Heredity and Evolution

  • Genetics deals with the study of Heredity and Variation.
  • The transmission of characters/traits from one generation to the next generation is called Heredity.
  • The differences in the characters/traits between the parent and offspring is called Variation.


Variation are of two types:
(i) Somatic Variation 
(ii) Gametic Variation 

(i) Somatic Variation

  • It takes place in the body cell.
  • It is neither inherited nor transmitted.
  • It is also known as acquired traits.
  • Examples: cutting of tails in dogs, boring of pinna etc.

(ii) Gametic Variation

  • Takes place in the gametes/Reproductive cells.
  • Inherited as well as transmitted.
  • Also known as inherited traits.
  • Example: human height, skin colour.


Variation occurs during reproduction whether organisms multiply sexually or asexually.


  • Variations are fewer.
  • Occurs due to small inaccuracies in DNA copying. (Mutation)


  •  Variations are large.
  • Occurs due to crossing over, separation of chromosomes, mutation.


  • Depending upon the nature of variations different individuals would have different kinds of advantage. Example, Bacteria that can withstand heat will survive better in a heat wave.
  • Main advantage of variation to species is that it increases the chances of its survival in a changing environment.
  • Free earlobes and attached earlobes are two variants found in human populations.


  • Gregor Johann Mendel (1822 & 1884) started his experiments on plant breeding and hybridisation. He proposed the laws of inheritance in living organisms.
  • Mendel was known as Father of Genetics.
  • Plant selected by Mendel: Pisum sativum (garden pea). He used a number of contrasting characters for garden pea.


  • He chose Garden Pea (Pisum sativum) as his experiment material because of:
  • Availability of detectable contrasting traits of several characters.
  • Short life span of the plant.
  • Normally allows self-fertilization but cross-fertilisation can also be carried out.
  • Large no. of seeds produced.
  • Mendel’s Experiments: Mendel conducted a series of experiments in which he crossed the pollinated plants to study one character (at a time).


  • Cross between two pea plants with one pair of contrasting characters is called a monohybrid cross.
    Example: Cross between a tall and a dwarf plant (short).
  • First-generation or F1 progeny are no ‘medium-height’ plants. All the plants were tall.
  • Second-generation or F2 are progeny (descendants) of the F1 tall plants are not all tall.
  • Both the tallness and shortness traits were inherited in the F1 plants, but only the tallness trait was expressed. Thus, two copies of the trait are inherited in each sexually reproducing organism.
  • These two may be identical or may be different depending on the parentage.

(a) Pure or homozygous condition:

(TT, tt) : Both are dominant traits, Both are recessive alleles

(b) Heterozygous condition (Hybrid):

  • Tt : One is dominant, one recessive trait
  • Phenotypic ratio → 3 : 1 (Three tall and one short)
  • Genotypic ratio → 1 : 2 : 1 (TT-one, Tt-two, tt-one)
  • Phenotype means Physical appearance either they are Tall or Short.
  • Genotype means Genetic makeup that is TT, Tt or tt.

Observations of Monohybrid Cross

(i) All F1 progeny were tall, no medium height plant. (Half way characteristic)
(ii) F2 progeny 1⁄4 were short, 3⁄4 were tall. 
(iii) Phenotypic ratio F2 – 3 : 1 (3 tall : 1 short) 


  • TT and Tt both are tall plants while tt is a short plant.
  • 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.
  • Characters/traits like ‘T’ are called dominant traits (because they express themselves) and ‘t’ are recessive traits (because they remain suppressed).


  • A cross between two plants having two pairs of contrasting characters is called a dihybrid cross.
  • Parent → Round green × Wrinkled yellow:
    (i) When RRyy was crossed with rrYY in F1 generation all were Rr Yy round and yellow seeds. 
    (ii) Self pollination of F1 plants gave parental phenotype and two mixtures (recombinants round yellow and wrinkled green) seeds plants in the ratio of 9 : 3 : 3 : 1. 


  • Round and yellow seeds are Dominant characters.
  • Occurrence of new phenotype combinations show that genes for round and yellow seeds are inherited independently of each other.


  • Cellular DNA is the information source for making proteins in the cell.
  • A section of DNA that provides information for one protein is called the gene for that protein.
  • Plant height can thus depend on the amount of a particular plant hormone.
  • The amount of the plant hormone made will depend on the efficiency of the process for making it.
  • Cellular DNA (Information source) → For synthesis of Proteins (Enzyme) → Works efficiently → More Hormone → produced Tallness of plant
  • Therefore, genes control characteristics/traits.


Determination of sex of an offspring is known as Sex Determination.


