Heredity and Evolution

Heredity
and
evolution
Radiate

Introduction
We have always seen that there are a lot of similarities between
us and our parents.
Also lot of dissimilarities between us and our friends or any
people who don’t belong to our family.
This is all due to an interesting phenomenon - heredity.
Heredity:- It is phenomenon of transfer of characters from
parents to offspring.
Variation:- The differences shown among individuals of the
same species or the differences exhibited by offspring as
compared to his parents.
Genetics:- It is the study of heredity and variations.
We all know that our body is made up of many cells.
In cells, we have nucleus that has chromatin material which
further condenses to form ribbon shaped chromosomes.

Chromosomes
These chromosomes are made up of DNA & Proteins and they
also have certain small segments (division) on them that are
called genes.
These genes are responsible for transferring characters from
parents to offspring.
The chromosome number is fixed in every species like in us, it is
23 pairs in each cells, except sex cells it have 23 (sperm & egg).

The cells in our
body are
generally of two
types
In our somatic cells, there are 23 pairs but in our sex cells they are
only 23 in number but not pairs.
During fertilization, when sperm and egg fuse to form zygote, at
that time, both of them contribute 23 chromosomes each.
The zygote formed inherits 46 chromosomes i.e., 23 pairs.
The chromosomes that passed from parents, have genes on them
which we know is a unit of heredity.
During mating, these chromosomes also undergo a phenomenon of
crossing over which causes blending of characters that is the reason
of offspring has few characters of mother, few of father and few
new characters.
The whole concept of heredity has been explained by a scientist;
Gregor Johaan Mendel (Father of Genetics).
• (Self replicating) 23 pairs1. Somatic cells
• (Sperm & egg) 23 [gamete cells]2. Sex cells

Experiment
plant: Pea Plant
Mendel actually performed very interesting experiments in order
to explain the concept.
For this, He took an experimental plant: Pea plant (Pisum
sativum)
As He saw a lot of different features in the same plant.
He actually predicted 7 main contrasting characters in it.

Let sum up
those
contrasting
features first

He selected pea
plant as an
experimental
plant because;
It has 7 distinct easily contrasting characters
Short life cycle
Easily available
Annual herb
Doesn’t require much care
It produces a large number of seeds

He formulated
various laws;
1. Law of unit
character:-
According to this law, for each character there is a pair of
contrasting genes.
Like-plant, it have genes.
Tall gene-T
Dwarf gene- t
TT
Tt
tT
tt
For e.g. For height, two contrasting genes are present that
correspond to either tallness in plants or dwarfness in plants.

2. Law of
dominance:-
According to this law, out of two contrasting genes controlling
each characters, one will express itself that is dominant and other
will remain suppressed (recessive) in the presence of dominant
gene.
If tall plant- Dominant
Dwarf plant- Recessive
Pure tall Pure & Dwarf Hybrid tall
TT tt Tt

3. Law of
segregation:-
Mendel’s first law of inheritance.
It states that ‘the two genes controlling each character separate
from each other during gamete formation’.
T,T t,t T,t
TT tt Tt

4. Law of
independent
assortment:-
Mendel’s second law of inheritance.
It states that, ‘the two genes controlling each characters separate
from each other during gamete formation and get randomly
arranged in offspring’.
To explain it, we will cross the parental generation in order to see
what possibilities of characters we can get in progeny.
tt (Dwarf) TT (Tall)
Progeny

Generally we
make two
types of
crosses:-
Let us understand the meaning of cross; it just refers to mating of
parents in order to form offspring.
In these crosses we will get the offspring possessing different
characters.
1. Monohybrid cross
2. Dihybrid cross

1. Monohybrid
cross
It is that in which one characters studied at a time.
Like we say that we consider character height and obtain different
results.
So, as character is only one that we are noticing, therefore, the
cross is monohybrid.
Now lets understand the laws with monohybrid cross; for e.g.- we
consider feature: height
Parental generation- Tall (TT) Dwarf (tt)
Formation of gametes-T,T t,t
Fuse
T- Dominant gene
Tt (all are tall)
First filial generation (F1 generation)

1. Monohybrid
cross
F1 Generation’s offspring:
Tt Tt
Tt Tt
Punnett square:-
Fuse
F2 Generation
T t
T TT (Tall) Tt (Tall)
t Tt (Tall) Tt (Dwarf)

