Tuesday, 2 May 2017
Wednesday, 1 March 2017
DAV Public
School
Sec.6,
Dwarka, New Delhi-75
HEREDITY AND
EVOLUTION
Biology Class-10
Heredity is
the transmission of characters or traits from parents to their offspring.
Genetics is
the science of heredity, i.e., the study of inheritance of characteristics in
offspring from their parents.
Variations –
The difference in the character among the individuals of a species is called
variation.
1.
Variations necessary for organic evolutions.
2.
It increases the chances of its survival in
changing environment.
The free earlobes and attached earlobes are the two
variation found in human population.
Accumulation of variations-
The significance of variations shows up only if its continues to be inherited
by the offspring for several generations.
Some important terms:-
1.
Genotype-the genetic make of an organisms
2.
Phenotype- the physical appearance of an
organisms
3.
Genes- unit of heredity which transfer traits
from parents to their offspring
4.
Diploid- two sets of chromosomes
5.
Haploid- one set of chromosomes
6.
Dominant genes- the gene which decides the
appearance of organisms even in presence of alternative genes is known as
dominant. Dominant gene represented by a capital letter.
7.
Recessive genes- the gene which decides the
appearance of organisms only in presence of another identical gene is called
recessive genes. Recessive genes represented by a small letter.
8.
F1 generation- when two parents cross
to produces to progeny then their progeny is called first filial generation.
9.
F2 generation- when first filial
progeny cross among themselves to produce second generation progeny, then this
progeny is called second filial generation.
10.
Hybrid- a new form of plants resulting from a
cross of different varieties of a plant is known as hybrid
11.
Monohybrid cross- crosses between one pair
contrasting characteristics.
12.
Dihybrid cross- crosses between two pair of
contrasting characteristics.
Rules of
inheritance of traits given by GREGOR JOHANN MENDEL
Gregor Johann
Mendel was an Austrian monk who grew garden peas (PISUM SATIVUM) in the church
garden. He identified the number of characteristics showing contrasting traits
follows-
1.
Pea plants are easy to cultivate and have large
flowers which help in cross pollination under controlled conditions.
2.
These plants are self pollinated in nature.
3.
These plants show discrete variations in
characters or traits.
4.
There are no intermediate forms produced.
1.
Mendel first crossed pure-bred tall pea plants
with pure-bred dwarf pea plants and found that only tall plants were produces
in the first generation or F1 generation. No dwarf pea plant (or
short pea plants) was obtained in the first generation of progeny. From this Mendel
concluded that the first generation showed the traits of only one of the parent
plants: tallness. The trait of other parent plant, dwarfness, did not show up
in the progeny of first generation.
2.
Mendel then crossed the tall pea plants of the
first generation and found that tall plants and dwarf plants were obtained in
the second generation in the ratio of 3:1. In other words in the F2
generation, three-fourth plants were tall and one- fourth were dwarf. Mendel
noted that the dwarf trait of the parent pea plant which had seemingly
disappeared in the first generation progeny reappeared in the second
generation. Mendel said that the trait of dwarfness of one of the parent pea
plant had not been lost; it was merely conceals or suppressed in the first
generation to remerge in the second generation. Mendel also noted that all the
pea plants produces from the hybrid tall parents of F2 generation,
were either tall or dwarf. There were no plants with intermediate height (or
medium height) in between the tall and dwarf plants. In this way, Mendel’s
experiment showed that the traits (like tallness and dwarfness) are inherited
independently.
Tall
plants: Dwarf plants=3:1
Dihybrid
inheritance involve the inheritance of two pairs of contrasting characteristics
( or contrasting traits)at the same time. The two pairs of contrasting
characteristics chosen by Mendel were shape and color of seeds: round-yellow
seeds, and wrinkled- green seeds.
Mendel
first crossed pure-bred pea plants having round- yellow seeds with pure- bred
pea plants having wrinkled green seeds and found that only round-yellow seeds
were produces in the first generation. No wrinkled – green seeds were obtains
in the F1 generation from this it was concluded that round shape and
yellow color of the seed were dominant traits over the wrinkled shape and green
color of the seeds.
