Class-10
HEREDITY AND EVOLUTION
Term-2
IMPORTANT CONCEPTS
☞ Genetics. Branch of science that deals with heredity and variation.
☞ Heredity. It means the transmission of features/characters/traits from one generation to the next generation.
☞ Variation. The differences among the individuals of a species/population are called variations.
☞ Mendel and his work on Inheritance.
Gregor Johann Mendel started
his experiments on plant breeding and hybridisation. Mendel was known as the
Father of Genetics.
The plant selected by Mendel was Pisum sativum (garden
pea). Mendel used a number of contrasting characters for garden pea.
☞ Sex Determination. Phenomenon of decision or determination of sex of an offspring.
☞ Factors Responsible for Sex Determination:
(i) Environmental. In some animals the temperature
at which the fertilised eggs are kept decides the gender. Example, in
turtle.
(ii) Genetic. In some animals like humans gender of
individual is determined by a pair of chromosomes called sex chromosomes (XX – female;
XY – male).
Sex Chromosomes. In human
beings there are 23 pairs of chromosomes. Out of these 22 chromosome pairs are
called autosomes and the last pair of chromosomes that help in deciding gender
of that individual are called sex chromosome.
XX – female; XY – male
The cross done shows that half the children will be boys and half
will be girls. All children will inherit an X chromosome from their mother
regardless whether they are boys or girls. Thus sex of children will be
determined by what they inherit from their father, and not from their mother.
☞ Acquired Traits:
(i) These are the traits which are developed in an
individual due to special conditions.
(ii) They cannot be transferred to the progeny.
(iii) They cannot direct evolution, example, low
weight of starving beetles.
☞ Inherited Traits:
(i) These are the traits which are passed from one
generation to the next.
(ii) They get transferred to the progeny.
(iii) They are helpful in evolution, example, colour
of eyes and hair.
☞ Micro evolution. It is the evolution which takes place on a small scale. Example, change in body colour of beetles.
☞ Speciation. It is the process of formation of new species.
Species is a group of
similar individuals that belong to a population that can interbreed and produce
fertile offspring. Speciation takes place when variation is combined with
geographical isolation.
☞ Gene flow. It is exchange of genetic material by interbreeding between populations of same species or individuals. Gene flow occurs between populations that are partly but not completely separated.
☞ Genetic Drift. It is the random change in the frequency of alleles (gene pair) in a population over successive generations.
Genetic drift
takes place due to:
— severe changes
in the DNA.
— change in
number of chromosomes.
☞ Natural Selection. The process by which nature selects and consolidates those organisms which are more suitably adapted and possesses favorable variations.
☞ Evolution and classification. Both evolution and classification are interlinked.
— Classification
of species is a reflection of their evolutionary relationship.
— The more
characteristics two species have in common the more closely they are related.
— The more
closely they are related, the more recently they have a common ancestor.
— Similarities
among organisms allow us to group them together and to study their
characteristics.
☞ Tracing Evolutionary Relationships:
I. Homologous
Organs. Morphological
and anatomical evidences. These are the organs that have same basic structural
plan and origin but different functions.
Example, forelimb of horse
(running), wings of bat (flying), paw of a cat (walk/scratch/attack)— same
basic structure but different functions.
II. Analogous
Organs. These
are the organs that have different origin and structural plan but same functions.
Example, wings of bat
(elongated fingers with skin folds), wings of bird (feathery covering along the
arm)—different structures but same functions.
III. Fossils. The remains and
relics of dead organisms that lived in the remote past. Fossils provide evidence
of evolution. Example, a fossil called Archaeopteryx has feathered wings
like birds but teeth and tail like reptiles hence suggesting that birds and
reptiles had a common ancestor.
☞ Artificial Selection. Humans have been a powerful agent in modifying wild species to suit their own requirement throughout ages by using artificial selection. Example, wheat (many varieties obtained due to artificial selection).
SHORT ANSWER QUESTIONS
Q. 1. How does use and disuse of an organ help in evolution
of a new species?
Ans. According
to Lamarck, those organs which are used regularly become strong and more
developed. On the other hand, those organs which are not used regularly become
weak and degenerate. Such characters are inherited to the offsprings and so on.
In the due course of time, such characters become permanent in later
generations and become quite different from their ancestors, resulting in the formation
of a new species.
