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Figure 8.1Figure 8.1

Figure 8.1

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

CHAPTER

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 differ

ent individuals. In this chapter, we

shall be studying the mechanism by which variations are created and

inherited.

8.18.1

8.1

CCUMULCCUMUL

CCUMUL

TION OF VTION OF V

TION OF V

ARIAARIA

ARIA

TIONTION

TION

DURING REPRODUCTIONDURING 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. 8.1).

Figure 8.1 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

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8.2 HEREDITY8.2 HEREDITY

8.2 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.

8.2.1 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 8.1Activity 8.1

Activity 8.1

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. 8.2). 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.

8.2.2 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 8.2Figure 8.2

Figure 8.2

(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)

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130

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.

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 or

ganisms 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 8.3Figure 8.3

Figure 8.3

Inheritance of traits

over two generations

Activity 8.2Activity 8.2

Activity 8.2

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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RR yy

(round, green)

rr YY

(wrinkled, yellow)

Rr Yy

(round, yellow)

Rr Yy

315 round, yellow

108 round, green

101 wrinkled, yellow

32 wrinkled, green

556 seeds

Figure 9.5 Independent inheritance of two

separate traits, shape and colour of seeds

RRYY RRYy RrYY RrYy

RRYy RRyy RrYy Rryy

RrYY RrYy rrYY rrYy

RrYy Rryy rrYy rryy

Figure 8.5Figure 8.5

Figure 8.5

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. 8.5). Thus, the tall/short trait and the round

seed/wrinkled seed trait are independently inherited.

8.2.3 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. 8.5 cannot work. This is because the two characteristics ‘R’ and

‘y’ would then be linked to each other and cannot be independently

Figure 8.4Figure 8.4

Figure 8.4

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132

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?

8.2.4 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. 8.6 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 8.6Figure 8.6

Figure 8.6

Sex determination in

human beings

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QUESTIONS

1. How do Mendel’s experiments show that traits may be dominant or

recessive?

2. How do Mendel’s experiments show that traits are inherited

independently?

3. A man with blood group A marries a woman with blood group O and

their daughter has blood group O. Is this information enough to tell you

which of the traits – blood group A or O – is dominant? Why or why not?

4. How is the sex of the child determined in human beings?

What you have learnt

n Variations arising during the process of reproduction can be inherited.

n These variations may lead to increased survival of the individuals.

n Sexually reproducing individuals have two copies of genes for the same trait. If the

copies are not identical, the trait that gets expressed is called the dominant trait

and the other is called the recessive trait.

n Traits in one individual may be inherited separately, giving rise to new combinations

of traits in the offspring of sexual reproduction.

n Sex is determined by different factors in various species. In human beings, the

sex of the child depends on whether the paternal chromosome is X (for girls) or Y

(for boys).

EXERCISES

1. A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers

with short pea plants bearing white flowers. The progeny all bore violet flowers,

but almost half of them were short. This suggests that the genetic make-up of the

tall parent can be depicted as

(a) TTWW

(b) TTww

(d) TtWw

2. A study found that children with light-coloured eyes are likely to have parents

with light-coloured eyes. On this basis, can we say anything about whether the

light eye colour trait is dominant or recessive? Why or why not?

3. Outline a project which aims to find the dominant coat colour in dogs.

4. How is the equal genetic contribution of male and female parents ensured in

the progeny?

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