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INHERITANCE
Major concepts:
Introduction to genetics
Chromosomes and genes
Mendel’s laws of inheritance
Co-dominance and incomplete dominance
Variations and evolution
For much of the human history people were
unaware of the scientific details of how
babies got the characteristics of their parents
...
Many answers to the questions
about how offspring get the characteristics
from their parents came from George
Mendel’s work
...


Introduction to genetics:
Genetics is the branch of biology in which we
study inheritance
...
These characteristics are called
the traits
...

Parents pass characteristics to their young
through gene transmission
...
The
chromosomes carry the units of inheritance
called the genes
...
They contain specific
instructions for protein synthesis
...


The body cells have a constant number of
paired chromosomes
...
In human body cells, there are
23 pairs of homologues chromosomes for a
total of 46 chromosomes
...

Chromosomes are made of chromatin
material (simply as chromatin)
...
DNA wraps around
histone proteins and forms round structures,
called nucleosomes
...
In this way, the
nucleosomes and the DNA between them
look like “beads on a string” (on below fig)the
fibers consisting of nucleosomes condense
into compact forms and get the structure of
chromosomes
...
According to
the Watson crick
...
These strands are
called around each other in the form of a
double helix
...
In
double helix the nitrogenous bases of
opposite nucleotides from pair through
hydrogens bonds
...

The nitrogenous base adenine of one
nucleotide forms pair with the thymine of
opposing nucleotide, while cytosine forms
pair with guanine, there are two hydrogen

bonds between adenine and thymine while
there are three hydrogen bonds between
cytosine and guanine
...
It is done to make the copies of
the chromatids of chromosomes
...
The DNA double helix is
unwounded and the two strands are
separated
...
Each strand acts as a template to
produce other strands
...
In this
way, both template strands make new
polynucleotide strands in front of them
...


How the DNA of chromosomes does Works?
DNA is the genetic material i
...
it contains the
instructions to direct all the functions of cells
...
Some
proteins perform structural roles while the
others act as enzymes to control all
biochemical reactions of cells
...
Let
us see how DNA is responsible for this
...
Specific proteins have specific
number and sequence of their amino acids
...
During

protein synthesis, the sequence of DNA
nucleotides decides that what will be the
sequence of amino acids
...
This process is called
transcription
...
The ribosome
reads this sequence and joins specific amino
acids according to it, to form protein
...

The part of DNA (sequence of nucleotides)
that contains the instructions for the
synthesis of a particular protein is known as a
gene
...
Like chromosomes, genes
also occur in pairs, one on each homologous
chromosome
...


Each gene determines a particular trait in an
organism
...
For convenience,
pairs of genes are represented by a letter or
symbol
...
It means
that a gene exist in more than one alternate
forms
...
The
alternate forms of gene are called alleles
...
A and a are
the alleles of one another
...
A and a are the alleles of
one another
...
when
chromosomes separate during meiosis, alleles

also separate and each gamete gets one of
the two alleles
...


Genotype and its Types:
The specific combination of genies in an
individual is known as genotype
...
e homozygous and heterozygous
...
e
...
Like other traits, it is also
controlled by one pair of genes
...
Three combinations i
...
e AA, and aa
...
The genotype in which the gene pair
contains two identical alleles (AA or aa) is
called homozygous genotype
...

When in the heterozygous condition one
alleles masks or prevents the expression of
the other, it is called the dominant allele
...
The dominant alleles are
represented by capital letters and recessive

alleles by lower case letters
...
e
...
In humans, alleles A
produces normally body pigments while
alleles a does not produces pigments
...
On the other hand if
genotype is aa, no pigments will be produced
and the individual will be albino, in this
example you see that the allele
...
The expression of this genotype in the form
of trait (in our example, being albino or
having normal pigmentation) is known as the
Phenotype
...

He developed the fundamental principles of
genetics
...
These factors were
eventually termed genes
...
In
his writings, he gave reasons for this
selection
...

There should be a number of different traits
that can be studied on below fig
...
g for
the trait of height there should be only two
very different phenotypes i
...

The organism (if it is a plant) should be selffertilized but cross fertilization should also be
possible
...


All these features present in pea plant
...
Cross pollination can also be done
by transferring the pollen grains form the
flower on one plant to the flower on another
plant
...
Mendel’s succeeded in his
work not only because he selected the right
organism for his experiments but also
because he analyzed the results by using the
principles of statistics (ratios)
...
For this purpose he crossed
(Reproduced) two plants having one
contrasting trait i
...
Across in
which only one trait is studied at a time, is
called as a Monohybrid cross
...


All resulting seed of the next generation were
round
...
As
dominant, while “wrinkled seed” as recessive
...
As a result
he got 7324 seeds; 5474 round and 1850 wrinkled
(3 rounds: 1 wrinkled)
...
The offspring of P1 generation are F1

generation (first filial), the cross in F1 generation
produces F2 generation (2nd filial)
...
e tallness was a dominant trait
...
Mendel got the ratio of tall to short plant
in F2 as 3:1
...

In each organism
...

During gametes formation, the genes (alleles) of
each pair segregate from each other and each
gamete receives one gene from the pair
...