Q. 2. Why offsprings differ from parents in certain characters?
Ans. It
is due to biparental percentage. The genes on chromosomes which pass over to
the next generation is partly derived from both the parents (mother and father).
During fertilisation of egg by the sperm, new combination of chromosomes enter
the zygote, due to which certain variations occur in the offsprings. Thus,
brothers and sisters show variations in their complexion, habits and behaviour.
Q. 3. (i) Why is variation important for a species?
(ii) What are the causes of variations?
[CBSE 2017]
Ans. (i)
Variation is important for species to survive.
(ii) Following are the causes of variations:
(a) Dual percentage: Offsprings inherit
some features from mother and some from father, hence no offspring will exactly
resemble to either of the parent or each other.
(b) Mutation in gene or chromosomal pattern
also causes variations.
Q. 4. Give the pair of contrasting traits of the
following characters in pea plant and mention which is dominant and recessive
(i) yellow seed
(ii) round seed
Ans. (i)
yellow — dominant
green — recessive
(ii) round — dominant
wrinkled — recessive
Q. 5. What is the contribution of Mendel to genetics?
Ans. Mendel
observed the occurrence of contrasting characters of garden pea in various
generations. On this basis, he interpreted that these contrasting characters
are controlled by factors. He considered each and every character as a unit,
which is controlled by a ‘factor’. Factors are carriers of hereditary information.
Now, factors are known as genes.
Q. 6. A very small population of a species faces a
greater threat of extinction than a larger population. Provide a suitable
genetic explanation.
Ans.
Fewer individuals in a species
impose extensive inbreeding among them. This limits the appearance of
variations and puts the species at a disadvantage if there are changes in the
environment. Since the individuals fail to cope up with the environmental changes,
they may become extinct.
Q. 7.
A change in DNA that is useful for one property to start with, can become
useful later for a different function. Explain.
Ans.
A change/feature/property of an
organism that may have helped it to adopt to an environmental condition can
also become useful for a completely different function in the future. For
example, feathers in birds, a character developed and selected during natural
selection for providing insulation in cold weather, became useful in later
stages for flight. Some dinosaurs had feathers, but they could not fly. Birds
later adapted the feathers to fly.
Q. 8.
Does the occurrence of diversity of animals on earth suggest their diverse
ancestry also? Discuss this point in the light of evolution.
[NCERT
Exemplar]
Ans.
Though animals have a vast
diversity in structures they probably do not have a common ancestry, because
common ancestry may greatly limit the extent of diversity. As many of these
diverse animals are inhabiting the same habitat, their evolution by geographical
isolation and speciation is also not likely. Thus, a common ancestry for all
the animals is not the likely theory.
Q. 9.
All the human races like Africans, Asians, Europeans, Americans and others
might have evolved from a common ancestor. Provide a few evidences in support
of this view.
[NCERT
Exemplar]
Ans.
All human races have evolved from
a common ancestor because everybody has: (i) Common body design,
structure, physiology and metabolism.
(ii) Constant chromosome number.
(iii)
Common genetic blue print.
(iv)
Freely inter-breeding species.
Q.
10. List two differences in tabular form between dominant trait and recessive
trait. What percentage/ proportion of the plants in the F2 generation/ progeny
were round, in Mendel’s cross between round and wrinkled pea plants?
Ans.
|
Dominant
trait |
Recessive
trait |
|
1. The trait which
appears in the F1 progeny is dominant. 2. It appears in
more numbers. |
1. The trait which
remains hidden or which does not appear in the F1 progeny is the recessive
trait. 2. It appears in less number. |
Q.
11. “We cannot pass on to our progeny the experiences and qualifications earned
during our life time”. Justify the statement giving reason and examples.
[CBSE
2015]
OR
With
the help of two suitable examples, explain why certain experiences and traits
earned by people during their lifetime are not passed on to their next
generations. When can such traits be passed on?
[CBSE
2017]
Ans.
We acquire knowledge and skills in
our lifetime such as learning dance, music, physical fitness, etc. But these
skills cannot be passed to our progenies because (i) Such characters or
experiences acquired during one’s lifetime do not bring any change in the DNA
of the germ cell. (ii) Only germ cells are responsible for passing on
the characters from the parents to the progeny. These traits can be passed to
the next generation when the changes are in the DNA of the germ cell.
Q.
12. (i)How many pairs of chromosomes are present in human beings? Out of
these how many are sex chromosomes? How many types of sex chromosomes are found
in human beings?
