1.
· Lethal gene of human:
- Sickel cell anaemia.
- Congenital ichthyosis
- Tay Sachs syndrome.
- Huntington's chorea.
- Epilepsy
- Retinoblastoma
a. Sickel cell anaemia
· Neel and beats 1st discovered that sickle cell anaemia is hereditary.
· It is caused by a defective mutant gene represented by HbS.
(normal and wild) (point mutation) → Hbs, (Defective and mutant)
· So HbA codes for normal haemoglobin
· HbS codes for Defective Haemoglobin

Fig: Inheritance pattern of Sickle cell anaemia

· Sickel cell trait can survive if does not work hard.
· Sickle cell patient never suffers from malaria i.e has natural resistance against malaria.
Thus HbS is a pleiotropic gene that regulates two characteristics i.e
i) Causes sickle cell anaemia
ii) Gives immunity against malaria

Genetic Basis of sickle anaemia

Sickle cell anaemia is due production of valine in place of glutamic acid in position 6 of the β-chain.

Fig: Genetic Basis of sickle cell anaemia

2. Non-allelic Gene Interaction:
· All of these interactions have independently assorting genes.
· Thus genotypic ratio in F2 generation of these interactions is the same as the genotypic ratio in F2 generation of dihybrid cross of Mendel i.e. 1:2:2:4:1:2:1:2:1
· It is broadly classified into two types
a) Epistatic gene intraction
b) Non-epistatic gene interaction

a) Epistatic gene intraction

· One gene suppresses the action of another gene at a different locus.
· The gene that suppresses the effect of another gene is called the epistatic gene.
· The gene whole effect that gets suppressed is called as Hypostatic gene.
This can be:
i) Dominant epistasis:
· In this case expression of one dominant gene is inhibited by another non-allelic dominant gene.
Eg: Inheritance of grain colour in sorghum,
Ratio = 12:3:1

ii) Recessive epistasis (Supplementary Gene interaction)
· In this case, expression of one dominant gene is inhibited by another non-allelic recessive gene when present in homozygous condition.
Eg: Inheritance of coat colour in mice
Ratio = 9:3:4

iii) Dominant Duplicate gene with the cumulative effect
Ratio = 9:6:1

i) Dominant duplicate gene without cumulative effect.
Eg: Awn character in mice
Ratio = 15:1

ii) Recessive duplicate gene(Complementary gene interaction)
Eg: inheritance of flower colour in Lathyrus.
Ratio = 9:7

b) Non epistatatic gene intraction (Collaboratore gene intraction ratio=9:3:3:1) or 
Pleiotropy or Pleotropism

· Generally one gene regulates only one character. But sometimes one gene regulates more than one character. Such gene is called a pleiotropic gene and the phenomenon is known as pleiotropy or pleiotropism.
· Eg; gene of sickle cell anaemia.
· Ratio=2:1 (in the inheritance of coat colour in mice) and 1:2 (in sickle cell anaemia).

Polygenic Inheritance
· When regulation of a trait is carried out by genes of more than one Locus, it is called polygenic inheritance.
· Ratio= 1:4:6:4:1
· Kolreuter: Father of polygenic inheritance.
· Such gens are called polygenes.
· Polygenes are called cumulative as well as multiple genes.
· Polygenic inheritance is also called quantitative inheritance because it determines the degree of the trait.

· Eg; of Polygenic Inheritance
Plant Animal
Kernel colour of wheat Skin colour
No. and size of flowers fruit and seeds Height, IQ

· Complete absence of melanin is called albinism.
· Production of melanin is regulated by a dominant gene (A).
· Its recessive allele (a) can not produce melanin.
· Albinism is due to the absence of the enzyme tyrosinase regulated by Cu.
· Possibility of Albino = 25% = 1/4 = 0.4
· Production of melanin occurs or does not occur in a person regulated by monogenic inheritance.
· But, what amount of pigment is produced in a person is regulated by polygenic inheritance.

Multiple Allelism:
· Presence of more than two alleles for single characteristics is called multiple allelism.
· Multiple allelism can be detected in the study of population and not on the individual because a diploid individual can have only 2 alleles from a multiple allelic series.
Eg; Eye colour of Drosophila.
· ABO blood group.
· Coat colour of Rabbit.
· Self-incompatibility, i.e. self sterility in Nicotiana tobacco.


