Question 1.
How do Mendel’s experiments show that the traits may be dominant or recessive?
Answer.
Mendel’s Experiments: Monohybrid Inheritance (One Visible Con¬trasting Character): Mendel
first took pea plants with different characteristics such as a pure bred tall
plant and a dwarf plant and crossed them. He found that only tall pea plants
were produced in the F
1 progeny. Then he allowed the F
1, tall plants to reproduce by self-pollination, and found that the F
2 progeny of the F
1 tall plants were not all tall. Both tall and dwarf plants were obtained
in the ratio of 3 :1. This ratio is known as the monohybrid ratio.
This indicated that both tallness and shortness traits were inherited in F
1 generation but only tallness trait was expressed which was dominant
over the unexpressed trait, i.e. shortness.
Question 2.
If a trait A exists in 10% of a population
of an a sexually reproducing species and a trait B exisis in 60% of the same
population, which trait is likely to have arisen earlier?
Answer.
Trait B is likely to have arisen earlier
than trait A. The new traits in a population occurs due to inaccuracies while
DNA copying mechanism. Hence, the new traits would be present in a veiy small
proportion compared to the old one which is already exists.
Question 3.
How does the creation of variations in a
species promote survival?
Answer.
Variations enable a species to adapt
according to the changes and new needs and thus provide survival of species.
Question 4.
How do Mendel’s experiments show that
traits are inherited independently?
Answer.
Dihybrid Inheritance (Two Visible Contrasting Characters): Mendel took pea plants having two
different characteristics: of seeds such as round shape with yellow colour and
wrinkled shape with green colour, and crossed them. He found that only
round-yellow seeds were produced in the F
1 progeny. Then he used F
1 progeny to generate F
2 progeny by self-pollination. He got four different combinations of
seeds: round – yellow, round – green, wrinkled – yellow and wrinkled-green in
the ratio of 9 : 3 : 3 : 1. This ratio is known as dihybrid ratio.
Question 5.
A man with blood group A marries a woman
with blood group 0 and their daughter has blood group 0. Is this information
enough to tell you which of the trait—blood group A or 0—is dominant? Why or
why not?
Answer.
No, this information is not enough to tell
us which of the traits—blood group A or 0, is dominant. Because a pair of
genes determines the blood group. In this case child has inherited 1° from
mother as well as father. (Father having I°I° and mother having I°I°.)
Question 6.
How is the sex of a child determined in
human beings?
Answer.
Sex Chromosomes: A male has XY sex chromosome and produces two types of sperms; 50% of
them carrying X chromosomes and another 50% carrying the Y chromosomes. A
female carries XX sex chromosomes and hence produces only X – carrying eggs.
If X-carrying egg fuses with the X -carrying sperm, the child born will be a
girl. If X – carrying egg fuses with the Y carrying sperm, the child born will
be a boy.
Question 7.
What are the different ways in which
individuals with a particular trait may increase in a population?
Answer.
(a) Natural
selection
(b) Genetic
drift
(c) Geographical isolation
Question 8.
Why are traits acquired during the lifetime
of an individual not inherited?
Answer.
Inherited Trait: The transmission of certain genetically controlled traits from parents
to their offspring or from one generation to the next, which bear all the
basic features with a great deal of variation are called inherited traits.
Question 9.
Why are the small numbers of surviving
tigers a cause of worry from the point of view of genetics?
Answer.
As the number of surviving tigers is small,
the sets of genes will be limited. This results in limited variation in
characters during sexual reproduction, thereby causing danger for their
survival in changing conditions.
Question 10.
What factors could lead to the rise of a
new species?
Answer.
(a) Natural
selection
(b) Genetic
drift
(c) Geographical isolation
Question 11.
Will geographical isolation be a major
factor in the speciation of a self-pollinating plant species? Why or why not?
Answer.
In a self-pollinating plant species,
geographical segregation cannot be a major factor in speciation. This is
because in self-pollinating plants, pollination is occurring in the same
plant.
So a natural barrier cannot cause
hindrance.
Question 12.
Will geographical isolation be a major
factor in the speciation of an organism that reproduces asexually? Why or why
not?
Answer.
In case of an asexually reproducing
organism, geographical isolation cannot be a major factor in speciation. This
is because in asexual reproduction, only one parent is involved and natural
barrier cannot pose a problem.
Question 13.
Give an example of characteristics being
used to determine how close two species are in evolutionary terms.
Answer.
Homologous characteristics are being used
to determine how close two species are in evolutionary terms. For example, the
forelimbs of human and bird have similar basic structure
though they are modified to perform
different functions in them. The basic structure or anatomical similarity
points to the existence of a common ancestor from these have evolved.
Question 14.
Can the wing of a butterfly and the wing of
a bat be considered homologous organs? Why or
why not?
Answer.
No, they are not to.be considered as
homologous organs because they perform the same function but they do not have
same structures and origins. In fact, they are the examples of analogous
organs.
Question 15.
What are fossils? What do they tell us
about the process of evolution?
Answer.