(ii)
A woman has only daughters, analyse the situation genetically and provide a
suitable explanation.
[NCERT
Exemplar]
Ans.
(i) 23 pairs of chromosomes
are present in human beings. One pair of these are sex chromosomes. Two types
of sex chromosomes are there: XX and XY.
(ii)
The woman produces ova with ‘X’ chromosome. The man produces sperms with ‘X’
and ‘Y’ chromosome which actually determines the sex of the baby. In this case,
every time the male X chromosome only fuses with the female X chromosome thus
producing XX combination.
Q. 13.‘Different species use different strategies to determine sex of a new born individual. It can be Explain the statement by giving example for each strategy.
[CBSE
2016]
Ans.
Environmental cue: In some animals, the temperature at which fertilised
eggs are kept determines whether the developing animal in egg is male or
female. In some animals like snail, individual can change sex.
Genetical
cue: A child who inherits an X
chromosome from the father will be a girl and one who inherits a Y chromosome
from the father will be a boy.
Q.
14. What is DNA copying? State its importance.
[CBSE
2015]
Ans.
A process where a DNA molecule
produces two similar copies of itself in a reproducing cell is called DNA
copying. Its importance are:
(i)
It makes the transmission of characters from parents to the next generation
possible.
(ii)
It causes variation in the population.
Q.
15. What is speciation? List four factors responsible for speciation.
[CBSE
2015]
Ans.
Speciation is the formation of new
species from the pre-existing population. Factors responsible for speciation:
(i)
Genetic drift (ii) Natural selection (iii) Geographical isolation
(iv) Mutation
Q. 16. Why is variation beneficial to the species but not necessary for the individual?
Ans. The importance of variation in organism introduced
during reproduction is that it helps the species of various organisms to
survive and flourish even in adverse environment. If all the organisms of a
population living in that habitat are exactly identical, then there is a danger
that all of them may die and no one would survive under those conditions. This
will eliminate the species from the habitat completely. However, if some
variations are present in some individual organism to tolerate excessive heat or
cold, then there is a chance for them to survive and flourish even in adverse
excessive heat or cold. Thus, variation is useful for the survival of a species
over time.
Q.
17. List the two types of reproduction. Which one of the two is responsible for
bringing in more variations in its progeny and how?
[CBSE
2017]
Ans.
The two types of reproduction are
sexual reproduction and asexual reproduction. Sexual reproduction is
responsible for bringing in more variations because of the process of DNA
copying which may result in some error in it. Also, it involves fusion of male
and female gametes from two different parents.
Q.
18. How do variations occur in an offspring?
Ans.
Dissimilarities between members of
the same species is called variations. Two offsprings of the same parents show
certain variations. Variations occur due to sex chromosomes. Variations arising
in germplasm (genes) of the organism are heritable. Mother and father
contribute to the gene pattern of the offsprings through their chromosomes, in
which recombination occurs at the time of gametogenesis. In zygote formation,
gene pattern of both parents come together, that causes some variations between
parents and offsprings and amongst offsprings also.
Q.
19. List in tabular form, two distinguishing features between the acquired
traits and the inherited traits with one example of each.
[CBSE
2015]
OR
List
three distinguishing features, in tabular form, between acquired traits and the
inherited traits.
[CBSE
2015]
OR
Distinguish
between the acquired traits and the inherited traits in tabular form, giving
one example for each.
[CBSE
2017]
OR
Differentiate
between inherited and acquired characters. Give one example for each type.
[NCERT
Exemplar]
OR
List
two differences between acquired traits and inherited traits by giving an
example of each.
[CBSE
2019]
Ans.
|
Acquired Traits |
Inherited Traits |
|
1. Does not bring
about change in the DNA of the germ cell. 2. Cannot be
passed on to the progeny. 3. Cannot direct
evolution. Examples: Acquiring knowledge,
loss of weight, etc. |
1. Brings about
change in the DNA of the germ cell. 2. Can be passed
on to the progeny. 3. Can direct
evolution. Examples: Skin colour, colour
of the eye, etc. |
Q. 20. In one of
his experiments with pea plants Mendel observed that when a pure tall pea plant
is crossed with a pure dwarf pea plant, in the first generation, F1 only tall
plants appear.
(i) What
happens to the traits of the dwarf plants in this case?