· Sudden change in the genetic material or genetic make-up of an organism.
· Mutagens are any chemical or physical agent that causes mutation.
· Discovered by – Hugo de Vries in Oenothera lamarkina (evening prime rose)
· Scientific study of mutation was given by Morgan.
· He worked on the fruit fly (Drosophila melanogasterand reported white-eyed male individuals among red-eyed males.
· Mutation may be natural or artificial. Artificail mutation is due to mutagens.
· Physical mutagens (u.v. rays, X – rays, a, b and gamma rays)
· Chemical mutagens (Colchicine and Mustard oil)
· Best organism to study mutation is haploid (n)
· Organisms frequently used for mutational studies are Drosophila (fruit fly) and Neurospora (fungi)
Note: One gene-one enzyme hypothesis was given by Beadle and Tatum while working on 
Neurospora (Pink mould or Pink Bread mould but experimentally proved by Morgan)

Various Types of Mutation
i. Gene/point mutation
It's a small mutation
Types of Gene Mutation:
a. Deletion – loss of a pair of nucleotides.
b. Inversion 
– Adition addition of a pair of nucleotides
c. Transition – Purine base is replaced by another purine and pyrimidine is replaced by another pyrimidine base.
d. Transversion – A purine base is replaced by pyrimidine and vice versa
Purine bases in DNA/RNA
a. Adenine (A)
b. Guanine (G)
Pyrimidine bases in DNA
a. Cytosine (c)
b. Thymine (T)
Pyrimidine bases in RNA
a. Cytosine (c)
d. Uracil (U)

ii. Chromosomal mutation
a. Change in the structure of chromosome
i. Deletion – Loss of a part of chromosome
ii. Duplication – Duplication of a section of chromosome so that a set of genes is repeated.
iii. Inversion – A portion of a chromosome breaks off and is re-united in reversed order.
iv. Translocation – A section of a chromosome breaks up and joins to another non-homologous chromosome.

b. Change in no. of chromosome
i. Aneuploidy
 – Change in individual no. of the chromosome.
a. Monosomy (2n – 1) – Loss of one single chromosome from a complete set of chromosomes of an organism.
e.g. Turners syndrome (45✖O)
Turner's syndrome in females is due to the loss of the 'X' chromosome.
b. Trisomy (2n + 1)
 – Presence of an extra chromosome in a complete set.
e.g. Down's syndrome (often called Mongolism is the trisomy of the 21st chromosome.
Note: Baby with Down's syndrome shows Mongolian face, mental retardation and they are fond of music.
Patau's syndrome – Trisomy of 13th chromosome
Edward's syndrome – Trisomy of 18th chromosome
Note: Sex chromosomal Trisomy Klinefelter's syndrome (47 XXY) is due to the presence of one extra x-chromosome in males.
c. Doube trisomy (2n + 1 + 1)  Presence of two extra dissimilar chromosomes.
d. Tetrasomy (2n + 2)  Presence of two extra pairs of chromosomes.
e. Nullisomy (2n – 2)  Loss of a pair of chromosomes.

ii. Euploidy
  Change in the entire set of chromosome
a. Monoploidy/haploid (2n – n = n)  Have one set of chromosomes or one genome (n) in the nuclei of their body cell.

b. Polyploidy (2n + n = 3n, 4n, 5n, 6n, 6n, 7n, 8n)
2 types
i. Autopolyploid
· Multiple sets of chromosomes are from the same genome (n).
Note: Genome is the haploid set of chromosomes.
E.g. orange, banana, pineapple.

ii. Allopolyploidy
· Multiple sets of chromosomes from different species.
E.g. Rhaphanobrassica (Allopolyploidy)
· Triticum (common wheat variety) is Allohexaploid.


· It is an exception to Mendel's law of Independent Assortment.
· Persistent association of genes located on the same chromosome, which are inherited together in the same position from one generation to another generation.
· Linked genes are the genes present on the same chromosome and wouldn't assort independently.
· Phenomenon of Linkage was discovered by Morgan working on the fruit fly (Drosophila Melanogaster.)
· Gene involved in the linkage is called a linked gene.

Sex-Linked Inheritance
Inheritance of non-sexual characters through sex chromosome.
i. X-linked inheritance
· Also called criss-cross inheritance.
· Characters are transferred from father to daughter and from mother to both daughter and son.
E.g. Haemophilia, Colorblindness.
· Haemophilia (It is the disease of blood clotting in which clotting of blood doesn't occur.
· It is Bleeder's/Royal disease.
· Queen Victoria was supposed to have muted gene for haemophilia.
· Is a lethal gene.
· Haemophilia is due to the lack of clotting factor VIII. (Haemophilia A)
Note: Gene that causes the death of the individual carrying it is called lethal genes.
· Carrier females is normal
· Predominantly males are sufferers and females are carriers.

