Sunday, November 4, 2018

CBSE E-Notes-Heredity and evolution - X Biology

Class 10 Heredity and Evolution – Gist of lesson
Genetics : Branch of science that deals with Heredity and variation.
Heredity : It means the transmission of features / characters/ traits from one generation to the next generation.
Variation : The differences among the individuals of a species/population are called variations.
Mendel and His Work on Inheritance
Gregor Johann Mendel started his experiments on plant breeding and hybridization. He proposed the laws of inheritance in living organisms. Mendel was known as Father of Genetics
Plant selected by Mendel : Pisum sativum (garden pea). Mendel used a number of contrasting characters for garden pea.
Following are the seven pairs of contrasting characters in Garden Pea
Flower colour
Flower position
Seed colour
Seed shape
Pod shape
Pod colour
Height of plant   

Mendels Experiments : Mendel conducted a series of experiments in which he crossed the pollinated plants to study one character (at a time)

Monohybrid Cross : Cross between two pea plants with one pair of contrasting characters is called a monohybrid cross. Example : Cross between a tall and a draft plant (short).
Observations of Monohybrid Cross
1. All F1 progeny were tall (no medium height plant (half way characteristic)
2. F2 progeny ¼ were short, ¾ were tall
3. Phenotypic ratio F2 – 3:1 (3 tall : 1 short)
Genotypic ratio F2 – 1 : 2 :1  = TT : Tt : tt
1. TT and Tt both are tall plants while tt is a short plant.
2. A single copy of T is enough to make the plant tall, while both copies have to be ‘t’ for the plant to be short.
3. Characters/Traits like 'T' are called dominant trait (because it express itself) and ‘t’ are recessive trait (because it remains suppressed)
Dihybrid Cross : A cross macle between two plants having two pairs of contrasting characters is called dihybrid cross.

Phenotypic Ratio
1. When RRyy was crossed with rrYY in F1 generation all were Rr Yy round and yellow seeds.
2. Self pollination of F1 plants gave parental phenotype and two mixtures (recombinants round yellow & wrinkled green) seeds plants in the ratio of 9:3:3:1
1. Round and yellow seeds are DOMINANT characters
2. Occurrence of new phenotypic combinations show that genes for round and yellow seeds are inherited independently of each other
SEX DETERMINATION : Phenomenon of decision or determination of sex of an offspring
FACTORS Responsible for Sex Determination
1. Environmental : In some animals the temperature at which the fertilised eggs are kept decides the gender. eg. in Turtle                                                                                  2. Genetic : In some animals like humans gender or individual is determined by a pair of chromosome called sex chromosome  XX – Female and XY – Male
This shows that half the children will be boys and half will be girls. All children will inherit an X chromosome from their mother regardless whether they are boys or girls. Thus sex of children will be determined by what they inherit from their father, and not from their mother.
Evaluation is the sequence of gradual changes which takes place in the primitive organisms, over millions of years, in which new species are produced.

Group of red beetles

Colour variation arises during reproduction

All beetles red except one that is green
One beetle Green Reproduction
Crows feed on red beetle
Progeny beetles green
No. of beetles reduces
Crow could not feed on green beetles as they got camouflaged in green bushes

Number of green beetles increases

Situation 1 : Green beetles got the survival advantage or they were naturally selected as they were not visible in green bushes. This natural selection is exerted by crows resulting in adaptations in the beetles to fit better in their environment  
Situation 2 : Blue beetles did not get survivals advantage. Elephant suddenly caused major havoc in beetle population otherwise their number would have been considerably large.
From this we can conclude that accidents can change the frequency of some genes even if they do not get survival advantage: This is called genetic drift and it leads to variation.
Characters or traits of an organism are controlled by the genes
A Section of DNA (cellular)


   Provides information

  For synthesis of Proteins

 Proteins controls a character
Example :
Gene T
responsible for synthesis of efficient enzyme (Protein)
More production
of growth hormone

Results in Tall Plants

Gene t
responsible for synthesis of less  efficient enzyme

Less production
of growth hormone

Results in short Plants

Genetic drift. It leads to diversity without any adaptation
Group of red beetles
Habitat of beetles (bushes)
Suffer from plant disease
Average weight of beetles
decreases due to poor nourishment
Number of beetles kept on reducing
Later plant disease gets eliminated
Number and average weight of the beetles increases again

Situation 3 : No genetic change has occurred in the population of beetle. The population gets affected for a short duration only due to environmental changes.

