NCERT Textbook - Heredity and Evolution Class 10 Notes | EduRev

Science Class 10

Class 10 : NCERT Textbook - Heredity and Evolution Class 10 Notes | EduRev

 Page 1


Science
142
Figure 9.1
Creation of diversity over succeeding
generations. The original organism at the top
will give rise to, say, two individuals, similar
in body design, but with subtle differences.
Each of them, in turn, will give rise to two
individuals in the next generation. Each of
the four individuals in the bottom row will be
different from each other . While some of these
differences will be unique, others will be
inherited from their respective parents, who
were different from each other.
Heredity and
Evolution
9 CHAPTER
W
e have seen that reproductive processes give rise to new individuals
that are similar, but subtly different. We have discussed how some
amount of variation is produced even during asexual reproduction. And
the number of successful variations are maximised by the process of
sexual reproduction. If we observe a field of sugarcane we find very little
variations among the individual plants. But in a number of animals
including human beings, which reproduce sexually, quite distinct
variations are visible among different individuals. In this chapter, we
shall be studying the mechanism by which variations are created and
inherited. The long-term consequences of the accumulation of variations
are also an interesting point to be considered. We shall be studying this
under evolution.
9.1 9.1 9.1 9.1 9.1 A A A A ACCUMUL CCUMUL CCUMUL CCUMUL CCUMULA A A A ATION OF V TION OF V TION OF V TION OF V TION OF VARIA ARIA ARIA ARIA ARIATION TION TION TION TION
DURING REPRODUCTION DURING REPRODUCTION DURING REPRODUCTION DURING REPRODUCTION DURING REPRODUCTION
Inheritance from the previous generation provides both
a common basic body design, and subtle changes in it,
for the next generation. Now think about what would
happen when this new generation, in its turn,
reproduces. The second generation will have differences
that they inherit from the first generation, as well as
newly created differences (Fig. 9.1).
Figure 9.1 would represent the situation if a single
individual reproduces, as happens in asexual
reproduction. If one bacterium divides, and then the
resultant two bacteria divide again, the four individual
bacteria generated would be very similar. There would
be only very minor differences between them, generated
due to small inaccuracies in DNA copying. However, if
sexual reproduction is involved, even greater diversity
will be generated, as we will see when we discuss the
rules of inheritance.
Do all these variations in a species have equal
chances of surviving in the environment in which they
find themselves? Obviously not. Depending on the
nature of variations, different individuals would have
2020-21
Page 2


Science
142
Figure 9.1
Creation of diversity over succeeding
generations. The original organism at the top
will give rise to, say, two individuals, similar
in body design, but with subtle differences.
Each of them, in turn, will give rise to two
individuals in the next generation. Each of
the four individuals in the bottom row will be
different from each other . While some of these
differences will be unique, others will be
inherited from their respective parents, who
were different from each other.
Heredity and
Evolution
9 CHAPTER
W
e have seen that reproductive processes give rise to new individuals
that are similar, but subtly different. We have discussed how some
amount of variation is produced even during asexual reproduction. And
the number of successful variations are maximised by the process of
sexual reproduction. If we observe a field of sugarcane we find very little
variations among the individual plants. But in a number of animals
including human beings, which reproduce sexually, quite distinct
variations are visible among different individuals. In this chapter, we
shall be studying the mechanism by which variations are created and
inherited. The long-term consequences of the accumulation of variations
are also an interesting point to be considered. We shall be studying this
under evolution.
9.1 9.1 9.1 9.1 9.1 A A A A ACCUMUL CCUMUL CCUMUL CCUMUL CCUMULA A A A ATION OF V TION OF V TION OF V TION OF V TION OF VARIA ARIA ARIA ARIA ARIATION TION TION TION TION
DURING REPRODUCTION DURING REPRODUCTION DURING REPRODUCTION DURING REPRODUCTION DURING REPRODUCTION
Inheritance from the previous generation provides both
a common basic body design, and subtle changes in it,
for the next generation. Now think about what would
happen when this new generation, in its turn,
reproduces. The second generation will have differences
that they inherit from the first generation, as well as
newly created differences (Fig. 9.1).