(ii)
When the F1 generation plants were self fertilised, he observed that in the
plants of second generation, F2 both tall plants and dwarf plants were present.
Why it happened? Explain briefly.
[CBSE
2016]
Ans.
(i) The dwarf traits of the
plants is not expressed in the presence of the dominant tall trait.
(ii)
In the F2 generation, both the tall and dwarf traits are present in the ratio
of 3 : l. This showed that the traits for tallness and dwarfness are present in
the F1 generation, but the dwarfness, being the recessive trait does not
express itself in the presence of tallness, the dominant trait.
Q.
21. A pea plant with blue colour flower denoted by BB is cross-bred with a pea
plant with white flower denoted by ww.
(i)
What is the expected colour of the flowers in their F1 progeny?
(ii)
What will be the percentage of plants bearing white flower in F2 generation,
when the flowers F1 plants were selfed?
(iii)
State the expected ratio of the genotype BB and Bw in the F2 progeny.
[CBSE
2015]
Ans.
(i)
Blue
(ii)
25%
(iii)
BB : Bw = 1 : 2
Q.
22. “Two areas of study namely ‘evolution’ and ‘classification’ are
interlinked”. Justify this statement.
[CBSE
2015]
OR
“Evolution
and classification of organisms are interlinked.” Give reasons to justify this
statement.
[CBSE
2017]
Ans.
Different forms of organisms have
evolved during the course of evolution. Classification deals with grouping of
these organisms into groups and subgroups based on their similarities and
differences. The more characteristics any two species have in common more
closely they are related. In other words, they will have a more recent
ancestor. Thus, classification helps is tracing the evolutionary relationships
between the two organisms. Hence classification and evolution are interlinked.
Q.
23. What are chromosomes? Explain how in sexually reproducing organisms the
number of chromosomes in the progeny is maintained.
[CBSE
2015, Delhi 2017]
Ans.
Chromosomes are thread-like
structures, made of proteins and DNA, found in the nucleus at the time of cell
division. In sexually reproducing organisms, the gametes undergo meiosis, and
hence, each gamete contains only half a set of chromosomes. When two gametes
fuse, the zygote formed contains the full set of chromosomes. Hence, the
formation of gametes by meiosis helps to maintain the number of chromosomes in
the progeny.
Q.
24. Mention the total number of chromosomes along with the sex chromosomes that
are present in a human female and a human male. Explain how in sexually
producing organisms the number of chromosomes in the progeny remains the same
as that of the parents.
[CBSE
2017]
Ans.
Human male has 22 pairs of
chromosomes along with XY sex chromosome. Human female has 22 pairs of
chromosomes along with XX sex chromosomes. The original number of chromosomes
(the amount of DNA) becomes half during gamete formation. When the gametes
fuse, the original number of chromosomes (the amount of DNA) is restored in the
progeny.
Q. 25. “Natural selection and speciation leads to evolution.” Justify this statement.
[CBSE 2017]
Ans.
Natural selection is defined as
the change in frequency of some genes in a population, which gives survival
advantage to a species. Whereas speciation is the development of a new species
from pre-existing ones. This leads to a sequence of gradual change in the
primitive organisms over millions of years, to form newer species which are
very different from older ones. This is called evolution.
Q.
26. Explain analogous organs and homologous organs. Identify the analogous and
homologous organs amongst the following:
Wings
of an insect, wings of a bat, forelimbs of frog, forelimbs of a human.
Ans.
Analogous organs are those organs which
have different structural designs and origin but perform similar functions. Homologous
organs are those which have the same basic structural design and origin but
perform different functions.
Analogous
organs: Wings of an insect, wings
of a bat.
Homologous
organs: Forelimbs of a frog,
forelimbs of a human.
Q.
27. Explain the following:
(i)
Speciation [CBSE 2019]
(ii)
Natural Selection [CBSE
2015]
Ans.
(i) The process by which
new species develop from the existing species is known as speciation. The factors
which could lead to speciation are:
(a)
Geographical isolation of population caused by various types of barriers such
as mountain ranges, rivers and seas. This leads to reproductive isolation
because of which there is no flow of genes between separated groups of
population.
(b)
Genetic drift caused by drastic changes in the frequencies of particular genes
by chance alone.
(c)
Variations caused in individuals because of natural selection.
