Heredity and Evolution | Important Notes for Board Examination

Genetics deals with the study of heredity and the variation of inherited characteristics.  The father of genetics is Gregor Mendel, a late 19th-century scientist and Augustinian friar. Mendel studied “trait inheritance”, patterns in the way traits are handed down from parents to offspring. He observed that organisms (pea plants) inherit traits by way of discrete “units of inheritance”. This term, still used today, is a somewhat ambiguous definition of what is referred to as a gene.

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Accumulation of Variations during Reproduction

The variations appear during reproduction based on whether organisms multiply Asexually or Sexually

  1. Asexually

    a) Variations are fewer
    b) Occurs due to small in accuracies in DNA copying. (Mutation)

  2. Sexually

    a) Variations are large
    b) Occurs due to crossing over, separation of chromosomes, mutations

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Importance of Variation

(i) 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.
(ii) Main advantage of variation to species is that it increases the chances of its survival in a changing environment.
Free ear lobes and attached ear lobes are two variants found in human populations.

Mendel and His Work on Inheritance

  • 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). Mendel used a number of contrasting characters for garden pea.

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  • Medel’s Experimental Material : He chose Garden Pea (Pisum sativum) as his experiment material because of :
    (i) Availability of detectable contrasting traits of several characters.
    (ii) Short life span of the plant.
    (iii) Normally allows self-fertilisation but cross-fertilisation can also be carried out.
    (iv) 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).

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 dwarf plant (short).

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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)

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).

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Dihybrid Cross

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

Phenotypic Ratio

Round, yellow : 9
Round, green : 3
Wrinkled, yellow : 3
Wrinkled, green : 1


(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.


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

How do these traits get expressed

Cellular DNA (Information source)
↓ For synthesis of
Proteins (Enzyme)
↓ Works efficiently
More Hormone
↓ produced
Tallness of plant

Therefore, genes control characteristics/traits.

Sex Determination

Factors responsible for Sex Determination are:

  • Environmental : In some animals, the temperature at which the fertilized eggs are kept decides the gender. E.g., in turtle
  • Genetic :In some animals like humans gender or individual is determined by a pair of chromosomes called sex chromosome.

XX – Female
XY – Male

Sex Chromosomes : In human beings, there are 23 pairs of chromosome. 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

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

Acquired and Inherited Traits

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

Ways by which speciation take place

Speciation takes place when variation is combined with geographical isolation.

1. Gene flow : Occurs between population that are partly but not completely separated.
2. Genetic drift : It is the random change in the frequency of alleles (gene pair) in a population over successive generations.
3. Natural selection : The process by which nature selects and consolidate those organisms which are more suitable adapted and possesses favourable variations.
4. 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.

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 for 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 (Running), Winds of bat (Flying) and Paw of a cat (Walk/scratch/attack) has same base structure, but performs different functions
II. Analogous Organs : These are the organs that have different origin and structural plan but same function.
Example : Analogous organs provide mechanism for evolution. Wings of Bat has elongated fingers with skin folds and wing of birds has feathery covering along the arms has different basic structure but performs similar functions.
III. Fossils : (Paleontological evidences)
The remains and relics of dead organisms of the past.


Fossils are preserved traces of living organisms. 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 RA-
JASAURUS – Fossil-dinosaur skull







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. E.g.,
(i) From wild cabbage many varieties like broccoli, cauliflower, red cab bage, kale, cabbage and kohlrabi were obtained by artificial selection.
(ii) Wheat (many varieties obtained due to artificial selection).

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.

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