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Heredity and Evolution


  • Genetics deal with study of Hereditary and Variation.

  • The transmission of traits/character from one generation to next generation is called Hereditary.

  • The difference in characters/traits between parents and offspring is called Variation.

Accumulation of Variation during reproduction

Variations appearing during reproduction depends whether organisms multiply sexually or asexually.

If multiply asexually

If multiply sexually

- Variations are fewer.

- Occurs due to small inaccuracies in DNA copying. (Mutation)

- Variations are large.

- Occurs due to crossing over, separation of chromosomes, mutation.

Importance of Variation:

  • Depending upon the nature of variations different individuals would have different kinds of advantages.

Examples: Bacteria that can withstand heat will survive better in heat wave.

  • Main advantage of variation to species is that it increases the chances of its survival in a changing environment.

- Free ear lobe and attached ear lobe are two variants found in human populations.

Some important terms


Chromosomes are long thread-like structures present in the nucleus of a cell which contain hereditary information of the cell in the form of genes.


DNA is a chemical in the chromosome which carries the traits in a coded form.


Gene is the part of a chromosome which controls a specific biological function.

Dominant trait:

The character which expresses itself in a (Ft) generation is dominant trait. Example: Tallness is a dominant character in pea plant.

Recessive trait:

The character which does not express itself but is present in a generation is recessive trait. Ex. dwarfism in the pea plant.

Contrasting characters:

A pair of visible characters such as tall and dwarf, white and violet flowers, round and wrinkled seeds, green and yellow seeds etc.


A condition in which both the genes of same type are present for example; an organism has both the genes for tallness it is expressed as TT and genes for dwarfness are written as tt.


A condition in which both the genes are of different types for example; an organism has genes Tt it means it has a gene for tallness and the other for dwarfness only tall character is expressed.


It is genetic make-up of an individual for example; A pure tall plant is expressed as TT and hybrid tall as Tt.


It is external appearance of the organism for example; a plant having Tt composition will appear tall although it has gene for dwarfness.

Homologous pair:

Homologous pair of characters are those in which one member is contributed by the father and the other member by the mother and both have genes for the same character at the same position.

Mendel and his experiments on Inheritances

- Gregor Johann Mendel, started his experiment started his experiments on plant breeding and hybridisation. He proposed the laws of inheritance in living organism.

- Mendel is known as Father of Genetics.

- Plants selected: Pisum sativum (Garden pea). He used number of contrasting characters for garden pea plant.

Character trait

Dominant trait

Recessive trait

Flower colour



Flower position



Seed colour



Seed shape



Pod shape



Pod colour



Height of plant


Dwarf / Short

Why Mendel chose pea plants?

Features of pants: availability of detectable traits of several characters.

short life-span of plant.

normally allows self-fertilization but cross-fertilization is carried out.

Large number of seed produced.

Monohybrid Cross:

Cross between two pea plant with one pair of contrasting character is called monohybrid cross.

Example: Cross between a tall (TT) and dwarf (short) plant (tt).

  • First generation or F1 progeny are no ‘medium-height’ plant, all plants were tall.

  • Second generation or F2 progeny are descendant of F1 tall plants. All plants are not tall.

  • Phenotypic ratio – 3:1 (three tall and on short)

  • Genotypic ratio – 1:2:1 (one TT: two Tt: one tt)


  1. All F1 progeny were tall, no medium height plants.

  2. In F2 progeny ¼ were short and ¾ were tall.


  1. TT and Tt both are tall plants while tt is short plants. TT and tt are homozygous traits while Tt are heterozygous trait.

  2. A single copy of T is enough to make plant tall while both copies must be tt for the plan to be short.

  3. Character/trait (allele) like T is dominant trait while t is recessive trait.

Another example for monohybrid cross: Cross between purple flower and white flower.

- Parents with dominant trait AA, purple flower cross-fertilized with recessive traits aa, white flower. The F1 progeny, all plants bear purple flowers (Aa).

- In F1 progeny, self-fertilization is carried with trait Aa, purple flower.

- The F2 progeny obtained with

Phenotypic ratio - 3:1 (purple flower): (white flower)

Genotypic flower – 1:2:1 (AA, purple flower) : (Aa, purple flower) : (aa, white flower).

Dihybrid Cross:

A cross between two plants having two pair of contrasting characters is called dihybrid cross.

Example: Parent – Round Green and Wrinkled Yellow seed.

Phenotypic ratio:

- Round, Yellow – 9

- Round, Green – 3

- Wrinkled, Yellow – 3

- Wrinkled, Green – 1



  1. When RRyy was crossed with rrYY, in F1 generation all were RrYy round and yellow seed.

  2. Self-pollination in F1 plants gave parental phenotype and two mixture (recombinant round yellow and wrinkled green) seeds plants in the ratio 9:3:3:1.