(ii)
According to Darwin’s theory of natural selection, the individuals who are most
suitable and fit are successful in struggle for existence for food, space,
mate, etc. Their offsprings are also better developed and adapted to the
environment. Whereas one who are less adapted to the environment may die. Thus,
nature selects better adapted organisms. This is called natural selection of
the well adapted, better developed individuals of species.
Q.
28. Describe the contribution of Lamarck.
Ans.
The gradual unfolding of organisms
from pre-existing organisms through changes since the beginning of life is
called evolution. The theory proposed by J.B. Lamarck is known as the theory of
inheritance of acquired characters. According to this theory, the use and
disuse of an organ leads to acquiring of change in that organ. These changes or
variations can be passed on from one generation to the next but this idea of
inheritance of acquired characters was soon discarded.
Q.
29. “Fossils are related to evolution”, justify this statement. Give the two
ways by which age of the fossils can be estimated.
Ans.
Fossils are the remains of the
organisms that once existed on earth, i.e., they are the preserved
traces of living organisms. They provide evidence of evolution by revealing the
characteristics of the past organisms and the changes that have occurred in these
organisms to give rise to the present organisms. Two ways of determining age of
fossils are:
(i)
Relative method-fossils closer to the surface are more recent.
(ii)
Dating-finding the ratio of different isotopes of the same element.
Q.
30. (i) Planaria, insects, octopus and vertebrates all have eyes.
Can we group eyes of these animals together to establish a common evolutionary origin?
Justify your answer.
(ii)
“Birds have evolved from reptiles”. State evidence to prove the statement.
[CBSE
2015]
Ans.
(i) No we cannot group them
together because the structure of the eye in each of the organisms is different.
(ii)
Fossils of certain dinosaurs and reptiles show imprints of feathers along with
their bones but they could not fly presumably, using the feathers for
insulation only. Later they evolved and adapted feathers for flight, thus
becoming the ancestors of present day birds.
Q.
31. Explain with the help of an example each, how the following provide
evidences in favour of evolution:
[CBSE
2017]
(i)
Homologous organs
(ii)
Analogous organs
(iii)
Fossils
Ans.
(i) Homologous
organs: The study of these organs suggests
that these organisms with organs having same structure but performing different
functions have evolved from a common ancestor, e.g., forelimbs of
different vertebrates.
(ii)
Analogous organs: The
study of these apparently similar organs suggests that the organisms with apparently
similar organs do not share common ancestory. Similarity in these organs is
superficial or design and the structure of these organs are very different, e.g.,
wings of bird and wings of butterfly.
(iii)
Fossils: They provide the missing
link between the species, e.g., Fossils of dinosaurs with feathers or
fossils of prehistoric horse, etc.
Q.
32. Mention three important features of fossils which help in the study of
evolution.
[NCERT
Exemplar]
Ans.
(i)
Fossils represent modes of preservation of ancient species.
(ii)
Fossils help in establishing evolutionary traits among organisms and their
ancestors.
(iii)
Fossils help in establishing the time period in which organisms lived.
Q.
33. “The sex of a newborn child is a number matter of chance and none of the
parents may be considered responsible for it.” Justify this statement with the
help of a flow chart showing sex determination in human beings.
[CBSE
2019]
Ans.
In human beings, sex of the child
depends upon which kind of male gamete fertilises with the female gamete. If
sperm carrying X chromosome fertilises the ovum carrying X chromosome, then the
child born will be a girl. If a sperm carrying Y chromosome fertilises the ovum
which carries X chromosome, then the child born will be a boy. So, it is the
male who is responsible for the sex of the child.
Q.
34. Name two homologous structures in vertebrates. Why are they so called? How
do such organs help in understanding an evolutionary relationship?
Ans.
Two homologous structures in
vertebrates are:
(i)
Limbs of birds and reptiles.
(ii)
Limbs of reptiles and amphibians.
These
are called so because the organs have similar structure to perform different
functions in various vertebrates. The homologous characteristics of such organs
indicate common ancestory. Thus these exist an evolutionary relationship.
Q.
35. Name the plant Mendel used for his experiment. What type of progeny was
obtained by Mendel in F1 and F2 generations when he crossed the tall and short
plants? Write the ratio he obtained in F2 generation plants.
[CBSE
2019]
Ans.
Mendel used garden pea (Pisum
Sativum) for his experiment.
When
Mendel crossed a pure tall plant (TT) with a dwarf plant (tt), the progeny thus
obtained was called F1 progeny/generation.
