(CHAPTER – 9):-HEREDITY
AND EVOLUTION
BASIC
TERMINOLOGY TO UNDERSTAND GENETICS
BASIC TERMINOLOGY TO UNDERSTAND GENETICS
GENE:
A
segment of DNA capable of self expression eg. Gene for height of plant
ALLELE:
Alternate
forms of a gene eg Gene expressing differently as Tall or Dwarf
CHARACTER:
Unique feature regulated by a gene eg. Eye colour
TRAIT:
Various
expressible forms of a character eg. Blue/black/green eye colour
PHENOTYPE:
Observable
or visible features eg. Height, colour, shape
GENOTYPE:
Genetic
makeup of phenotype eg. Pure Tall (TT), Hybrid Tall (Tt)
HOMOZYGOUS:
Both
the alleles of a gene, are same in the cell eg. TT or tt for height of plant
HETEROZYGOUS:
Both the alleles of a gene, are different in a cell eg. Tt
SELFING:
Reproduction
between organisms with similar genetic trait eg. TT/TT, or Tt/Tt
CROSSING:
Reproduction
between organisms with different
genetic trait eg. TT/tt, Tt/TT
PURE/TRUE
BREED:
A
condition of homozygosity giving 100% same offspring, on selfing eg TT, tt
HYBRID:
A
condition of heterozygosity giving different ratio of offsprings, on selfing
eg. tt
FIRST
FILIAL (F1 -Generation):
The
probability ratio/nature of offsprings for the cross between Parent
generation. For a Mendelian cross, it is always a Hybrid (100%)
SECOND
FILIAL (F2 -Generation):
The
probability ratio/Nature of offsprings for the cross between F1 – generation
(selfing). For a Mendelian nature, it is always 3:1 in monohybrid cross.
MONO-HYBRID
CROSS:
A
genetic cross, in which inheritance of only one character is studied.
DI-HYBRID
CROSS:
A
genetic cross, in which inheritance of two characters are studied
simultaneously.
MONOHYBRID
RATIO:
PHENOTYPIC
(3:1) GENOTYPIC (1:2:1)
DIHYBRID
RATIO:
PHENOTYPIC
(9:3:3:1) GENOTYPIC (1:2:1:2:4:2:1:2:1)
DOMINANT:
Out
of the two different alleles/Traits in cell, only one expresses
phenotypically, it is called Dominant allele/ Trait.
RECESSIVE:
Out
of the two different alleles/traits, in the cell, the one which is masked or
hidden in the presence of other is called Recessive allele/Trait. It
expresses only in Homozygous condition
BACK
CROSS:
A
cross involving a progeny/offspring and any of the parent.
TEST
CROSS:
A
cross involving F1 – generation and
Recessive parent only ( A Type of Back Cross) where zygosity of the offspring
is identified. 100% result means Homozygous and 50% : 50% or 1:1 result means
Heterozygous
1a) Heredity :-
The transfer of characters or traits from the parents to their off
springs is called heredity.
b) Variations :-
The
differences between the characters or traits among the individuals of the same
species are called variations.
2) Accumulation of
variations during reproduction
When organisms reproduce, the off springs
show minor variations due to inaccuracies in DNA copying. These variations are
less in asexual reproduction and more in sexual reproduction.
Some variations are useful variations and
they help the organism to adjust to the changes in the environment. Some
variations do not help the organism to adjust to the changes in the environment
and they may die and become extinct.
3) Rules for inheritance of characters (traits) :-
Characters are transferred through genes
present in the DNA molecules in the chromosomes present in the nucleus of the
cell.
The inheritance of characters is due to the
fact that both the father and mother contributes equal amount of genetic
material to the child. So for each trait there are two factors one from the
father and one from the mother.
Gregor Johann Mendel conducted experiments
with garden pea plants and determined the rules for the inheritance of traits.