When
the F1 generation pea plants having round yellow seeds were
cross-bred by self pollination, then four types of seeds having different
combinations of shape and color were obtained in second generation. These were
round-yellow, round- green, wrinkled- yellow and wrinkled- green seed. Mendel
collected a total of 556 F2 seeds and counted them shape wise and color
wise. He got the following result:
Round-
Yellow seeds-315
Round-green
seeds- 108
Wrinkled-
yellow seeds-101
Wrinkled-green
seeds-32
The
phenotypic ratio of different types of seed can be written as:
Round Round Wrinkled Wrinkled
Yellow: Green: Yellow : Green = 315 :108 :101 :32
Seeds Seeds Seeds Seeds
=
9 :3 :3 :1
Thus,
the ratio of each phenotype (or appearance) of the seeds in the F2
generation is 9:3:3:1. This known as the dihybrid ratio.
According
to Mendel’s second law of inheritance: in the inheritance of more than one pair
of traits in a cross simultaneously, the factors responsible for each pair of
traits are distributed independently to the gametes.
How
do traits get expressed?
Cellular
DNA has the blueprint(information) to make cellular proteins. A fragment of DNA
that gives information for synthesis of one protein is called a gene for that
protein.
All
biochemical reactions in the cell are controlled by protein/ enzyme. An enzyme
is necessary for protein production.
The
tallness or characteristics of the pea plant is controlled by hormone that
triggers the growth. The height/ tallness will depend upon the amount of the
particular hormone in the plant. The amount of the growth hormone will depend
on the efficiency of the enzymes making it.
If
the enzymes is working efficiently than the amount of growth hormone will be
more, resulting in tall plant. And if the enzyme is less efficient then the
amount will be less, resulting in short height plant. The gene controls the
efficiency of the enzyme. Thus, genes control the characteristics or traits.
SEX DETERMINATION
The process by which the sex of a
child is determined is called sex determination.
Genetic factors - genetics is
involved in the determination of sex of a person. This can be explained as
follows.
The chromosomes which determine the sex of
a person are called sex chromosomes. There are two types of sex
chromosomes, one is called X chromosome and the other is called Y chromosome.
A. male( man or
father) has one X chromosome and one Y chromosome. This means that half the male gametes or half the
sperms will have X chromosome and the other half will have Y chromosome.
A female (women or mother) has two
X chromosome (but no Y chromosome). This means that all the female gametes
called ova (or eggs) will have only X chromosome.
The sex of a child depends on what happens
at fertilization:
(a)
if a sperm carrying one chromosome fertilizes an
ovum (or egg) which carries X chromosome, then the child born will be a girl
(or female). This is because the child will have XX combination of sex
chromosome.
(b)
If a sperm carrying Y chromosome fertilizes an
ovum (or egg) which carries X chromosome, then the child born will be a boy (or
male). This is because the child will have XY combination of sex chromosomes.
Please
note that it is the sperm which
determines the sex of a child. This is because half of the sperms have X
chromosomes and the other half have Y chromosomes. Thus, there is 50% chance of
a boy and 50% chance of a girl being born to the parents. This is why the human
population is roughly half males and half females
Environmental factors –Environmental factors play an important role in sex determination.
1.In turtles Chrysema picta High
incubation temperature -- Results in
development of female.
2. In lizard Agama agama high
incubation temperature – Results in development of female.
Organic
Evolution
It is the gradual changes in the
structure and function of organism which may
Result in change in existing body
design of the organism or result in formation of new species.
Microevolution- Structure and
functions at small scale. Result in change in existing body design.
Macroevolution- Structure and function changes at a large scale
involving long period of time result in
formation of new species.
Factors/sources of evolution
1. Variations during sexual reproduction
2. Mutations
3. Natural selection
4. Genetic drift
Natural selection-the process by which nature selects organism with best
possible variations to survive is called as natural selection .it occurs
gradually and continuously in any population at any time.
Genetic drift- chance elimination of certain genes from a population due
to an accident or natural calamity is called genetic drift .it occurs in a
small population only .
Inherited trait
Acquired trait
1. The trait which a person inherit 1.The trait
which a person acquire
from his parents. from
environment.
2.Inherited traits involve DNA of a 2. They are not
incorporated
Person. in
the DNA of a person
Eg. Eye colour etc eg.
The son of the wrestler
not be a wrestler
Specition- A process of formation of new species from already existing one is called Speciation.
1. Existence of sub – populations which
are separated by a geographical barrier.
Hence restriction to genus flow.
2. Action of different forces of natural selection and gemetic dirift on sub-population ,in their separate ways.
3.