Figure 9.1 would represent the situation if a single
individual reproduces, as happens in asexual
reproduction. If one bacterium divides, and then the
resultant two bacteria divide again, the four individual
bacteria generated would be very similar. There would
be only very minor differences between them, generated
due to small inaccuracies in DNA copying. However, if
sexual reproduction is involved, even greater diversity
will be generated, as we will see when we discuss the
rules of inheritance.
Do all these variations in a species have equal
chances of surviving in the environment in which they
find themselves? Obviously not. Depending on the
nature of variations, different individuals would have
2020-21
Heredity and Evolution 143
9.2 HEREDITY 9.2 HEREDITY 9.2 HEREDITY 9.2 HEREDITY 9.2 HEREDITY
The most obvious outcome of the reproductive process still remains the
generation of individuals of similar design. The rules of heredity determine
the process by which traits and characteristics are reliably inherited. Let
us take a closer look at these rules.
9.2.1 Inherited Traits
What exactly do we mean by similarities and differences? We know that
a child bears all the basic features of a human being. However, it does
not look exactly like its parents, and human populations show a great
deal of variation.
QUESTIONS
?
1. If a trait A exists in 10% of a population of an asexually reproducing
species and a trait B exists in 60% of the same population, which trait
is likely to have arisen earlier?
2. How does the creation of variations in a species promote survival?
Activity 9.1 Activity 9.1 Activity 9.1 Activity 9.1 Activity 9.1
n Observe the ears of all the students in the class. Prepare a list of
students having free or attached earlobes and calculate the
percentage of students having each (Fig. 9.2). Find out about the
earlobes of the parents of each student in the class. Correlate the
earlobe type of each student with that of their parents. Based on
this evidence, suggest a possible rule for the inheritance of earlobe
types.
9.2.2 Rules for the Inheritance of Traits – – – – –
Mendel’s Contributions
The rules for inheritance of such traits in human beings are related to
the fact that both the father and the mother contribute practically equal
amounts of genetic material to the child. This means that each trait can
be influenced by both paternal and maternal DNA. Thus, for each trait
there will be two versions in each child. What will, then, the trait seen in
the child be? Mendel (see box) worked out the main rules of such
inheritance, and it is interesting to look at some of his experiments from
more than a century ago.
Figure 9.2 Figure 9.2 Figure 9.2 Figure 9.2 Figure 9.2
(a) Free and (b) attached
earlobes. The lowest part
of the ear, called the
earlobe, is closely attached
to the side of the head in
some of us, and not
in others. Free and
attached earlobes are two
variants found in human
populations.
different kinds of advantages. Bacteria that can withstand heat will survive
better in a heat wave, as we have discussed earlier. Selection of variants
by environmental factors forms the basis for evolutionary processes, as
we will discuss in later sections.
(a)
(b)
2020-21
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Science
142
Figure 9.1
Creation of diversity over succeeding
generations. The original organism at the top
will give rise to, say, two individuals, similar
in body design, but with subtle differences.
Each of them, in turn, will give rise to two
individuals in the next generation. Each of
the four individuals in the bottom row will be
different from each other . While some of these
differences will be unique, others will be
inherited from their respective parents, who
were different from each other.
Heredity and
Evolution
9 CHAPTER
W
e have seen that reproductive processes give rise to new individuals
that are similar, but subtly different. We have discussed how some
amount of variation is produced even during asexual reproduction. And
the number of successful variations are maximised by the process of
sexual reproduction. If we observe a field of sugarcane we find very little
variations among the individual plants. But in a number of animals
including human beings, which reproduce sexually, quite distinct
variations are visible among different individuals. In this chapter, we
shall be studying the mechanism by which variations are created and
inherited. The long-term consequences of the accumulation of variations
are also an interesting point to be considered. We shall be studying this
under evolution.
9.1 9.1 9.1 9.1 9.1 A A A A ACCUMUL CCUMUL CCUMUL CCUMUL CCUMULA A A A ATION OF V TION OF V TION OF V TION OF V TION OF VARIA ARIA ARIA ARIA ARIATION TION TION TION TION
DURING REPRODUCTION DURING REPRODUCTION DURING REPRODUCTION DURING REPRODUCTION DURING REPRODUCTION
Inheritance from the previous generation provides both
a common basic body design, and subtle changes in it,
for the next generation. Now think about what would
happen when this new generation, in its turn,
reproduces. The second generation will have differences
that they inherit from the first generation, as well as
newly created differences (Fig. 9.1).
