Online Notes on Chapter=9 ( Heredity and Evolution) Part 1

This is Online Notes on Chapter=9 ( Heredity and Evolution)  Part 1 for preparation of CBSE BOARD Examination, NTSE etc.

Accumulation of Variation during Reproduction :

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.

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).

Observations of Monohybrid Cross:

(i) All F1 progeny were tall, no medium height plant. (Half way
characteristic)

(ii) F2 progeny ¼ were short, ¾ were tall.

(iii) Phenotypic ratio F2 – 3 : 1 (3 tall : 1 short)

Genotypic ratio F2 – 1 : 2 : 1

TT : Tt : tt = 1 : 2 : 1

Conclusions:

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).

Share this

AEI Online Tutor