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| Father of Genetics: Sir Gregor Johann Mendel (1822-1884) |
a) When plants having one
pair of character (Eg:- tall and short plant) was crossed (Monohybrid
cross) :-
Mendel selected pea plants having one pair
of character – a tall pea plant and a short pea plant. He selected pure tall
(TT) and pure short (tt) pea plants and cross pollinated them. He obtained all
tall plants (Tt) in the first generation (F1 ). When the first
generation plants were self pollinated, he obtained tall and dwarf plants in
the ratio 3:1 in the second generation. (F2)
The ratio of pure tall (TT), hybrid tall
(Tt) and pure dwarf (tt) was in the ratio 1:2:1
The trait that is expressed in the F1
generation is called the dominant
trait and the trait that is supressed in the F1 is called the
recessive trait.
 |
| (Mendelian Mono-Hybrid Cross) |
b) When plants having two
pairs of characters (Eg:- shape and colour of seeds) were crossed (Dihybrid
cross) :-
Mendel selected pea plants having two
pairs of characters – shape and colour of seed. He selected plants having round
yellow seeds (RRYY) and wrinkled green seeds (rryy) and cross pollinated them.
He obtained all plants with round yellow
seeds (RrYy) in the F1 generation. When these plants were self
pollinated in the F2
generation out of 16 plants, 9 had round yellow (RrYy), 3 had round
green (Rryy), 3 had wrinkled yellow (rrYy) and 1 had wrinkled green (rryy)
seed. In the ratio 9:3:3:1.
4) Sex determination in human beings :-
Human beings have 23 pairs of chromosomes
in the nucleus of the cell. Out of this two chromosomes are sex chromosomes X
and Y. The female has two X chromosomes (XX) and male has one X and one Y
chromosome (XY). The sperms and eggs have one set of sex chromosomes. Some
sperms have X chromosome and some have Y
chromosome. All eggs have X chromosome.
If a sperm having X chromosome fuses with
an egg having X chromosome the child will be a girl. If a sperm having Y
chromosome fuses with an egg having X chromosome the child will be a boy.
5. Variations may or may not help organisms to survive :-
Let us study with the help of an example. A colony of green beetle living on grassy ground or green branches of a plant. Due to error in DNA copying slight variations accumulate in nature. These variations give different directions to organism in terms of evolution and speciation, like:
a) Some variations help organisms to
survive :-
Eg :- There are some beetles living in
green bushes. They increase their numbers by reproduction. Crows can easily see
the red beetles and they are eaten by the crows. During reproduction due to
some variation some green beetles are produced instead of red beetles. The
green beetles are not visible to crows and are not eaten by them. Then
gradually the population of the red beetles decreases and the population of the
green beetles increases. This variation has helped the organisms to survive.
b) Some variations do not help organisms to survive:-
Eg :- During sexual reproduction a colour
variation occurs in red beetles and some blue beetles are produced instead of
red beetles. Both the red and blue beetles are visible to crows and are eaten
by them. Then the population of both red and blue beetles decreases. This
variation has not helped the organisms to survive.
sometimes such variations helped by calamity or disaster, may results in formation of new species by chance. (case of Elephant Foot in beetle example)
c) Acquired traits cannot be passed from one generation to the next:-
Eg :- If the population of beetles
increases and plants are affected by
diseases, then the food available for the beetles decreases and their body
weight also decreases. If after a few years the availability of food increases
then the body weight of the beetles also increases. This acquired trait cannot
be passed from one generation to the next because there is no change in their
genetic composition.
[ beetle example- video]
[ beetle example- video]
6. Speciation :- ( Formation of new species from existing species
The formation of new species from existing
species is mainly due to one or more of the following factors. They are :-
Accumulation of variations, Physical barriers, Genetic drift, Natural selection,
Mutation and migration.
i) Accumulation of variations :- The differences between the individuals of the same species is called
variations. The accumulation of variations over several generations produce new
species.
ii) Physical barriers (Isolation):- Populations may get separated by physical barriers
like mountains, rivers, lakes etc. These isolated groups produce variations
which can produce new species.
iii) Genetic drift :- Natural calamities
or introduction of new members of the same species in an area can
produce changes in the gene pool of the population and new variations are
produced which can produce new species.
iv) Natural selection :- Only those individuals of a species which have useful
variations and can adapt to the changes in the environment survive and the
others die. These organisms can produce variations and new species.