Figure 9.1 would represent the situation if a single
individual reproduces, as happens in asexual
reproduction. If one bacterium divides, and then the
resultant two bacteria divide again, the four individual
bacteria generated would be very similar. There would
be only very minor differences between them, generated
due to small inaccuracies in DNA copying. However, if
sexual reproduction is involved, even greater diversity
will be generated, as we will see when we discuss the
rules of inheritance.
Do all these variations in a species have equal
chances of surviving in the environment in which they
find themselves? Obviously not. Depending on the
nature of variations, different individuals would have
2020-21
Heredity and Evolution 143
9.2 HEREDITY 9.2 HEREDITY 9.2 HEREDITY 9.2 HEREDITY 9.2 HEREDITY
The most obvious outcome of the reproductive process still remains the
generation of individuals of similar design. The rules of heredity determine
the process by which traits and characteristics are reliably inherited. Let
us take a closer look at these rules.
9.2.1 Inherited Traits
What exactly do we mean by similarities and differences? We know that
a child bears all the basic features of a human being. However, it does
not look exactly like its parents, and human populations show a great
deal of variation.
QUESTIONS
?
1. If a trait A exists in 10% of a population of an asexually reproducing
species and a trait B exists in 60% of the same population, which trait
is likely to have arisen earlier?
2. How does the creation of variations in a species promote survival?
Activity 9.1 Activity 9.1 Activity 9.1 Activity 9.1 Activity 9.1
n Observe the ears of all the students in the class. Prepare a list of
students having free or attached earlobes and calculate the
percentage of students having each (Fig. 9.2). Find out about the
earlobes of the parents of each student in the class. Correlate the
earlobe type of each student with that of their parents. Based on
this evidence, suggest a possible rule for the inheritance of earlobe
types.
9.2.2 Rules for the Inheritance of Traits – – – – –
Mendel’s Contributions
The rules for inheritance of such traits in human beings are related to
the fact that both the father and the mother contribute practically equal
amounts of genetic material to the child. This means that each trait can
be influenced by both paternal and maternal DNA. Thus, for each trait
there will be two versions in each child. What will, then, the trait seen in
the child be? Mendel (see box) worked out the main rules of such
inheritance, and it is interesting to look at some of his experiments from
more than a century ago.
Figure 9.2 Figure 9.2 Figure 9.2 Figure 9.2 Figure 9.2
(a) Free and (b) attached
earlobes. The lowest part
of the ear, called the
earlobe, is closely attached
to the side of the head in
some of us, and not
in others. Free and
attached earlobes are two
variants found in human
populations.
different kinds of advantages. Bacteria that can withstand heat will survive
better in a heat wave, as we have discussed earlier. Selection of variants
by environmental factors forms the basis for evolutionary processes, as
we will discuss in later sections.
(a)
(b)
2020-21
Science
144
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. 9.3.
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.
Figure 9.3 Figure 9.3 Figure 9.3 Figure 9.3 Figure 9.3
Inheritance of traits
over two generations
Activity 9.2 Activity 9.2 Activity 9.2 Activity 9.2 Activity 9.2
n In Fig. 9.3, 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. 9.4.
Page 4


Science
142
Figure 9.1
Creation of diversity over succeeding
generations. The original organism at the top
will give rise to, say, two individuals, similar
in body design, but with subtle differences.
Each of them, in turn, will give rise to two
individuals in the next generation. Each of
the four individuals in the bottom row will be
different from each other . While some of these
differences will be unique, others will be
inherited from their respective parents, who
were different from each other.