1. Monohybrid
cross
For ratio:-
A. Phenotype: Physical expression of organism
Tall : Dwarf
3 : 1
B. Genotype: Genetic constitution of an organism (Genes
combination like;Tt, TT, Tt, tt)
TT :Tt : tt
1 : 2 : 1

2. Dihybrid
cross
A cross in which two characters are studied at a time.
Dominant:-YY RR
YY RR × yy rr
Yellow, round Green, wrinkled
Gametes formation
Both character include
(YR,YR) YR yr (yr, yr)
Yy Rr (F1 Generation)
Yellow, round

2. Dihybrid
cross
Like if we make cross considering two features; like, height and
flower colour in plant.
Now lets understand the laws with dihybrid cross:-
For e.g., we consider feature; height and seed colour.
tall with round seed dwarf with wrinkled seed
TT RR tt rr
TR,TR tr, tr
Tt Rr (All are tall with round seed)
First filial generation

2. Dihybrid
cross
Tt Rr, Gametes are;TR :Tr :Tr :TR
9 : 3 : 3 : 1
Phenotype:- TR :Tr : tR : tr
9 : 3 : 3 : 1
TR Tr tR tr
TR TTRR TTRr TtRR TtRr
Tr TTrR TTrr TtrR Ttrr
Tr Ttrr tTRr ttRR ttRr
Tr tTrR tTrr ttrR ttrr

Sex
determination
It is the process by which sex of a child is determined.
Father Mother
Parental generation- XY XX
Gametes- X,Y X,X
Fuse
Punnett square:-
It was concluded that;
There is 50-50% chance of boy or a girl.
The sex of child depend on father because father produces different
types of gametes.
X X
X XX (Girl) XX (Girl)
Y XY (Boy) XY (Boy)

Evolution
It is the gradual sequence of changes that took place over years
and led to development of entirely new species.
We must be wondering that we should have some evidence to
prove the occurrence of evolution.
Different evidences are;
Fossils: The dead remains of plants and animals that got buried
under earth millions of years ago.
The study of fossils is called palaeontology.

Fossils reveals
the order of
origin, i.e.,
Protozoans
(water)
Fishes
AmphibiansReptiles
Birds Mammals

Evidences of
evolution
1. Archaeopteryx: It is fossil that resembles bird as well as
reptiles. We can say that it is link between reptiles and birds.
2. Homologous organs: are those which have same structure but
perform different functions. For e.g.; Fore limbs of man &
cow.
3. Analogous organs: are those which have different structure
but perform same functions. For e.g.; Wings of birds and
insects.
4. Vestigial organs: are those which were functional in our
ancestor but now they are not functional. For e.g.; Nictitating
membrane, Pinna muscles & Vermiform appendix, tail bone
(man).
Notes Nictitating membrane in the eyes of man (left) and ape
(right).
5. Embryology: It is the study of development of embryo. This
study reveals that all organisms of different species have
common ancestors.

Ernst Haeckel’s
Biogenetic Law
According to this, Ontogeny Recapitulates Phylogeny, this
means that during the development of embryo, complete
history is repeated.
Ernst Haeckel’s phrase "ontogeny recapitulates phylogeny"—is
a historical hypothesis that the development of the embryo of
an animal, from fertilization to gestation or hatching
(ontogeny), goes through stages resembling or representing
successive adult stages in the evolution of the animal's remote
ancestors (phylogeny).

Theory of
evolution;
1. Lamarck
theory:-
According to this;
Use or disuse of organ makes the organ efficient or less
efficient.
Favourable characters are passed on to next generation.
Accumulation of new characters leads to evolution.

Theory of
evolution;
2. Darwin
theory:-
According to this;
Population is increasing day by day, but still, the population
rate is constant.
Individuals of species struggle for food, space and mate.
Those who can struggle, survive and others estimate.
Favourable characters passed on to next generation.
Accumulation of new characters leads to evolution.

Speciation
It is the formation of new species from pre-existing ones.
A new species may be formed from the population occurring in
different areas of distribution; Allopatric speciation.
A new species formed from fragment of population in the same
area due to mutation; Sympatric speciation.
Microevolution: It is the formation of species due to small but
significant changes like gene mutation and gene
recombination.
Macroevolution: It is the development of different genera and
taxa.

Heredity and Evolution

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