Page 135 - NCERT Science Class 10 English Medium

Gregor Johann Mendel (1822–1884)
Mendel was educated in a monastery and went on to study science and
mathematics at the University of Vienna. Failure in the examinations for a
teaching certificate did not suppress his zeal for scientific quest. He went
back to his monastery and started growing peas. Many others had studied
the inheritance of traits in peas and other organisms earlier, but Mendel
blended his knowledge of science and mathematics and was the first one
to keep count of individuals exhibiting a particular trait in each generation.
This helped him to arrive at the laws of inheritance.

Mendel used a number of contrasting visible characters of garden
peas – round/wrinkled seeds, tall/short plants, white/violet flowers and
so on. He took pea plants with different characteristics – a tall plant and
a short plant, produced progeny by crossing them, and calculated the
percentages of tall or short progeny.
In the first place, there were no halfway characteristics in this first-
generation, or F1 progeny – no ‘medium-height’ plants. All plants were
tall. This meant that only one of the parental traits
was seen, not some mixture of the two. So the next
question was, were the tall plants in the F1
generation exactly the same as the tall plants of the
parent generation? Mendelian experiments test this
by getting both the parental plants and these F1 tall
plants to reproduce by self-pollination. The progeny
of the parental plants are, of course, all tall. However,
the second-generation, or F2, progeny of the F1 tall
plants are not all tall. Instead, one quarter of them
are short. This indicates that both the tallness and
shortness traits were inherited in the F1 plants, but
only the tallness trait was expressed. This led Mendel
to propose that two copies of factor (now called genes)
controlling traits are present in sexually reproducing
organism. These two may be identical, or may be
different, depending on the parentage. A pattern of
inheritance can be worked out with this assumption,
as shown in Fig. 8.3.
Figure
Figure 8.3
Figure 8.3
Figure 8.38.3
Figure 8.3
Inheritance of traits
Activity
Activity 8.2
Activity
Activity 8.2
8.2
over two generations Activity 8.28.2
n In Fig. 8.3, what experiment would we do to confirm that the F2
generation did in fact have a 1:2:1 ratio of TT, Tt and tt trait
combinations?
In this explanation, both TT and Tt are tall plants, while only tt is a
short plant. In other words, 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. Traits
like ‘T’ are called dominant traits, while those that behave like ‘t’ are
called recessive traits. Work out which trait would be considered
dominant and which one recessive in Fig. 8.4.

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