Colour blindness
· Also called Daltonism
· Non-Lethal
· Colour blind person can not recognize red and green colour.
ii. Y – linked inheritance
· Y – Y-linked genes are called holandric genes.
· Characters are transferred from father to son.
E.g. Hypertrichosis (excessive hair growth in the external air).

Nucleic acid
1. DNA (Deoxyribonucleic acid)

· Found in Nucleus, mitochondria and chloroplast.
· Double stranded.
· Sugar: Deoxyribose sugar (6c)
Note: Ribose sugar (5c) in RNA.
· Nitrogen bases
· Purine – Adenine (A), Guanine (G)
· Pyrimidine – Cytosine (C), Thymine (T).
· Watson and crick give the famous Double Helical structure of DNA.
· The width or diameter of DNA molecule is 20 A𝆩.
· One complete turn of DNA helix contains 10 base pairs and its length is 34 A𝆩.
· Distance between two adjacent base pairs is 3.4 A𝆩.
· Nucleoside = Nitrogen bases (purine or pyrimidine) + sugar
· Nucleotide = Nitrogen base + sugar + phosphate group
Nucleotide = Nucleoside + Phosphate group
· The two strands of DNA are complementary and antiparallel.
· Two strands of DNA are joined by each other by a Hydrogen bond (H-bond).
    a. Between A and T, 2 H - bonds
    b. Between G and C, 3 H – bonds
· Bond between sugar and nitrogen base is glycosidic bond.
· In a DNA helix, the amount of adenine is always equal to the amount of thymine and the amount of guanine is always equal to the amount of cytosine.

DNA Replication
· Semi-conservative and bidirectional

· Semi-conservative mode of replication was given by Watson and crick.
· They also proposed a model of DNA molecule for which they were awarded the Noble Prize.
· Both strands of DNA are involved in the replication.
· Replication starts with the breakdown of H-bonding in presence of the enzyme Helicase.
· Unwinding (uncoiling) of DNA strand creates coiling tension which is removed by the enzyme topoisomerase that enzyme that helps in DNA replication is DNA polymerase.
· Replication always occurs in the '5 – 3' direction.
· Sugar and phosphate molecules form the backbone of DNA duplex.
· Replication is continuous in one strand called the leading strand and discontinuous in another strand called the lagging strand.
· Fragments of DNA in lagging strand are Okazaki fragments that are joined by DNA ligase.
· Main function of DNA is the transformation of hereditary characters. DNA as the hereditary material was concluded by Frederich Griffith experimenting on the bacteria Diplococcus, Pheunmoniae which causes pneumonia in mammals. His experiment is known as Griffith's experimenting.
· The phenomenon by which DNA is isolated from one type of cell and introduced into another cell is called Transformation. In a bacterial case, it is called Bacterial Transformation.

2. RNA (Ribonucleic acid)
· Single-Stranded
· Polyribonucleotide is similar to ATP.
· Formed from DNA by the process of transcription.
· Transcription occurs in only one strand of DNA in the 5 '-3' direction.
· Nitrogen bases are
    · Purine – Adenine (A), Guanine (G)
    · Pyrimidine – Cytosine (C) and Uracil (U)
· RNA lacks Thymine (T)
· Main function of RNA is protein synthesis but it acts as genetic material in some viruses.

i. Ribosomal RNA (rRNA)
· A coiled molecule closely bound to protein fraction.
· Largest RNA
· Found in Ribosome, so it is responsible for protein synthesis

ii. tRNA or Soluble RNA (sRNA)
· Single-stranded clover leaf-shaped molecule with an anticodon end.
· Found in cytoplasm
· Smallest RNA.
· Carries genetic code.
· Transfer amino acid from amino acids from the cytoplasm to the site of protein synthesis (ribosome)

iii. Messenger RNA (mRNA)
· Long chain of RNA made up of several codons.
· Produced in the nucleus and then found in cytoplasm attached to ribosomes carries genetic information (genetic code) for protein synthesis.
· Each protein has specific mRNA.