Then
he self-pollinated F1 generation and found out 75% tall plants and 25% dwarf.
F2 generation ratio:-
1 : 2
: 1
Pure
tall : Hybrid tall : Pure dwarf
(TT)
(Tt) (tt)
Q.
36. How did Mendel’s experiments show that different traits are inherited
independently? Explain.
[CBSE
2017]
Ans.
Mendel conducted a dihybrid cross;
and observed that though he started with two types of parents , he obtained
four types of individuals in F2. The appearance of new recombination in F2
generations along with parental type characters show that traits are inherited
independently of each other.
Q. 37.
What is an organic evolution? It cannot be equated with progress. Explain with
the help of a suitable example.
[CBSE
2017]
Ans.
Organic evolution is a sequence of
gradual changes which take place in the organism over millions of years
resulting in the formation of new organisms or species. Evolution is not the
progress from lower form of life to higher. It has given rise to more complex
body design even while simpler body designs continue to flourish. For example,
human beings who have not evolved from chimpanzees, but both have common
ancestors.
Q. 38.
Does geographical isolation of individuals of a species lead to formation of a
new species? Provide a suitable explanation.
[NCERT
Exemplar]
Ans.
Yes, geographical isolation
gradually leads to genetic drift. This may impose limitations to sexual
reproduction of the separated population. Slowly the separated individuals will
reproduce among themselves and generate new variations. Continuous accumulation
of those new variations through a few generations may ultimately lead to the
formation of a new species.
Q. 39.
(i) What is meant by characteristics?
(ii)
Give an example of the characteristics being used to determine how close two
species are in evolutionary terms.
[CBSE
2019]
Ans.
(i) It is the detail of
appearance or behaviour; in other words, a particular form or a particular
function.
(ii)
Study of homologous organs such as forelimbs of mammals, birds, reptiles and
amphibians shows that though they perform different functions but they have
similar basic or internal structure. This is because they have evolved from
common ancestor and help us in determining the closeness between two species in
evolutionary terms.
Q. 40.
List three factors that provide evidences in favour of evolution in organisms
and state the role of each in brief.
[CBSE
2016]
Ans.
Three factors that provide
evidences are:
(i)
Analogous organs — organisms with similar looking organs may have different
origin.
(ii)
Homologous organs — organisms with apparently different looking organs may have
similar origin.
(iii)
Fossils — allow us to make estimates of how far back evolutionary relationships
go. Fossils when chronologically arranged help in tracing the evolutionary
history of an organism.
Q.
41.Mendel, in one of his experiments with pea plants, crossed a variety of pea
plant having round seeds with one having wrinkled seeds. State Mendel’s
observations giving reasons of F1 and F2 progeny of this cross. Also, list any
two contrasting characters, other than round seeds of pea plants that Mendel
used in his experiments.
[CBSE
2019]
Ans.
Mendel crossbred round seed plants
with wrinkled seed plants. The F1 progeny so obtained had heterozygous round
seed plants and no wrinkled seed plants. When he self crossed the F1
generation, both round and wrinkled seed plants were obtained in the ratio of 3
: 1.
LONG ANSWER QUESTIONS [5 Marks]
Q. 1.
“It is possible that a trait is inherited but may not be expressed.” Give a
suitable example to justify this statement.
[CBSE
2015]
OR
With
the help of an example justify the following statement: “A trait may be
inherited, but may not be expressed”.
[CBSE
2016]
OR
How
did Mendel explain that it is possible that a trait is inherited but not
expressed in an organism?
[CBSE
2017]
OR
(i)
What are dominant and recessive traits? (ii) “Is it possible that a
trait is inherited but may not be expressed in the next generation?” Give a
suitable example to justify this statement.
[CBSE
2019]
Ans.
(i)
Dominant traits are those which expresses itself in the F1 generation.
Recessive traits are those traits which remains hidden or does not express
itself in the F1 generation.
(ii)
Yes, it is possible that a trait is inherited but may not be expressed. For
example, when pure tall pea plants are crossed with pure dwarf pea plants, only
tall pea plants are obtained in F1 generation. On selfing tall plants of F1,
both tall and dwarf plants are obtained in F2 generation in the ratio 3 : 1.
Reappearance of the dwarf character, a recessive trait in F2 generation shows
that the dwarf trait was present in individuals of F1 but it did not
express.