  1. Round and yellow seeds are dominant traits/characters.

  2. Occurrence of new phenotype combinations shows that genes for round and yellow seeds are inherited independently of each other.

How do these Traits get expressed?

o Cellular DNA (information source) - a section of DNA, Gene – for synthesis of protein (enzymes) – works efficiently – hormone produced – produced tallness of plant.

The amount of plant hormone made will depend on the efficiency of the process for making it.

So, Genes controls traits/characters.

Sex Determination

Determination of sex of offspring is known as Sex determination.


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




Group red and green beetles

  • All beetles red except one green.

  • Crows feed on red beetles.

  • No. of beetles reduces.

  • One green beetle-progeny green beetle-crows do not feed on green beetles as they are not able to see green beetle in bushes.

  • Number of green beetle increase.

  • Green beetles got survival advantage as they were not visible in the green bushes.

  • This natural selection is exerted by crows resulting in the adaption of beetles to fit in the environment.

Group red and blue beetles

  • Reproduction in group of red beetles.

  • Number of red beetles increase.

  • All are red except one blue.

  • One blue beetle reproduces and the no of blue beetles also increases.

  • Crow can see both red and blue beetles and so can feed on both.

  • Number reduces, but still red ones are more and blue are few.

  • Suddenly elephant stamps on bush and where few beetles are left and that too blue.

  • Blue beetle did not get survival advantage, elephant suddenly, caused a major havoc.

  • These accidents can change frequency of genes even if they do not get survival advantages. This is called Genetic Drift.

Group of red beetles and bushes

  • Group of red beetles- habitat (bushes) suffer from plant diseases.

  • Average weight of beetle reduces due to poor nourishment.

  • Number of beetles keep on reducing.

  • Later plant disease gets eliminated.

  • Number and average weight of beetle increase again.

  • No genetic change has occurred in the population of beetle.

  • The population gets affected for short duration only due to environmental changes.

Acquired and Inherited Traits

Acquired traits

Inherited traits

These are the traits which are developed in an individual due to special conditions.

These are traits which are passed from generation to next generation.

They cannot be transferred to the progeny.

They can be transferred to progeny.

They cannot direct evolution

Examples: low weight of starving beetles.

They are helpful in studying evolution

Example: colour of eyes and hair.


Speciation occurs in following different ways:

a) Gene Flow: Occurs between population that are partly but not completely separated.

b) Genetic Drift: It is the random change in the frequency of alleles (gene pair) in a population over successive generations.

c) Natural Selection: The process by which nature selects and consolidates those organisms which are most suitably adapted and possesses favorable variations.

d) Geographical isolation: It is caused by mountains, rivers, etc. It leads to reproductive isolation due to which there is no flow of genes between separated groups of populations.

Evolution and Classification

Evolution and classification are interlinked

- Classification of species is reflection of their evolutionary relationship.

- The more characteristic two species have on common more closely they are related and more recent ancestor they have.

- Similarities among organism allow us to group them together and study their characteristics.

Evidences for Evolution

1. Homologous Organs:

These are the organs that have same basic structural plan and origin but different functions.

They provide evidences for evolution indicating that they are derived from the same ancestors.

Examples: limb of frog, wings of bird, hands of human, forelimb of lizard.

2. Analogous Organs:

These are the organs that have different origin and structural plans but same functions.


Wings of bat – elongated fingers with skin fold.

Wings of birds - feathery covering along the arms.

Different basic structure but form similar functions i.e., Flight

It provides for mechanism for evolution.

3. Fossils:

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 bird have evolved from reptile.

Age of Fossil:

- Deeper the fossil, older it is.

- Detecting the ratio of differences of the same element in the fossil material is Radio-carbon Dating (C14 dating).

Evolution by Stages

- Evolution takes place in stage i.e., bit by bit generation

- Evolution is simply the generation of diversity and the shaping of the diversity by environmental selection.

- Humans have been the most powerful agent in modifying wild species to suit their own requirement throughout the ages by using artificial selection.

- From wild cabbages many varieties like broccoli, cauliflower, red cabbage, kale, cabbage and kohlrabi were obtained by artificial selection.

Molecular phylogeny

- It is based on the idea that organisms which are more distantly related will accumulate a greater number of differences in their DNA.

- Organism which are most distantly related will accumulate greater difference in their DNA

Human Evolution

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

  • They do not do in a single line

  • They went forward and backward

  • Moved in and out of Africa

  • Sometimes came back to mix with each other


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