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
factors.
• One trait is expressed (dominant trait) and the
other remains hidden (recessive trait) in F1
generation.
• 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.
DNA RNA
Number of strand 2 1
Type of sugar Deoxyribos Ribos
Nitrogen bases Adenine,
thymine,
cytosine,
guanine
Adenine,
uracil,
cytosine,
guanine
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
nucleotide.
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
chromosomes.
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.