Fossils are the remains or traces of dead
plants and animals that lived in the past They provide the proof of changes
and the relationship between various groups of organisms. They show how one
species gives rise to another species with certain modifications. They allow
us to make estimates of how far back evolutionary relationships go.
Question 16.
Why are human beings who look so different
from each other in terms of size, colour and looks said to belong to the same
species?
Answer.
Human beings who look so different from
each other in terms of size, colour and looks said to belong to the same
species because of the following reasons:
(a) They have
same number of chromosomes.
(b) They have
a common ancestor.
(c) They
interbreed to produce fertile offsprings.
Question 17.
In evolutionary terms, can we say which
among bacteria, spiders, fish and chimpanzees have a
‘better’ body design? Why or why not?
Answer.
Chimpanzees have more complex body design
than fish, followed by spiders and then bacteria. Yet, we cannot say that
chimpanzees have a better body design, because evolution does not necessarily
mean progress. Evolution simply means the generation of diversity and
selection by nature. It is the adaptability of an organism to the environment
that supports its survival, not its complexity. Bacteria which are the
simplest life forms still survive today. They are found in varied habitats
like hot springs, in ice sheets, etc. only because of their adaptability to
the ‘ changing environment. Thus, in evolutionary terms, we cannot say that a
particular organism has a better body design.
Chapter End Questions
Question 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
(c) TtWW
(d) TtWw
Answer.
(c) TtWW
Question 2.
An example of homologous organs is
(a) our arm
and a dog’s fore leg
(b) our teeth
and an elephant’s tusks
(c) potato
and runners of grass
(d) all of
the above
Answer.
(d) all of
the above
Question 3.
In evolutionary terms, we have more in
common with
(a) A Chinese
school-boy.
(b) A
chimpanzee.
(c) A spider.
(d) A bacterium.
Answer.
(a) A Chinese
school-boy
Question 4.
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?
Answer.
Yes, we can say that the light eye colour
trait is dominant because children born from the parents having light-coloured
eyes also had light-coloured eyes.
Question 5.
Outline a project which aims to find the
dominantcoat colour in dogs.
Answer.
Let a dog with ‘black colour homozygous
trait (BB)’ mate with dog with ‘white homozygous trait (bb)’. If the offspring
of Ft generation are black, then the black trait is dominant. If the offspring
of F
1 generation are all white, then the white trait is dominant.
Question 6.
How are the areas of study—evolution and
classification, interlinked?
Answer.
Classification of organisms into groups is
based on the similarities and differences between them. The more
characteristics two species or organisms will have in common, the more closely
they are related and the more recently they will have had a common ancestor.
Classification shows that how closely organisms are related with respect to
evolution. It is in fact a reflection of their evolutionary relationship.
Question 7.
Explain the importance of fossils in
deciding evolutionary relationships.
Answer.
Fossils are the remains or traces of dead
plants and animals that lived in the past They provide the proof of changes
and the relationship between various groups of organisms. They show how one
species gives rise to another species with certain modifications. They allow
us to make estimates of how far back evolutionary relationships go.
Question 8.
What evidence do we have for the origin of
life from inanimate matter?
Answer.
In 1953, Miller and Urey assembled an early
earth atmosphere which consisted gases like NH
3, CH
4 and H
2S, etc. except oxygen, over water. They maintained it at the temperature just
below the 100°C and passed electric sparks in the mixture of gases to
stimulate lightning. This was continued about one week. After that they found
that 15% carbon from CH
4 had been converted to simple organic compound like sugar and amino
acids which constitute into protein molecules. This experiment gives the
evidence for origin of life from inanimate matter.
Question 9.
Explain how sexual reproduction gives rise
to more viable variations than asexual repro-duction. How does this affect the
evolution of those organisms-that reproduce sexually?
Answer.
In sexual reproduction, the DNA comes from
two parents. Therefore, the variations are more
obvious. When the next generation
reproduces, it passes on the characteristics inherited from the parents as
well as its own individual traits. This results in similarities and
variations.
Question 10.
How is the equal genetic contribution of
male and female parents ensured in the progeny?
Answer.
Individually both male and female parents
possess 23 pairs of chromosomes, i.e. they have a total of 46 chromosomes.
During gametes formation, this diploid chromosome number gets halved. The
female gamete has 22 + X chromosomes and the male gamete has 22 + X or 22 + Y
chromosomes. When the female and the male gametes fuses, the diploid condition
again get restored in the zygote.
The zygote has 44 + XX or 44 + XY chromosomes.
In this way, the equal genetic contribution of male and female parents is ensured in the progeny.
Question 11.
Only variations that confer an advantage to
an individual organism will survive in a population. Do you agree with this
statement? Why or why not?
Answer.
Yes, we agree with this statement because
advantageous variations enable an organism to cope with changes in the
environment. For example, there is a sudden drop in environ-mental temperature
of an area where bacteria live. Under such condition only those bacteria that
can tolerate low temperature will survive in a cold wave, while others do not.
These survivors pass on their advantageous characters to their offspring
resulting in the formation of new species.