Accumulation of variations in the
sub-population over a period of time making incompatable and unable to interbreed
and therefore they form two different species.
Geographical
isolation:
Two sub-population of the same species which are separated by
some physical barrier like river ,mountain range etc. are unable to interbreed
,tough in the absence of physical barrier, interbreeding is possible. The
sub-populations are said to be geographical.
Evidence of
evolution
1.
Homologus organs (similar organs)- the organs of different organism
having similar origin but they perform different functions are called homologus
organs .these organs have same arrangement of bones ,similar blood vessels and
nerves . Eg. hands of man ,floppres of whale ,, forelimbs of frog
2.
Analogous organs- The organs that
differ in their origin and structural details but perform `the same
function are called analogous organs.
e.g. the wings of birds
and wings of insects are analogous organs.
3. Evidences from fossils- Fossils
Fossils are impressions/preserved traces of organisms that once lived on
earth. These impressions can be on different materials such as rocks
,gums/resins.
Organisms that die ,get buried in moist soil ,clay or any other material.
When the material hardens in due course of time ,the soft body parts decay. But
impressions of hard body parts such as bones ,are left on material. It gets
converted to fossil. Formation of fossils is called Fossilisation.
How Old is a Fossil?
The age of fossil is determined by:
1.The depth at which fossil is
found. The fossils found close to the earth surface are of recent origin
and the ones in deeper layers old.
2.Dating of the fossils. It is
done by the detection of different isotopes of the same element found in the
fossil material.
Evolution by Stages
Evolution is not all of by sudden. It takes through stages. For example
,from primitive to intermediate then to developed and then advanced stage. Like
development and adaptation of wings in animals to fly ,eyes also developed.
1.Eyes. Eyes ,the sense organs were
developed to’identify’. Some animals have rudimentary eyes just to detect
light. Some have identification of objects ,in insects and image formation in
vertebrates like man.
You know that flatworm has rudimentary eyes insects have compound eyes
,octopus has eyes ,structure of eye is different in each case. They have
separate evolutionary origins.
2.Feathers on body. Feathers o body appeared for
different purposes but later were used for other function also.
Initially ,it might have started for providing insulation in cold weather
,such as in some dinosaurs. But later ,it was used for flight by birds.
Dinosaurs did not use it for flying. This means that bird are closely related
to reptiles. Since dinosaurs were reptiles.
3.Descendants of wild cabbage. Over two thousand years farmers have
cultivated wild cabbage as a source of food plant. They produced different
vegetables from it by selection as we see today, six varieties of vegetables
were produced by this artificial selection.
Some farmers selected for swollen stem and formed kohlrabi.
Some selected for sterile flowers and formed cauliflower.
Some selected for arrested flowers and formed broccoli.
Some farmers cultivated for short distance between the leaves cabbage.
Some preferred for colored red leaves for forming red cabbage.
While some others grew it for large leafy type called kale.
We can also trace the evolutionary relationship by studying the changes
in DNA during the reproduction. Comparing the DNA of different species gives us
a direct estimate of how much the DNA has changed from its original form during
formation of new species. This method is now extensively used to define
evolutionary relationships.
Evolution should not be equated with
progress
Evolution as said is still in progress. The organisms observed today are
the descendants of past and due accumulation of past changes. It does not mean
that past organisms/ancestors were not good or the present ones are better. It
all depends on environmental selection for survival. Still today we have very
simple organisms like bacteria surviving in all habitats and inhospitable
habitats. Neither the human beings are pinnacle of evolution. They are just one
species of evolutionary life.
Human Evolution
Study of human evolution has been done by use of same tools for study of
evolutionary relationships.
1.
Excavation-digging
the soil.
2.
Time
dating.
3.
Study
of fossils.
4.
Study
of DNA sequences.
There is a great diversity of human beings and features across the world.
People used to talk about ‘races’. These races were identified on the basis of
skin color as – white, black, brown and yellow. Actually all are one species.
Regardless the places where humans lived for past thousand years they all
originated from Africa. Homo sapiens, the earliest human species lived there.
From here our ancestors spread in all directions. Though they kept going there
for mixing with each other. Some stayed there only and spread across Africa.
The other people migrated from Africa to West Asia, Central Asia, Eurasia,
South Asia and East Asia. They also travelled to Indonesia and Philippines to
Australia and America. Like all other species, the humans also evolved accidently
and were tried to live their lives the best as they could.
Thursday, 9 February 2017
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