Q. 2.
How do Mendel’s experiments show that the (i) traits may be dominant or recessive, (ii)
traits are inherited independently?
[CBSE
2015, Delhi 2017]
OR
How
do Mendel’s experiments show that
(i)
traits may be dominant or recessive?
(ii)
inheritance of two traits is independent of each other?
[CBSE
2017]
Ans.
(i)
When Mendel cross-pollinated pure tall pea plants with pure dwarf pea plants,
only tall plants were obtained in F1 generation. On self pollinating the F1
progeny, both tall and dwarf plants appeared in F2 generation in the ratio 3 :
1. Appearance of tall character in both the F1 and F2 shows that it is a
dominant character. The absence of dwarf character in F1 generation and its
reappearance in F2 shows dwarfness is the recessive character.
(ii)
When Mendel first crossed pure-breed pea plants having round-yellow seeds with
pure-breed pea plants having wrinkled-green seeds, he found that only
round-yellow seeds were produced in the first-generation. No wrinkled-green
seeds were obtained in the generation. From this, it was concluded that round
shape and yellow colour of the seeds were dominant traits over the wrinkled
shape and green colour of the seeds.
When
the F1 generation pea plants having roundyellow seeds were cross-bred by
self-pollination, then four types of seeds having different combinations of
shape and colour were obtained in second generation (F2). These were
roundyellow, round-green, wrinkled-yellow and wrinkled-green seeds.
Such
a cross is known as dihybrid cross as two sets of corresponding characters are
considered. Mendel observed that along with round-yellow and wrinkled-green,
two new combinations of characteristics, round-green and wrinkled-yellow, had
appeared in the F2 generation. On the basis of this observation, Mendel
concluded that though the two pairs of original characteristics (seed colour
and shape) combine in the F1 generation, they get separated and behave
independently in the subsequent generation.
Q. 3.
Explain Mendel’s experiment with peas on inheritance of characters considering
only one visible contrasting character.
[CBSE
2015]
Ans.
Mendel conducted breeding
experiments with garden peas:
(i)
He studied plants (pure) of a tall/short varieties.
(ii)
He crossed them and obtained F1 progeny.
(iii)
He found that F1 progeny was all tall plants.
(iv)
He selfed the (hybrid) plants of F1 progeny.
(v)
He found that in F2 progeny there were tall as well as short plants.
(vi)
The three quarter plants were tall and one quarter was short. (or any other
contrasting character may be taken.)
Q. 4.
What are fossils? How are they formed? Describe in brief two methods of
determining the age of fossils. State any one role of fossils in the study of
the process of evolution.
[CBSE
2015]
OR
What
are fossils? How are they formed? List two methods of determining the age of
fossils. Explain in brief the importance of fossils in deciding the
evolutionary relationships.
[CBSE
2016, 2017]
OR
What are fossils? Describe briefly two methods of determining the age of fossils.
[CBSE 2019]
Ans.
Fossils are dead remains of
animals and plants from remote past. Fossils are formed when dead organisms are
not completely decomposed. The organisms may get trapped in resins of tree,
lava of volcanoes or hot mud, which when hardens retains the animal’s parts
thus forming fossils. Two methods of determining the age of fossils are:
(i)
Relative method: By
estimating the age of the layer of Earth’s crust where the fossil is found.
Fossils near the surface are recent and those in the deeper layers are more
ancient.
(ii)
Radio-carbon dating method: By
detecting the ratios of different isotopes of carbon in the fossils.
Fossils play the following roles:
(i)
By
determining the age of fossils we come to know the type of Earth strata present
at that time.
(ii)
We
can also know the type of animals and plants present on the earth at that time.
(iii)
They
help in establishing evolutionary relationship by providing connecting links. (Any
one)
Q. 5.
What is speciation? List four factors that could lead to speciation. Which of
these cannot be a major factor in the speciation of a self-pollinating plant
species? Explain. Give reason to justify your answer.
[CBSE
2016, 2015]
Ans.
Speciation is the formation of new
species from the pre-existing population. Factors responsible for speciation:
(i)
Genetic drift
(ii)
Natural selection
(iii)
Geographical isolation
(iv)
Mutation Geographical isolation cannot be a major factor in the speciation of a
self pollinating plant species because physical barrier cannot be created in
selfpollinating plants.
No comments:
Post a Comment