CBSE Class 10 Science Heredity and Evolution Assignment Set F

Very Short Answer Type Questions :

Question. Give an example where sex determination is regulated by environmental factors.
Answer: In snail, sex is determined by environmental factors (temperature).

Question. Define speciation.
Answer: Speciation means origin of new species from the existing one.

Question. What is the effect of DNA copying which is not perfectly accurate on the reproduction process?
Answer: It will result in less variations which may further lead to less adaptation to changing environment, thus, giving less chances of survival.

Question. Name the plant on which Mendel performed his experiments.
Answer: Garden pea (Pisum sativum).

Question. What are the basic events in evolution?
Answer: The changes in DNA during reproduction are the basic events in evolution.

Question. a. What is the genetic constitution of human sperm?
b. Mention the chromosomes pair present in zygote determining the sex of a male child.
Answer: a. 22 + Y or 22 + X
b. (22 + Y + 22 + X)

Question. a. How many gene sets should a germ cell have?
b. Mention two factors causing evolution.
Answer: a. One gene set.
b. (i) Reproduction isolation (ii) Natural selection

Question. What term did Mendel use for genes? Where are the genes located?
Answer: Mendel used the term ‘factors’ for genes. Genes are portions of DNA which code for a single protein.

Question. What are analogous organs? Explain with an example.
Answer: Analogous organs have the same function but have different structural design and origin. For example, wings of birds and insects have the same function but have different structural design and origin.

Question. State the importance of variations.
Answer: Variations are the differences in the characters among the individuals of a species. Variations enable the organisms to adapt themselves in the changing environmental conditions. Variations form the raw materials for evolution and formation of new species.

Question. What is evolution?
Answer: Evolution is the sequence of gradual changes which take place in living organisms over millions of years to give rise to new species.

Question. What type of reproduction gives rise to more number of successful variations?
Answer: Sexual reproduction.

Question. A pea plant with blue colour flower denoted by BB is cross-breed with a pea plant with white flower denoted by ww.
a. What is the expected colour of the flowers in their F1 progeny?
b. What will be the percentage of plants bearing white flower in F₂ generation, when the flowers of F₁ plants were selfed?
c. State the expected ratio of the genotypes BB and Bw in the F₂ progeny.
Answer: a. F1 generation — blue.
b. 25%
c. BB : Bw = 1 : 2.

Question. Which of the following are homologous and analogous organs?
(a) Wings of birds and insects.
(b) Flippers of whale and fins of fish.
(c) Flippers of whale and wings of bat.
(d) Our teeth and elephants tusks.
(e) Potato and runners of grass.
Answer: (a) — Analogous organs
(b) — Analogous organs
(c) — Homologous organs
(d) — Homologous organs
(e) — Homologous orgAnswer 

Question. a. What function is performed by human arms, forelimbs of dog and forelimbs of whales?
b. Which type of organs are these?
c. Why do we call them so?
Answer: a. Human arm : holding things Forelimbs of dog : running Forelimbs of whales : paddles
b. Homologous,
c. Same origin, different functions.

Question. How do traits or characters get expressed?
Answer: Genes control the expression of a trait or a character in an organism. Genes produces proteins. The proteins act as enzymes which can directly control a character or help in the formation of a hormone which can control the expression of a particular character.

Question. Which of the two sperm or egg–decides the sex of the child?
Answer: The sperm decides the sex of the child.

Question. What is genetic drift?
Answer: Accidents in small populations can change the frequency of some genes in a population even if they give no survival advantage. This is genetic drift, which provides diversity without any adaptations. So, it is the random change in the frequency of alleles in a population over successive generation.

Question. In a pea plant, find the contrasting trait if:
a. the position of flower is terminal.
b. the flower is white in colour.
c. shape of pod is constricted.
Answer: a. Axial position of flower.
b. Purple colour of flower.
c. Inflated shape.

Question. What is a trait?
Answer: A characteristics feature is called a trait.

Question. Name two human traits that show variations.
Answer: Colours of eyes and shape of external ears.

Short Answer Type Questions :

Question. How is the equal genetic contribution of male and female parents ensured in the progeny?
Answer: During sexual reproduction, a female gamete or egg cell fuses with a male gamete or sperm cell which are haploid to form zygote. Zygote is diploid which contains 23 chromosomes from mother and 23 from father. In this way, an equal genetic contribution of male and female parents is ensured in the progeny.

Question. What is meant by analogous organs? Taking a suitable example, explain how they support the theory of Organic Evolution. 
Answer: Analogous organs are those organs which I have different basic structural design and developmental origin but have similar appearance and perform similar functions.
Example: The wings of birds and bats look similar but have different design in their structure. They have a common function of flying but their origins are not common. So, birds and bats are not closely related.

Question. A true breeding tall plant is crossed with a true breeding short plant. All the offsprings of the F1 generation are tall. Of these two characters which one is dominant and which is recessive.
Answer: 

Question. What are fossils? What is their significance in the study of evolution?
Answer: Fossils are impressions of the body/ body parts or the remains of organisms living in the past, which got preserved in sediments of earth.
a. Study of fossils helps in working out evolutionary relationships.
b. Fossils provide one of the most acceptable evidences in support of evolution, because we can study the evolutionary past of individuals in the form of their fossils.
c. Age of fossils can also be found by time dating using isotopes of carbon! carbon dating.
d. By studying fossils occurring in different strata of rocks, geologists are able to reconstruct the time course of evolutionary events.

