Heredity is the transmission of characters from
parents to their offsprings.
Variations are the features seen in offsprings that are
different from their parents.
The branch of science which deals with heredity and
variations is known as Genetics.
Gregor Johann Mendel is known as ‘the
Father of Genetics’ because of his inferences
from the hybridization experiments in pea
plants (Pisum sativum) from 1856 to1863.
Considering one pair of contrasting traits (tallness-
dwarfness) in garden pea plants, Mendel got plants in
3:1 ratio in the second (F2) generation:
He conducted several hybridization experiments
considering the following traits;

  • Height of the plant (tall/dwarf) – Position of the
    flower (axial/terminal) – Shape of the seed (round/
    wrinkled) – Colour of seed coat – Colour of
    cotyledon – Colour of fruit and – Shape of fruit.
    The main inferences of Gregor Mendel:
    • A trait is controlled by the combination of two
    • One trait is expressed (dominant trait) and the
    other remains hidden (recessive trait) in F1
    • The trait which remains hidden in the first
    generation appears in the second generation.
    • The ratio of the dominant and recessive traits
    in the second generation is 3:1 .
    Considering two pairs of contrasting traits (tallness-
    dwarfness and round-wrinkled seeds), Mendel got
    plants in 9:3:3:1 ratio in the second (F2) generation:
  • Mendel inferred that the difference is due to the
    independent assortment of each character or trait.
    The hereditary factors, described by him as letters,
    are now known as the genes.
    Genes are the specific parts of DNA that control
    metabolic activities and responsible for specific
    characteristic feature of any organism.
    The two different forms of a gene that controls a trait
    is called as ‘alleles’. (Eg:- Suppose ‘Tt’ is the factors
    responsible for the trait, ‘height’, the allele ‘T’ is for
    tallness and the allele ‘t’ determines dwarfness. DNA, RNA -the nucleic acids :
    DNA (deoxyribo nucleic acid) and RNA (ribo nucleic
    acid) are made of nucleotides containing sugar,
    phosphate groups and 4 types of nitrogen bases.
    Number of strand 2 1
    Type of sugar Deoxyribos Ribos
    Nitrogen bases Adenine,
    Nucleotides are the basic units of nucleic acids.
    A nucleotide is made up of a nitrogen base, a sugar
    molecule and a phosphate group.
    (Nitrogen bases are molecules that contain nitrogen
    and are alkaline in nature )
    DNA is a double helical structured molecule. In 1953,
    James Watson and Francis Crick proposed this model.

The two long strands of DNA contain deoxyribose
sugar and phosphate groups and its steps are made of
pairs of nitrogen bases. The nitrogen base, adenine
pairs with thymine and cytosine pairs with guanine.
In other words, DNA molecule is made up of four
kinds of nucleotides, namely adenine nucleotide,
thymine nucleotide, cytosine nucleotide and guanine
The single stranded nucleic acid, RNA, is made of 4
different types of nucleotides that contain ribose sugar,
phosphate and nitrogen bases (adenine, uracil, guanine
and cytosine).
RNA involve in the protein synthesis as directed by
the DNA.
Gene action (protein synthesis)
Genes, which are the specific parts of DNA,
act through synthesizing proteins.
Proteins are formed in the ribosomes. Different types
of RNA involve in this process.
The stages of protein synthesis of DNA:
• DNA unwinds and mRNA forms.
• mRNA reaches outside the nucleus.
• mRNA reaches ribosomes.
• Based on the information in mRN, amino acids
are transferred to ribosomes by tRNA.
• Ribosomes bind amino acids to form protein.
Different kinds of RNAs involved in this are :
mRNA (messenger RNA),
tRNA (transfer RNA),
rRNA (ribosomal RNA).
Sex determination in man
There are 46 (23 pairs) chromosomes seen in each
cell of human being. Out of which, 44 (22 pairs) are
somatic chromosomes and 2 (one pair) are sex
Sex chromosomes are two types: X and Y. [XY in
males and XX in females]The total chromosomes can be represented as
44+XX in female and 44+XY is male.
Reasons for variations in offsprings
Crossing over, combination of allele during
fertilization and mutation occur in chromosomes etc.
may cause variation.

  • Crossing over
    The process of pairing of chromosome and
    exchanging their parts, during the initial
    phase of meiosis, is called crossing over.
    As a result of this, part of one DNA
    becomes the part of another DNA, causing
    a difference in the normal distribution of
    genes. This may causes expression of new characters
    (variations) in the offsprings. .
  • Fertilization
    When gametes undergo fusion (fertilization), the
    combination of allele changes. This causes the
    expression of characteristics in offsprings that are
    different from parents.
  • Mutations
    Mutation is a sudden inheritable change in the genetic
    constitution of an organism. It may occur due to the
    defects in the duplication of DNA, certain chemicals,
    radiations etc.
    Mutation causes changes in genes, that lead to
    variations in characters.
    Melanin, a pigment protein,imparts colour to the skin.
    The difference in gene function is the reason for
    colour differences of skin. The differences in skin
    colour is mere an adaptation to live under sun.

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