Heredity and
Evolution
9 CHAPTER
W
e have seen that reproductive processes give rise to new individuals
that are similar, but subtly different. We have discussed how some
amount of variation is produced even during asexual reproduction. And
the number of successful variations are maximised by the process of
sexual reproduction. If we observe a field of sugarcane we find very little
variations among the individual plants. But in a number of animals
including human beings, which reproduce sexually, quite distinct
variations are visible among different individuals. In this chapter, we
shall be studying the mechanism by which variations are created and
inherited. The long-term consequences of the accumulation of variations
are also an interesting point to be considered. We shall be studying this
under evolution.
9.1 9.1 9.1 9.1 9.1 A A A A ACCUMUL CCUMUL CCUMUL CCUMUL CCUMULA A A A ATION OF V TION OF V TION OF V TION OF V TION OF VARIA ARIA ARIA ARIA ARIATION TION TION TION TION
DURING REPRODUCTION DURING REPRODUCTION DURING REPRODUCTION DURING REPRODUCTION DURING REPRODUCTION
Inheritance from the previous generation provides both
a common basic body design, and subtle changes in it,
for the next generation. Now think about what would
happen when this new generation, in its turn,
reproduces. The second generation will have differences
that they inherit from the first generation, as well as
newly created differences (Fig. 9.1).
Figure 9.1 would represent the situation if a single
individual reproduces, as happens in asexual
reproduction. If one bacterium divides, and then the
resultant two bacteria divide again, the four individual
bacteria generated would be very similar. There would
be only very minor differences between them, generated
due to small inaccuracies in DNA copying. However, if
sexual reproduction is involved, even greater diversity
will be generated, as we will see when we discuss the
rules of inheritance.
Do all these variations in a species have equal
chances of surviving in the environment in which they
find themselves? Obviously not. Depending on the
nature of variations, different individuals would have
2020-21
Heredity and Evolution 143
9.2 HEREDITY 9.2 HEREDITY 9.2 HEREDITY 9.2 HEREDITY 9.2 HEREDITY
The most obvious outcome of the reproductive process still remains the
generation of individuals of similar design. The rules of heredity determine
the process by which traits and characteristics are reliably inherited. Let
us take a closer look at these rules.
9.2.1 Inherited Traits
What exactly do we mean by similarities and differences? We know that
a child bears all the basic features of a human being. However, it does
not look exactly like its parents, and human populations show a great
deal of variation.
QUESTIONS
?
1. If a trait A exists in 10% of a population of an asexually reproducing
species and a trait B exists in 60% of the same population, which trait
is likely to have arisen earlier?
2. How does the creation of variations in a species promote survival?
Activity 9.1 Activity 9.1 Activity 9.1 Activity 9.1 Activity 9.1
n Observe the ears of all the students in the class. Prepare a list of
students having free or attached earlobes and calculate the
percentage of students having each (Fig. 9.2). Find out about the
earlobes of the parents of each student in the class. Correlate the
earlobe type of each student with that of their parents. Based on
this evidence, suggest a possible rule for the inheritance of earlobe
types.
9.2.2 Rules for the Inheritance of Traits – – – – –
Mendel’s Contributions
The rules for inheritance of such traits in human beings are related to
the fact that both the father and the mother contribute practically equal
amounts of genetic material to the child. This means that each trait can
be influenced by both paternal and maternal DNA. Thus, for each trait
there will be two versions in each child. What will, then, the trait seen in
the child be? Mendel (see box) worked out the main rules of such
inheritance, and it is interesting to look at some of his experiments from
more than a century ago.
Figure 9.2 Figure 9.2 Figure 9.2 Figure 9.2 Figure 9.2
(a) Free and (b) attached
earlobes. The lowest part
of the ear, called the
earlobe, is closely attached
to the side of the head in
some of us, and not
in others. Free and
attached earlobes are two
variants found in human
populations.
different kinds of advantages. Bacteria that can withstand heat will survive
better in a heat wave, as we have discussed earlier. Selection of variants
by environmental factors forms the basis for evolutionary processes, as
we will discuss in later sections.
(a)
(b)
2020-21
Science
144
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. 9.3.
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.
Figure 9.3 Figure 9.3 Figure 9.3 Figure 9.3 Figure 9.3
Inheritance of traits
over two generations
Activity 9.2 Activity 9.2 Activity 9.2 Activity 9.2 Activity 9.2
n In Fig. 9.3, 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. 9.4.
