Page 136 - NCERT Science Class 10 English Medium

What happens when pea plants showing two different
characteristics, rather than just one, are bred with each other?
What do the progeny of a tall plant with round seeds and a short
plant with wrinkled-seeds look like? They are all tall and have
round seeds. Tallness and round seeds are thus dominant traits.
But what happens when these F1 progeny are used to generate
F2 progeny by self-pollination? A Mendelian experiment will find
that some F2 progeny are tall plants with round seeds, and some
were short plants with wrinkled seeds. However, there would also
be some F2 progeny that showed new combinations. Some of them
would be tall, but have wrinkled seeds, while others would be short,
but have round seeds. You can see as to how new combinations of Figure 8.4
Figure 8.4
Figure 8.4
Figure 8.48.4
Figure
traits are formed in F2 offspring when factors controlling for seed
shape and seed colour recombine to form zygote leading to form x
F2 offspring (Fig. 8.5). Thus, the tall/short trait and the round RR yy rr YY
(round, green) (wrinkled, yellow)
seed/wrinkled seed trait are independently inherited.
Ry rY
8.2.3 How do these Traits get Expressed?
How does the mechanism of heredity work? Cellular DNA is F1
the information source for making proteins in the cell. A section Rr Yy
(round, yellow)
of DNA that provides information for one protein is called the
gene for that protein. How do proteins control the x
characteristics that we are discussing here? Let us take the Rr Yy Rr Yy
F1 F1
example of tallness as a characteristic. We know that plants
have hormones that can trigger growth. Plant height can thus RY Ry rY ry
depend on the amount of a particular plant hormone. The F2
amount of the plant hormone made will depend on the RY
efficiency of the process for making it. Consider now an enzyme RRYY RRYy RrYY RrYy
that is important for this process. If this enzyme works
Ry
efficiently, a lot of hormone will be made, and the plant will be
RRYy RRyy RrYy Rryy
tall. If the gene for that enzyme has an alteration that makes
rY
the enzyme less efficient, the amount of hormone will be less,
and the plant will be short. Thus, genes control characteristics, RrYY RrYy rrYY rrYy
or traits. ry
If the interpretations of Mendelian experiments we have been RrYy Rryy rrYy rryy
discussing are correct, then both parents must be contributing
equally to the DNA of the progeny during sexual reproduction. 315 round, yellow 9
We have disscussed this issue in the previous Chapter. If both 108 round, green 3
parents can help determine the trait in the progeny, both parents 101 wrinkled, yellow 3
must be contributing a copy of the same gene. This means that 32 wrinkled, green 1
each pea plant must have two sets of all genes, one inherited from
556 seeds 16
each parent. For this mechanism to work, each germ cell must
have only one gene set. Figure 9.5 Independent inheritance of two
separate traits, shape and colour of seeds
Figure 8.5
Figure 8.5
Figure
Figure 8.5
Figure 8.58.5
How do germ-cells make a single set of genes from the normal two Independent inheritance
copies that all other cells in the body have? If progeny plants inherited a of two separate traits,
single whole gene set from each parent, then the experiment explained shape and colour of seeds
in Fig. 8.5 cannot work. This is because the two characteristics ‘R’ and
‘y’ would then be linked to each other and cannot be independently
Heredity 131

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