These conclusions were called the Law of
segregation
...
Such crosses are called dihybrid
crosses
...
e shape colour
...
Similarly yellow seed
colour (controlled by Y) was dominant over green
(controlled by Y)Mendel crossed a true-breeding
plant that had round yellow seeds (RRYY) with a
true-breeding plant having wrinkled green seeds in
F1 generation were round yellow
...
This cross produced seeds with four
phenotypes
...
The ratio of these
phenotypes was 9:3:3:1
...
It is named after R
...
A checker board is used to cross all the

possible gametes of one parent with all the gametes
of other parent
...

Mendel explained that the two traits i
...
The
segregation of R and r alleles happens
independently of the segregation of Y and y alleles
...
This principle is known as the law of
independent assortment
...

CO-DOMINANCE AND INCOMPLETE DOMINANCE:
After the discovery of Mendel’s work
...
These experiments proved that all the
traits in organism do not fallow Mendel’s laws
...
Similarly for many traits there are more than
two alleles in a gene pair
...

Co-dominance: is the situation where two different
alleles of a green pair example themselves
completely, instead of showing a dominantrecessive relationship
...

An example of co-dominance is the expression of
human blood group AB
...
This gene has
three alleles i
...
the allied Iᵃ, produces
antigen A in blood and the phenotype in blood
group A
...
the allele i
does not produce any antigen and the phenotype is

blood group O
...

when there is a heterozygous genotype of Iᵃ, Iᵇ,
each of the two alleles produces the respective
antigen and neither of them dominates over the
others
...
As a result of the blending, as
intermediate phenotype is expressed
...


In four O clock plant, the trait of flower colour is
controlled by two alleles (let us say them R and r)
...
When a homozygous red

flowered plant (RR) is crossed with homozygous
white flowered plant(rr) the heterozygous (Rr)
plants of F1 generation produce pink flowers (pink
is a blend of red and white colours)
...
However,
when two heterozygous plants with pink flowers
(Rr) are crossed
...


Initiating and planning:

Predict from pedigree charts the passage of
traits from one generation to the other
...

VARIATIONS AND EVOLUTION:
In the previous chapter, we studied that sexual
reproduction produces variations in the next
generation
...
The
main sources of variations in sexually reproducing
populations are described next
...

Mutations (changes in DNA) are important
source of variations
...


During fertilization, one of the millions of
sperms combines with a single egg
...

Gene flow i
...

Variations also caused by different
combinations of chromosomes in gametes
and then in zygote
...
In other words, a couple
can produce more than 70 trillion genetically
different children!
...
e
...

Discontinuous variations show distinct
phenotypes
...
The
individuals of a population either have distinct
phenotypes
...
Blood groups
are a good example of such variations
...
Discontinuous
variations are controlled by the alleles of a
single gene pair
...

In continuous variations, the phenotypes
show a complete range measurement from
one extreme to the other
...
are example of
continuous variations
...
No
population can show only two or three
distinct heights
...

Practical:
Record the heights of class fellows to predict
which kind of variations is it
...

Variations Lead to Evolution:
Organic evolution (biological evolution) is the
change in the characteristics of a population
or species of organism over the course of
generations
...
The changes in an
individual are not considered as evolution,
because evolution refers to populations and
not to individuals
...

Alternation in genetic characteristics (traits)
of a types of organism over time; and

Creation of new types of organism from a
single type
...
The antievolution ideas
support that all living things had been created
in their current form only a few thousand
years ago
...
But the scientific work in eighteen
century led to the idea that living things might
change as well
...
It
was called as “the theory of natural
selection”
...
He also published a book
...

Darwin’s theory of evolution was not widely
accepted because of lack of sufficient
evidence
...
some

scientist proved that the theory of natural
selection and Mendelian genetics are the
same ideas just as Darwin had proposed
...
There are
morphological and physiological variations in
all populations
...

Different population face different
environments and they have to adapt to
different conditions
...
Fitness
means an organism’s ability to survive and
reproduce
...
These conditions produce
struggle for survival among the organisms of
population
...
On
the other hand, the rate of the transmission
of unfavorable to next generations is low
...

in the example mentioned next, we can see a
mouse population with variations in skin
colour
...
Only
medium and dark coloured mouse can make
their next generations, in next generation,
population again contains light medium and
dark coloured mouse
...
Now only the
dark coloured mouse makes new generation
...
We will
see only the dark coloured (favorable
variation) mouse in the population
...

So, the individuals with favorable variations
become a major part of population while the
individuals with harmful or unfavorable
variations become rarer
...
e
dark and white coloured moths
...
In the
19th century when industries were established
in England, the lichens on tree trunks died
(due to polluted air) and the naked tree
trunks turned dark
...
The natural
selection selected dark months to reproduce
...
In this case, the
dark colour variation in moth may be
considered an adaptation to environment
...
Charles Darwin also used this term in his
work on natural selection
...


Artificial selection (or selective breeding)
means intentional breeding between
individuals for certain traits, or combination
of traits
...

Animals or plants having desirable
characteristics are selected for breeding
...
In
artificial selection, the bird animals are known
as breeds
...

Numerous breeds of sheep,goat,cow,hen,etc
have been produced by artificial selection to
increase the production of
wool,meat,milk,eggs etc
...


In artificial selection, humans favor specific
variations for selection while in natural
selection
...




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