Heredity and Evolution 145
x
RR yy
(round, green)
rr YY
(wrinkled, yellow)
Ry rY
Rr Yy
(round, yellow)
F1
x
Rr Yy
F1
Rr Yy
F1
315 round, yellow
108 round, green
101 wrinkled, yellow
32 wrinkled, green
9
3
3
1
556 seeds
16
Figure 9.5 Independent inheritance of two
separate traits, shape and colour of seeds
RY Ry rY
ry
RY
Ry
rY
ry
RRYY RRYy RrYY RrYy
RRYy RRyy RrYy Rryy
RrYY RrYy rrYY rrYy
RrYy Rryy rrYy rryy
F2
Figure 9.5 Figure 9.5 Figure 9.5 Figure 9.5 Figure 9.5
Independent inheritance
of two separate traits,
shape and colour of seeds
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
traits are formed in F2 offspring when factors controlling for seed
shape and seed colour recombine to form zygote leading to form
F2 offspring (Fig. 9.5). Thus, the tall/short trait and the round
seed/wrinkled seed trait are independently inherited.
9.2.3 How do these Traits get Expressed?
How does the mechanism of heredity work? Cellular DNA is
the information source for making proteins in the cell. A section
of DNA that provides information for one protein is called the
gene for that protein. How do proteins control the
characteristics that we are discussing here? Let us take the
example of tallness as a characteristic. We know that plants
have hormones that can trigger growth. Plant height can thus
depend on the amount of a particular plant hormone. The
amount of the plant hormone made will depend on the
efficiency of the process for making it. Consider now an enzyme
that is important for this process. If this enzyme works
efficiently, a lot of hormone will be made, and the plant will be
tall. If the gene for that enzyme has an alteration that makes
the enzyme less efficient, the amount of hormone will be less,
and the plant will be short. Thus, genes control characteristics,
or traits.
If the interpretations of Mendelian experiments we have been
discussing are correct, then both parents must be contributing
equally to the DNA of the progeny during sexual reproduction.
We have disscussed this issue in the previous Chapter. If both
parents can help determine the trait in the progeny, both parents
must be contributing a copy of the same gene. This means that
each pea plant must have two sets of all genes, one inherited from
each parent. For this mechanism to work, each germ cell must
have only one gene set.
How do germ-cells make a single set of genes from the normal two
copies that all other cells in the body have? If progeny plants inherited a
single whole gene set from each parent, then the experiment explained
in Fig. 9.5 cannot work. This is because the two characteristics ‘R’ and
‘y’ would then be linked to each other and cannot be independently
Figure 9.4 Figure 9.4 Figure 9.4 Figure 9.4 Figure 9.4
2020-21
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142
Figure 9.1
Creation of diversity over succeeding
generations. The original organism at the top
will give rise to, say, two individuals, similar
in body design, but with subtle differences.
Each of them, in turn, will give rise to two
individuals in the next generation. Each of
the four individuals in the bottom row will be
different from each other . While some of these
differences will be unique, others will be
inherited from their respective parents, who
were different from each other.
Heredity and
Evolution
9 CHAPTER
W
e have seen that reproductive processes give rise to new individuals
that are similar, but subtly different. We have discussed how some
amount of variation is produced even during asexual reproduction. And
the number of successful variations are maximised by the process of
sexual reproduction. If we observe a field of sugarcane we find very little
variations among the individual plants. But in a number of animals
including human beings, which reproduce sexually, quite distinct
variations are visible among different individuals. In this chapter, we
shall be studying the mechanism by which variations are created and
inherited. The long-term consequences of the accumulation of variations
are also an interesting point to be considered. We shall be studying this
under evolution.
9.1 9.1 9.1 9.1 9.1 A A A A ACCUMUL CCUMUL CCUMUL CCUMUL CCUMULA A A A ATION OF V TION OF V TION OF V TION OF V TION OF VARIA ARIA ARIA ARIA ARIATION TION TION TION TION
DURING REPRODUCTION DURING REPRODUCTION DURING REPRODUCTION DURING REPRODUCTION DURING REPRODUCTION
Inheritance from the previous generation provides both
a common basic body design, and subtle changes in it,
for the next generation. Now think about what would
happen when this new generation, in its turn,
reproduces. The second generation will have differences
that they inherit from the first generation, as well as
newly created differences (Fig. 9.1).