  •  Environmental: In some animals, the temperature at which the fertilized eggs are kept decides the gender. Example: Turtle
  • Genetic: In some animals like humans gender or individual is determined by a pair of chromosomes called sex chromosome.
    XX – Female XY – Male


  • In human beings, there are 23 pairs of chromosomes.
  •  Out of these 22 chromosomes pairs are called autosomes and the last pair of chromosome that help in deciding gender of that individual is called sex chromosome. XX – Female 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.


Evolution is the sequence of gradual changes which takes place in the primitive organisms, over millions of years, in which new species are produced.

Situation I (Group of Red and Green Beetles)

  • Colour variation arises during reproduction
  • All beetles red except one that is green
  • Crows feed on red beetle → No. of beetles reduces
  • One beetle green → Progeny beetles green → Crows could not feed on green beetles as they got camouflaged (hide) in green bushes 
  • Number of green bettles increases


  • 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 II (Group of Red and Blue Beetles)

  • Reproduction in group of red beetles
  • All beetles are red except one that is blue
  • Number of red beetles increases as they reproduces
  • One blue beetle reproduces and no. of blue beetles also increases
  • Crows can see both blue and red beetles and can eat the
  • Number reduces but still red beetles are more and blue ones are few
  • Suddenly elephant comes and stamps on the bushes
  • Now beetles left are mostly blue


  • Blue beetles did not have a survival advantage. Elephants suddenly caused major havoc in the 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.

Situation III (Group of Red Beetles and Bushes)

  • 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 beetles increases again


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



Speciation takes place when variation is combined with geographical isolation. 
(i) Gene flow: Occurs between populations that are partly but not completely separated. 
(ii) Genetic drift: It is the random change in the frequency of alleles (gene pair) in a population over successive generations. Genetic drift takes place due to:
(a) Severe changes in the DNA
(b) Change in number of chromosomes
(iii) Natural selection: The process by which nature selects and consolidates those organisms which are more suitable, adapted and possess favourable variations. 
(iv) Geographical isolation: It is caused by mountain ranges, rivers etc. Geographical isolation leads to reproductive isolation due to which there is no flow of genes between separated groups of population. 


  • Both evolution and classification are interlinked.
  • Classification of species is a reflection of their evolutionary relationship.
  • The more characteristic two species have in common the more closely they are related.
  • The more closely they are related, the more recently they have a common ancestor.
  • Similarities among organisms allow us to group them together and to study their characteristics.


(i) Homologous Organs (Morphological and anatomical evidence)

  • 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 (Running), Winds of bat (Flying), Paw of a cat (Walk/scratch/attack)
  • Same basic structural plan, but different functions perform.

(ii) Analogous Organs:

  •  These are the organs that have different origin and structural plan but same function.
  • Analogous organs provide mechanism for evolution. Example:
    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: (Paleontological evidences)

  • The remains and relics of dead organisms of the past.
  • They are preserved traces of living-organisms.
  • Fossil Archaeopteryx possess features of reptiles as well as birds. This suggests that birds have evolved from reptiles. Example:
    Ammonite: Fossil-invertebrate, Trilobite: Fossil-invertebrate, Knightia: Fossil-fish, Rajasaurus: Fossil-dinosaur skull
  • Age of the fossils:
    (a) Deeper the fossil, older it is.
    (b) Detecting the ratios of difference of the same element in the fossil material Radio- carbon dating [C-(14) dating]


Evolution takes place in stages i.e. bit by bit 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. (Enough to give fitness advantage)
  • Insects have compound eyes.
  • Humans have binocular eyes.

(ii) Functional Advantage

  • Evolution of Feathers: Feathers provide insulation in cold weather but later they might become useful for flight. Example:
    (i) Dinosaurs had feathers, but could not fly using feathers.
    (ii) 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 requirements throughout ages by using artificial selection. Example: From wild cabbage many varieties like broccoli, cauliflower, red cabbage, kale, cabbage and kohlrabi were obtained by artificial selection. Wheat (many varieties obtained due to artificial selection)

(iii) Molecular Phylogeny

  • It is based on the idea that changes in DNA during reproduction are the basic events in evolution.
  • Organisms which are most distantly related will accumulate greater differences in their DNA.


  • Excavating, Time dating, Fossils and Determination of DNA sequences are the tools to study Human evolutionary relationship.
  • Although there is great diversity of human forms all over the world, yet all humans are a single species.
  • All humans come from Africa. The earliest members of the human species, Homo sapiens, can be traced there. Our genetic footprints can be traced back to our African roots.
  • The residents spread across Africa, the migrants slowly spread across the planet from Africa to West Asia, then to Central Asia, Eurasia, South Asia, East Asia. They travelled down the islands of Indonesia and the Philippines to Australia, and they crossed the Bering land bridge to the Americas.
  • They did not go in a single line.
  • Sometimes came back to mix with each other.

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