Heredity and Evolution
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1. The transmission of characters from parent to their off springs is known as heredity.
The study of heredity and variations is known as genetics.
♦ Clones are those organisms which are the carbon copies of one another.
♦ Variation in sexually reproducing organisms are caused due to the following factors like
environment, crossing over and recombination of genes and mutation.
♦ The first study of inheritance was done by Gregor Mendel on garden pea.
♦ Paired condition of chromosomes is known as diploid.
♦ Unpaired condition of chromosomes is known as haploid.
♦ DNA (Deoxyribo Nucleic Acid), RNA (Ribo Nucleic Acid) is the genetic material in
2. Mendel’s laws of inheritance are
(i) Law of Dominance
(ii) Law of Segregation (Law of purjty of gametes)
(iii) Law of Independent Assortment
3. Genotype is the composition of genes present in an organism and the characteristic which
is visible in an organism is called its phenotype.
4. When two parents cross (or breed) to produce progeny (or offsprings), then their progeny
is called F1-generation (First Filial Generation) and when the first generation progeny
cross among themselves to prod
or second Filial Generation.
♦ Mendel conducted his famous experiments on garden pea (Pisum sativum).
♦ He used a number of contrasting characters like round / wrinkled seeds, tall/ short
plants, white/ violet flowers and so on.
5. During Monohybrid Cross
♦ When tall pea plants are crossed with short pea plants then in Fi generation only tall
plants were obtained.
♦ F2 progeny ofFi tall plants are not all tall but one quarter of them are short indicating
that both tallness and shortness traits were inherited in F1 but only tallness trait was
expressed due to dominance.
♦ In dihybrid cross two pairs of contrasting characters were considered. Tall plant with
round seeds were crossed with short plant with wrinkled seeds. In Fi tall plants with
round seeds were obtained. On selfing these F, plants with F2 produced tall plants with
round seeds, short plant with wrinkled seeds and some new combinations (tall plant }
with wrinkled seeds and short plant with rounds seeds) were also obtained. The
tall/short trait and round wrinkled traits are independently inherited.
♦ The expression of a particular trait is controlled by gene.
6. DNA is the source of making protein in a cell.
The section of DNA that provides information for one protein is called gene.
7. Physical and Chemical Basis of Heredity
Mendel (1866) said that heredity was controlled by particles, called germinal units, or factors.
8 Sex determination is the process by which the sex of a person is determined.
All human chromosomes are not paired. 22 pairs are called autosomes. Women have a perfect
pair of sex chromosomes XX. But men have a mismatched pair XY.
It is the sequence of gradual changes which take place in the primitive organisms over
millions of years in which new species are produced
A. The evidences of evolution are :
i. Homologous organs,
ii. Analogous organs, and
B. Theories of Evolution
Jean Baptiste Lamarck gave the first theory of evolution.
Darwin’s Theory of Evolution
Charles Robert Darwin (1809-1882) explained the evolutionary principle in his famous book
“The origin of species”. The theory proposed by him is popularly known as theory of natural
selection or Darwinsim.
The main features of the theory of natural selection are as follows:
(i) Over production
(ii) Limited food and space
(iii) Struggle for Existence
(v) Natural Selection or Survival of the Fittest
10. Speciation : The process by which new species develop from the existing species is known
The factors which leads to speciation are :
♦ Geographical isolation
♦ Genetic drift and
Evolutions are of three types :-
(i) Convergent Evolution
(ii) Divergent Evolution
(iii) Parallel Evolution.
12. Fossils : The remains of dead plants or animals that lived in the remote past are known as
Various kinds of fossils are : Ammonite, Trilobite and Dinosaur.
13. Evolution by Stages : Evolution of complex organs have taken place bit-by-bit
For example eye, feathers of birds have evolved because of survival advantage of intermediate
Thus changes in DNA during reproduction are the main cause of evolution.
14. Human Evolution : All have beings belong to single speceis Homo sapiens, although
there were many races of humans.
They have originated in Africa, some ancestors left Africa and migrated to West Asia,
Central Asia, Eurasia South Asia, East Asia, Indonesia, Australia, America, while others stayed
Excavating, time-dating, studying fossils, determining DNA Sequences have been used for
studying human evolution.