Figure 9.1 would represent the situation if a single
individual reproduces, as happens in asexual
reproduction. If one bacterium divides, and then the
resultant two bacteria divide again, the four individual
bacteria generated would be very similar. There would
be only very minor differences between them, generated
due to small inaccuracies in DNA copying. However, if
sexual reproduction is involved, even greater diversity
will be generated, as we will see when we discuss the
rules of inheritance.
Do all these variations in a species have equal
chances of surviving in the environment in which they
find themselves? Obviously not. Depending on the
nature of variations, different individuals would have
2020-21
Heredity and Evolution 143
9.2 HEREDITY 9.2 HEREDITY 9.2 HEREDITY 9.2 HEREDITY 9.2 HEREDITY
The most obvious outcome of the reproductive process still remains the
generation of individuals of similar design. The rules of heredity determine
the process by which traits and characteristics are reliably inherited. Let
us take a closer look at these rules.
9.2.1 Inherited Traits
What exactly do we mean by similarities and differences? We know that
a child bears all the basic features of a human being. However, it does
not look exactly like its parents, and human populations show a great
deal of variation.
QUESTIONS
?
1. If a trait A exists in 10% of a population of an asexually reproducing
species and a trait B exists in 60% of the same population, which trait
is likely to have arisen earlier?
2. How does the creation of variations in a species promote survival?
Activity 9.1 Activity 9.1 Activity 9.1 Activity 9.1 Activity 9.1
n Observe the ears of all the students in the class. Prepare a list of
students having free or attached earlobes and calculate the
percentage of students having each (Fig. 9.2). Find out about the
earlobes of the parents of each student in the class. Correlate the
earlobe type of each student with that of their parents. Based on
this evidence, suggest a possible rule for the inheritance of earlobe
types.
9.2.2 Rules for the Inheritance of Traits – – – – –
Mendel’s Contributions
The rules for inheritance of such traits in human beings are related to
the fact that both the father and the mother contribute practically equal
amounts of genetic material to the child. This means that each trait can
be influenced by both paternal and maternal DNA. Thus, for each trait
there will be two versions in each child. What will, then, the trait seen in
the child be? Mendel (see box) worked out the main rules of such
inheritance, and it is interesting to look at some of his experiments from
more than a century ago.
Figure 9.2 Figure 9.2 Figure 9.2 Figure 9.2 Figure 9.2
(a) Free and (b) attached
earlobes. The lowest part
of the ear, called the
earlobe, is closely attached
to the side of the head in
some of us, and not
in others. Free and
attached earlobes are two
variants found in human
populations.
different kinds of advantages. Bacteria that can withstand heat will survive
better in a heat wave, as we have discussed earlier. Selection of variants
by environmental factors forms the basis for evolutionary processes, as
we will discuss in later sections.
(a)
(b)
2020-21
Science
144
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. 9.3.
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.
Figure 9.3 Figure 9.3 Figure 9.3 Figure 9.3 Figure 9.3
Inheritance of traits
over two generations
Activity 9.2 Activity 9.2 Activity 9.2 Activity 9.2 Activity 9.2
n In Fig. 9.3, 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. 9.4.
Heredity and Evolution 145
x
RR yy
(round, green)
rr YY
(wrinkled, yellow)
Ry rY
Rr Yy
(round, yellow)
F1
x
Rr Yy
F1
Rr Yy
F1
315 round, yellow
108 round, green
101 wrinkled, yellow
32 wrinkled, green
9
3
3
1
556 seeds
16
Figure 9.5 Independent inheritance of two
separate traits, shape and colour of seeds
RY Ry rY
ry
RY
Ry
rY
ry
RRYY RRYy RrYY RrYy
RRYy RRyy RrYy Rryy
RrYY RrYy rrYY rrYy
RrYy Rryy rrYy rryy
F2
Figure 9.5 Figure 9.5 Figure 9.5 Figure 9.5 Figure 9.5
Independent inheritance
of two separate traits,
shape and colour of seeds
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
traits are formed in F2 offspring when factors controlling for seed
shape and seed colour recombine to form zygote leading to form
F2 offspring (Fig. 9.5). Thus, the tall/short trait and the round
seed/wrinkled seed trait are independently inherited.
