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Heredity and Evolution

heredity and evolution

Heredity and Evolution:

Earlier we have studied about reproduction. Also, we saw the importance of variations to keep the existence of species on this earth. In this article we will understand the mechanisms by which variations are created or inherited.


  • The process of inheritance of traits (characters or features) from parents to their children is known as heredity.
  • Fertilized egg (zygote) contains the information by which continuity of features from the prior generation to the next is maintained.
  • A child gets practically equal amounts of genetic material from both the father and the mother.
  • Each trait present in child has two versionse. one from each parent.

Mendel’s experiment:

  • Gregor Johann Mendel also known as father of genetics has done his experiments on plants breeding and hybridization.
  • Mendel selected pea plants to conduct his experiment.
  • He conducted a series of experiments to study each one’s character.

Mendel chose pea plants to conduct his experiment because:

  • In pea plants contrasting traits of various character can easily be identified.
  • Lifespan of pea is very short, two generation of plants can be grown in a given year.
  • A large number of seeds are produced, so he can study about characteristics of each seed broadly.
  • Normally peas show self-fertilization but cross-fertilization is also carried out.

Monohybrid cross:

  • The cross between two plants having a pair of contrasting characters is monohybrid cross. As cross between a tall and a dwarf pea plant.
  • All plants obtained in the ‘F1’ progeny (first-generation) were tall. There were no medium sized plants.
  • All plants in F2 progeny (second-generation) of the cross between ‘F1 progeny tall plants are not all tall.
  • Both tallness and shortness traits were inherited in F1 progeny but only tallness were expressed.
  • Two copies of traits (characters) are inherited in organisms having sexual mode of reproduction. These pair of contrasting characters are called allele.
  • Depending upon parents these two traits may be similar or different.

In pea plants, Mendel expressed the traits as homozygous condition (pure) and heterozygous condition (Hybrid)

  • Homozygous condition: In (TT) both are dominant traits and in (tt) both are recessive alleles.
  • Heterozygous condition: In (Tt) one is dominant and other is recessive traits.

Phenotype and genotype:


  • It means their physical appearance either they are tall or short. Simply observable features are phenotype.
  • Phenotypic ratio in Mendel’s experiment – 3 : 1 (three tall and one short).


  • The complete set of genes in an organisms genome say, they show genetic makeup that are TT, Tt, tt are known as genotype.
  • Phenotypic ratio in Mendel’s experiment – 1 : 2 : 1 (TT – one , Tt – two, tt – one).

Conclusion of Mendel’s experiment:

  • TT and Tt are both tall plants while tt is short plant.
  • A single copy of ‘T’ is enough to make the plant tall on the other hand both copy of ‘t’ is required to make it short plant.
  • Characters or traits like ‘T’ are called dominant trait as it expresses itself.
  • Characters or traits like ‘t’ are known as recessive traits as it remains suppressed.

Dihybrid cross:

  • The cross between two plants in which two pairs of contrasting characters are studied is called dihybrid cross.
  • Round green X Wrinkled yellow, as here two characters are observed one is shape and other is colour in same experiment.
  • He observed that when RRyy was crossed with rrYY, in F1 generation all were Rr Yy round and yellow seeds.
  • Observed phenotypic ratio (Round, Yellow) – 9 : (Round, Green) – 3 : (wrinkled yellow) – 3 : (Wrinkled, Green) – 1.


  • Round and yellow seeds are dominant
  • Genes for different characters are inherited independently to each other that give rise to new phenotypic conditions.

Expression of these traits:

  • Inside the cell, It is cellular DNA which provides information for making proteins.
  • Gene of the protein is the section of DNA which provides information about that protein.
  • The Traits of a particular organism depend upon the amount of particular hormones present.
  • The amount of hormones produced will depend upon the efficiency of the process of production.
  • Gene has the main role in making these hormones, If there is any alteration in gene then there will be variation in traits.
  • In human beings, the gene for each trait is present as separate independent pieces which are called a chromosomes.
  • Each cell have two copies of each chromosome, one each from the male and female parents.

Sex Determination:

  • To know about the gender or sex of the offspring is said to be sex determination.
  • There are several factors which are responsible for sex determination, but two main factors are mainly seen, One is environmental factor and the other one is Genetic factor.
  • In some animals, the temperature at which the fertilized eggs are kept decides the sex of their offspring. Turtle has the same mode.
  • In humans and many animals the sex or gender of offspring is determined by a pair of chromosomes. They are called sex chromosomes.