Transcription: Process by which a single-stranded RNA is formed from DNA. This process is catalysed by RNA polymerase.
Translation: Protein synthesis from RNA in the cytoplasm.
· Gene: Part of DNA specific for a particular function.
· Each gene occupies a specific position in a specific chromosome called locus.
a. Cistron – Functional unit of gene-specific for the synthesis of one polypeptide.
b. Recon – Unit of recombination or crossing over.
c. Muton – Unit of Mutation.

Genetic Code
· There are 20 amino acids available in the cells which are coded by A, U, C, G letters. The genetic information coded in these letters are passed from DNA to mRNA and then to protein. These are called a genetic codon. So it’s a dictionary that helps in translating the language of nucleic acid into the language of protein.
· Genetic code is a triplet.
· Triplet code is called a codon.
· Total codons are 64.
· Out of 64, 3 codons are non-sense codons.
· Non-sense codons are UAA, UAG, UGA
· Initiation codon is AUG. ( for methionine)
· AUG is specific for amino acid, Methionine.
· Types of amino acids involves in protein synthesis is 20.

Protein synthesis

i. Transcription
· Formation for m-RNA from DNA.
· Reverse transcription occurs in Retrovirus.
    i.e. RNA → DNA
· Copying of RNA to DNA is done by the enzyme RNA dependent, DNA polymerase or Reverse Transcriptase.

ii. Translation
· Formation of protein from mRNA.
· Central dogma of molecular biology is: 
DNA → mRNA → Protein

Fig: Showing central Dogma process in human beings.

Fig: Showing translation

In a gene
· Extrons are the regions that contain the information necessary to form a protein.
· Introns are the regions that do not contain the information necessary for protein synthesis.
· Unwanted introns are removed from the gene by the process of splicing.
· Cancer is basically due to differential gene expression.
· Cancerous cells keep dividing and give rise to a mass called Tumor.
· The genes called oncogenes (cancer-causing genes) is found in the normal cells.
· However, in the normal cells, they remain inoperative and are called proto-oncogenes.
· Branch of biology that deals with ageing is called Gerontology.
· Genetic engineering is the combining of DNA from two different organisms and generating recombinant DNA.
· Restriction Endonuclease and DNA ligase are the enzymes responsible for genetic Engineering.
· Viral oncogene are those oncogenes that are present in the viruses and can trigger cancer in the normal cells when incorporated.
· Genepool – Total variety of genes and alleles present in sexually reproducing organisms.
· Clones are the offsprings produced by asexual reproduction
· The light stain area of the chromosome is called Euchromatin (genetically active) and the darkly stained area is called Heterochromatin (genetically inert).
· Eugenics – Science dealing with the improvement of the human race through the application of the laws of heredity.

High Yielding Points in Genetics

1. If the haploid chromosome number in an organism is 12, the chromosome number present in its trisomic is 25
i.e. N = 2n + 1 = 2 × 12 + 1 = 25

In the garden pea experiment, Mendel used round and wrinkled seeds.

. The RNA molecule is synthesized from a single-stranded DNA by the process of transcription.

The phenomenon of the exchange of segments between maternal and paternal chromosomes is called crossing over.

Foetal sex can be determined by examining cells from amniotic fluid looking for Barr body.

If two alleles come together, only one is able to express itself, it is known as the law of dominance.

When we cross the red-flowered variety of Mirabilis jalapa with the white-flowered variety, we get pink flowers. This is called Incomplete dominance.

Mendelian recombination is due to an Independent assortment of genes

In recombinant DNA technology, plasmid vectors must be cleaved by an enzyme of the same type.

. Down’s syndrome in human beings is due to three copies of chromosome 21st.

. Kleinfelter’s syndrome is due to XXY syndrome.

. The eukaryotic chromosomes differ from bacterial chromosomes in the presence of Introns.

The disease caused by the pleiotropic gene can be treated with gene therapy.

In operon view, an operator gene combines with regulator protein to switch on structural gene transcription.

 A colour blind father marries a mother whose father was colour blind. The resulting progeny is both the son and daughter colour blind.

When a hybrid of F1 generation is crossed with the homozygous recessive parents, it is called a test cross.

The process in which mRNA codes for a protein is Translation.

When an albino plant is crossed with a normal plant, the resulting offspring is normal because a normal character is dominant over an albino.

19. Codon is a sequence of three nucleotides that code for one amino acid.

Also, Read Notes of Other Lessons of Botany:

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