Acquired Traits
Inherited Traits
1. These are the traits which are developed in an individual due to special conditions
1. These are the traits which are passed from one generation to the next.
2. They cannot be transferred to the progeny
2. They get transferred to the progeny.
3. They cannot direct evolution eg. Low weight of starving beetles.
3. They are helpful in evolution. eg. Colour of eyes and hair

Micro evolution : It is the evolution which is on a small scale. eg. change in body colour of beetles.
The process by which new species develop from the existing species is known as speciation.
Speciation : it is the process of formation of new species. 
Species : A group of similar individuals within a population that can interbreed and produce fertile offspring.
Geneflow : It is exchange of genetic material by interbreeding between populations of same species or individuals
Speciation takes place when variation is combined with geographical isolation.
Gene flow : occurs between population that are partly but not completely separated
It is the random change in the frequency of alleles (gene pair) in a population over successive generations.
*Natural Selection : The process by which nature selects and consolidate those organisms which are more suitably adapted and possesses favorable variations 
Genetic drift takes place due to 
(a) Severe changes in the DNA  (b) Change in number of chromosomes
Evolution and classification
Both evolution and classification are interlinked.
1. Classification of species is reflection of their evolutionary relationship.
2. The more characteristic two species have in common the more closely they are related.
3. The more closely they are related, the more recently they have a common ancestor.

4. Similarities among organisms allow us to group them together and to study their characteristic.
(Evidences of Evolution)
I. Homologous Organs : (Morphological and anatomical evidences. These are the organs that have same basic structural plan and origin but different functions.
Homologous organs provides evidence for evolution by telling us that they are derived from the same ancestor.
Example :
Forelimb of Horse
Same basic structural but different functions perform.
Winds of bat
(flying) plan,
Paw of a cat
II. Analogous Organs : These are the organs that have different origin and structural plan but same function example :
Example : Analogous organs provide mechanism for evolution.
Wings of bat       elongated fingers with skin folds

Wings of bird     Feathery covering along the arm

Different basic structure, but perform similar function i.e., flight

III. Fossils : (Palaeontological evidences) : The remains and relics of dead organisms of the past.

Fossil Archaeopteryx possess features of reptiles as well as birds. This suggests that birds have evolved from reptiles. Examples of Fossils

AMMONITE - Fossil-invertebrate                
TRILOBITE - Fossil-invertebrate
KNIGHTIA - Fossil-fish                                 
RAJASAURUS - Fossil dinosaur skull


I. Deeper the fossil, older it is.                                                                                                           
II. Detecting the ratios of difference of the same element in the fossil material i.e. Radio-carbon dating [C-(14) dating)
Evolution by stages : Evolution takes place in stages ie bit by bit over generations.
I. Fitness advantage
Evolution of Eyes
Evolution of complex organs is not sudden it occurs due to minor changes in DNA, however takes place bit by bit over generations.

Flat worm has rudimentary eyes
Insects have compound eyes                 enough to give fitness advantage
Humans have binocular eyes

II. Functional Advantage
Evolutions of feathers :
Feathers provide insulation in cold weather but later they might become useful for flight.
Example : Dinosaurs had feathers, but could not fly using feathers. Birds seem to have later adapted the feathers to flight.
Evolution by Artificial Selection :
Humans have been a powerful agent in modifying wild species to suit their own requirement throughout ages by using artificial selection. eg
(i) From wild cabbage many varieties like broccoli, cauliflower, red cabbage, kale, cabbage and kohlrabi were obtained by artificial selection.
(ii) Wheat (many varieties obtained due to artificial selection).
Molecular Phylogeny
Þ  It is based on the idea that changes in DNA during reproduction are the basic events in evolution

Þ Organisms which are more distantly related will accumulate greater differences in their DNA

                                         HUMAN EVOLUTION

Tools to Study Human Evolutionary Relationship
Excavating         Time dating        Fossils    Determining DNA Sequences

Although there is great diversity of human forms all over the world get all humans are a single species.

Chapter - 9: Heredity and Evolution
Assignment  for practice before  exam
CBSE Notes-Heredity & evolution
Point to remember- To revise before exam
NCERT Solutions Heredity & evolution
CBSE Notes-Heredity & evolution
Heredity and Evolution Questions for practice
Heredity and Evolution  Guess questions 
Heredity and Evolution  Notes
Heredity and Evolution[Solved Questions]-1
Heredity and Evolution[Solved Questions]-2
Heredity and Evolution[Solved Questions]-3

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