Sex determination in human beings:

  • Human beings have 23 pairs of chromosome.
  • In these 23 pairs of chromosomes, 22 pairs are called autosomes.
  • The last pair of chromosomes help in deciding the gender of an individual and this chromosome is sex chromosome.
  • The sex chromosome in female is ‘XX’ and that in male is ‘XY’.
  • All the offspring inherit ‘X’ chromosome from mother either children is boy or a girl.
  • The sex of children totally depends on the chromosomes what they inherit from their father, not what they get from their mother.


  • Evolution is simply the change inherited to the next generation from previous one in biological population over a long period of time (millions of years).
  • As we already know that life on this earth is evolved from a single unicellular organism.
  • Every organisms present on this earth is said to be evolved from single ancestor.
  • No genetic change can occur in a particular niche for a short period.
  • Only due to environmental change, the population gets affected.

Acquired and Inherited Traits:

Acquired Traits Inherited Traits
These traits are acquired
during lifetime mainly from
These traits are inherited
from generation and passed
to the next generation.
It doesn’t bring changes in
the DNA of germ cell.
Genes of inherited traits are
present in DNA of germ cell.
Example: Knowledge,
resistance to environmental
drawback etc.
Example: Eye colour, Face
colour, hair colour etc.


  • The process by which new species originates is said to be speciation.
  • A group of organisms which have similar characters, also they can breed among themselves are said to be species.
  • If two groups of the same species are anyway prevented from interbreeding for many generations then speciation can occur.
  • Thus speciation covers a major part in heredity and evolution.

Speciation takes place by following ways:

  1. Gene flow: It takes place between population that are partially but not completely separated.
  2. Genetic drift: It takes place due to either severe change in DNA or change in number of chromosomes.
  3. Natural selection: Nature selects the organisms best suited under adaptable conditions and favorable variations. As, nature selects the best.
  4. Geographical isolation: When a group of species are separated due to geographical barriers such as mountain, river etc. so that gene flow stops in separate groups of population.

Evolution and Classification:

  • The modern system of classification is based on evolutionary relationship also, they are called phylogenetic classification.
  • When two species have similar characteristics then they are also closely related. This leads better to the fact that both must have same ancestor.
  • These similarities among organisms allows us to group them together also helps in studying each group characteristics wholly.
  • The kingdom appears as highest taxa, while the species appears as the lowest taxa.
  • Heredity and evolution go hand in hand.

Evidence of Evolution:

Everyday archeologists find a new evidence that proves evolution has taken place on this earth. To make it easy for research as well as studies they have classified something:

Homologous Organs:

Organs having common design but serve different functions in different animals. Homologous organ provides evidence for evolution as we identify wheather they are derived from the same ancestor.

Example: Forelimbs of all tetrapods i.e. forelimb of horse (running), wings of bat (flying), paw of cat (attack) etc.

Analogous organ:

Organs which have different design but serve a common function in different animals are said to be analogous organs.

Example: Wings of birds and wings of bat are examples of a pair of analogous organs.


The preserved remains of animals or plants or other organisms from past (min. 10,000 years) are called fossils. Fossils gives idea about many extinct animals and also give vision of how the evolution could have taken place.

Example: Ammonite: Fossil and Trilobite – invertebrate.


Evolution by stages:

Everything on this earth took time to evolve stage by stage from simple to complex.

Let’s understand the evolution of eyes:

  • Planaria is the first animal which shows ‘eye’ like structure, but they can’t differentiate between two objects.
  • Insect’s eyes are compound eyes which are made up of thousands of optical surfaces.
  • In higher animals we see that they have simple eyes which are composed of a single lens, also most animals can’t differentiate between colours.
  • Now come to human eyes, they are most advanced, easily we can identify colours and also have depth perception.

Human evolution:

  • The scientific name of modern human beings is Homo-sapiens.
  • Scientists proved that Homo sapiens have evolved in Africa. They migrated from there towards north.
  • After being a great diversity of humans on this earth but till today humans are single species.
  • Excavation, time-dating, studying fossils, as well as determining DNA sequences are the tools that have been used for studying human evolution.
  • People moved forwards and backwards, with groups sometimes separating from each other, sometimes coming back to mix with each other, even moving in and out of Africa.
  • Thus heredity and evolution in human are in core study.
Also read,
Life processes
Human respiratory
Nutrition in animals
Control and coordination Human Eye and colourful
How do organisms
Control and coordination
Light – Reflection and Refraction
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