Iv) Mutation: A sudden inheritable change in the genotype of an
organism. It help in quich speciation.
v) Migration :- Some
individuals of a species may migrate to a new geographical area and adjust to
the changes in the environment there and develop new variations and produce new
species.
7. Evolution :-
The gradual changes taking place in living
organisms giving rise to new organisms due to changes in their genetic
composition is called evolution.
a. Inherited traits :-
Inherited traits are traits in an
organism due to changes in the genetic
composition and it can be passed from one generation to the next and it results
in evolution.
b. Acquired traits :-
Acquired traits are traits which are
acquired by an organism during its lifetime and it cannot be passed from one
generation to the next and it does not result in evolution.
9. Evidences of evolution :-
There are a number of common features
in different organisms which provide evidence to show evolutionary
relationship. The main evidences of evolution are from the study of:-
Homologous organs, Analogous
organs and Fossils.
a) Homologous organs :- are organs which are similar in structure but different
in functions.
Eg :- The fore limbs of
amphibians, reptiles, birds, and mammals have similar structures but different
functions. Frog (amphibian) uses its fore limb to raise the front of the body.
Lizard (reptile) uses its fore limb for walking and running. Bird’s forelimbs
are modified as wings for flying. Mammals use the fore limbs for grasping,
walking, running, swimming, flying etc. This shows evolutionary relationship.
b) Analogous organs :- are organs
which are different in structures but similar in functions.
Eg :- The wings of butterfly,
bird and bat have different structures but similar functions. This shows
evolutionary relationship.
c) Fossils :- are the
remains of organisms which lived long ago.
From the study of fossils we
can know their structures and the time period in which they lived. The fossils
of complex and recent organisms are found closer to the surface of the earth
and the fossils of simpler organisms are found deeper inside the earth. The age
of fossils can be determined by Radio Carbon Dating. The study of fossils
(Palaeontology) shows evolution of simpler forms into complex forms and their
evolutionary relationship.
 |
| ( Fossil Stegosaurus from Jurassic period) |
 |
| ( Fossil Plant log from Cretaceous Period) |
 |
| (Frozen Wooly Mammoth from Pleistocene Deposits) |
 |
| (Fossil of Upper Himalayan Fish from Oligocene) |
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| (Invertebrate fossil Shell) |
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| (Fossil Trilobit from Early Cambrian) |
10. Evolution by stages :-
Complex organisms and its
organs developed from simpler organisms gradually over generations.
i) Evolution of eyes :- The eyes of planaria are just eye spots to detect
light. It developed gradually into a complex organ in higher animals.
ii) Evolution of feathers :- Feathers were first
developed in dinosaurs and used for protection from cold. Later birds used them
for flying.
iii) Evolution by
artificial selection: - Humans
cultivated wild cabbage for over 2000 years and produced different vegetables
from it by artificial selection.
Eg :- Cabbage – by selecting short distance between the
leaves.
Cauliflower – by selecting sterile
flowers.
Kale – by selecting large leaves
Kholrabi – by selecting the swollen
stem
Broccoli – by arresting flower growth
Embryology: Study of
structural, developmental and comparative aspects of embryos of organisms. It
help in establishing evidences of evolution by stages.
 |
| (Comparative Embryology in Evolution) [ Evolution by stages video lecture ] |
11. Evolution should not be equated with progress :-
Evolution has not resulted in progress.
Evolution has resulted in the formation of several complex species from simpler
species due to variations, genetic drift and natural selection. This does not
mean that one species gets eliminated when new species are formed or that the new
species are better than the older species. Species get eliminated only if they
are not able to adapt to the changes in the environment.
Several species which could
adapt to the changes in the environment still continue to survive for example
bacteria. Human beings have not
evolved from chimpanzees. They had a common ancestor from which they evolved
separately. Human beings are not the pinnacle of evolution but they are only
one species among the several evolving species.
12. Human evolution :- (Homo sapiens)
There is a great diversity among human
beings in their form and features around the world. Human beings evolved in
Africa. Some of them stayed there and others migrated to different parts of the
world. Then due to genetic variations and the environmental changes in different
geographical regions they developed changes in their forms and features.
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| (Skulls during Human Evolution) |
Courtesy: Google image and you tube videos
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