9.2.3 How do these Traits get Expressed?
How does the mechanism of heredity work? Cellular DNA is
the information source for making proteins in the cell. A section
of DNA that provides information for one protein is called the
gene for that protein. How do proteins control the
characteristics that we are discussing here? Let us take the
example of tallness as a characteristic. We know that plants
have hormones that can trigger growth. Plant height can thus
depend on the amount of a particular plant hormone. The
amount of the plant hormone made will depend on the
efficiency of the process for making it. Consider now an enzyme
that is important for this process. If this enzyme works
efficiently, a lot of hormone will be made, and the plant will be
tall. If the gene for that enzyme has an alteration that makes
the enzyme less efficient, the amount of hormone will be less,
and the plant will be short. Thus, genes control characteristics,
or traits.
If the interpretations of Mendelian experiments we have been
discussing are correct, then both parents must be contributing
equally to the DNA of the progeny during sexual reproduction.
We have disscussed this issue in the previous Chapter. If both
parents can help determine the trait in the progeny, both parents
must be contributing a copy of the same gene. This means that
each pea plant must have two sets of all genes, one inherited from
each parent. For this mechanism to work, each germ cell must
have only one gene set.
How do germ-cells make a single set of genes from the normal two
copies that all other cells in the body have? If progeny plants inherited a
single whole gene set from each parent, then the experiment explained
in Fig. 9.5 cannot work. This is because the two characteristics ‘R’ and
‘y’ would then be linked to each other and cannot be independently
Figure 9.4 Figure 9.4 Figure 9.4 Figure 9.4 Figure 9.4
2020-21
Science
146
inherited. This is explained by the fact that each gene set is present, not
as a single long thread of DNA, but as separate independent pieces,
each called a chromosome. Thus, each cell will have two copies of each
chromosome, one each from the male and female parents. Every germ-
cell will take one chromosome from each pair and these may be of either
maternal or paternal origin. When two germ cells combine, they will
restore the normal number of chromosomes in the progeny, ensuring
the stability of the DNA of the species. Such a mechanism of inheritance
explains the results of the Mendel experiments, and is used by all
sexually reproducing organisms. But asexually reproducing organisms
also follow similar rules of inheritance. Can we work out how their
inheritance might work?
9.2.4 Sex Determination
We have discussed the idea that the two sexes participating in sexual
reproduction must be somewhat different from each other for a number
of reasons. How is the sex of a newborn individual
determined? Different species use very different strategies
for this. Some rely entirely on environmental cues. Thus,
in some animals like a few reptiles, the temperature at
which fertilised eggs are kept determines whether the
animals developing in the eggs will be male or female. In
other animals, such as snails, individuals can change sex,
indicating that sex is not genetically determined. However,
in human beings, the sex of the individual is largely
genetically determined. In other words, the genes inherited
from our parents decide whether we will be boys or girls.
But so far, we have assumed that similar gene sets are
inherited from both parents. If that is the case, how can
genetic inheritance determine sex?
The explanation lies in the fact that all human
chromosomes are not paired. Most human chromosomes
have a maternal and a paternal copy, and we have 22
such pairs. But one pair, called the sex chromosomes, is
odd in not always being a perfect pair. Women have a
perfect pair of sex chromosomes, both called X. But men
have a mismatched pair in which one is a normal-sized X
while the other is a short one called Y. So women are XX,
while men are XY. Now, can we work out what the
inheritance pattern of X and Y will be?
As Fig. 9.6 shows, half the children will be boys and
half will be girls. All children will inherit an X chromosome
from their mother regardless of whether they are boys or
girls. Thus, the sex of the children will be determined by
what they inherit from their father. A child who inherits
an X chromosome from her father will be a girl, and one
who inherits a Y chromosome from him will be a boy.
Figure 9.6 Figure 9.6 Figure 9.6 Figure 9.6 Figure 9.6
Sex determination in
human beings
2020-21
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