Question. (a) On what rules inheritance is based?
(b) Is each trait influenced by both paternal and maternal DNA?
Answer: a. The rules of inheritance is based on the fact that the traits in the progeny are carried out by DNA’s of both the parents (mother and father). These rules are known s ‘Mendel’s Laws of Inheritance’.
The rules are:
(i) Law of dominance,
(ii) Law of segregation, and
(iii) Law of independent assortment.
b. Yes, it is true that each trait is influenced by both paternal and maternal DNA.

Question. Explain how gene expresses itself in a cell? Why are we somewhat similar to our parents yet not identical to them?
Answer: Genes are functional segments of DNA. They are units of heredity that gets passed on through reproduction from parents to progeny. It is the blue print of life.
DNA expresses itself through synthesis of proteins.
Proteins make structures in our body and also controls the functioning. In other words cells, tissues, organs and hence a living body expresses the traits inherited as genes/ DNA. Since we inherit the DNA, half from one and half from other parents, hence we somewhat resemble them. At the same time we show mixed traits of both so can not be identical to either of them.

Question. “We cannot pass on to our progeny the experiences and qualifications earned during our lifetime”. Justify the statement giving reason and examples.
Answer: Acquiring knowledge / skill in one’s lifetime such as learning dance, music, physical fitness or any other suitable example.
Reason:
a. Such characters / experiences acquired during one’s lifetime do not bring any change in the DNA of the reproducing c$ll/germ cell.
b. Only germ cells are responsible for passing on the characters from the parents to the progeny.

Question. a. Insects, octopus and vertebrates all have eyes.
Can we group eyes of these animals together to establish a common evolutionary origin? Justify your answer.
b. “Birds have evolved from reptiles”. State evidence to prove the statement.
Answer: a. No, the structure of the eye in each of the organisms is different.
b. Fossils of certain dinosaurs/reptiles show imprints of feathers along with their bones but they could not fly presumably using the feathers for insulation.
Later they developed / evolved and adapted feathers for flight, thus becoming the ancestors of present day birds. (OR any other suitable evidence/example)

Question. What are chromosomes ? Explain how in sexually reproducing organisms the number of chromosomes in the progeny is maintained.
Answer: Chromosomes – Thread like structures made up of DNA found in the nucleus. The original number of chromosomes becomes half during gamete formation.
Hence, when the gametes combine, the original number of chromosomes gets restored in the progeny. (or same thing explained in the form of a flow chart).

Question. Explain with an example for each, how the following provides evidences in favour of evolution in organisms:
a. Homologous organs
b. Analogous organs
c. Fossils
Answer: a. Homologous organs – study of homologous organs suggests that the organs having same structure but performing different functions have evolved from a common ancestor. Example: forelimbs of a frog, lizard, bird and man.
b. Analogous organs – show adoption of organs for common use. Example: wings of butterfly and wings of bat.
c. Fossils – provide the missing links between two species. Example: – Archeopteryx / fossils of some dinosaurs with feathers.

Question. a. “Chromosomes are heredity carriers.” Why do we say so?
b. Which vital function is not controlled by autosomes?
Answer: a. Chromosomes are made-up of DNA. Genes are located on the chromosomes. It is the DNA copy which transfers from both the parents to their offspring. Therefore, chromosomes made-up of DNA and containing genes are said to be the heredity carriers.
b. Sex of the child is not controlled by autosomes.

Question. Woman are often blamed for bearing daughters. As a student with knowledge in science how will you explain it to your fellow students that the sex of the child is not determined by mother’s genetic contribution?
Answer: Sex of the child is not determined by mother’s genetic contribution. The sex is determined by a specific pair of 7 chromosomes called sex-chromosomes. In female, this pair consists of two similar (homologous) chromosomes denoted as XX. Hence, females produce only one type of gametes (Ova/eggs) each having an ‘X’ chromosome.
Whereas male human being has two different types of sex chromosomes i.e., X, Y (heterologous) having different sizes and shapes. Hence male produces two different kinds of gametes (sperms). Half of them have “X’ chromosome and half have ‘Y’ chromosome.
If a sperm with ‘X’ fuses with the ovum, female child is born and if a sperm with ‘Y’ chromosome fuses with the ovum, male child is bom as shown below in the cross. Therefore, the father’s genes is responsible for the determination of the sex of the child.

Question. a. During evolution some changes may have occurred for a reason but proved to be useful for some other purpose. Cite one such example.
b. Has the evolutionary changes took place all of sudden. Elaborate.
Answer: a. Evolutions of feathers: Functional Advantage: A change that is useful for one property to start with can become useful later for quite a different function. Feathers, for example, can start out as providing insulation in cold weather. But later, they might become useful for flight. In fact, some dinosaurs had feathers, although they could not fly using the feathers. Birds seem to have later adapted the feathers to flight. This, means that birds are very closely related to reptiles, since dinosaurs were reptiles. Birds have evolved from reptile.
b. Evolution changes has not took place all of sudden. Evolution takes place in stages, i.e., bit by bit over generations. Even an intermediate stage, such as a rudimentary eye, can be useful to some extent. This might be enough to give a fitness advantage. In fact, the eye seems to be a very popular adaptation. Insects have them, so does an octopus, and so do vertebrates. And the structure of the eye in each of these organisms is different – enough for them to have separate evolutionary origins.

Question. (a) Explain giving examples how artificial selection has helped in the formation of newer varieties of cauliflower.
(b) List the steps involved in the formation of new species.
(c) How different races of human beings belong to the same species?
Answer: a. Formation of newer varieties, e.g., Broccoli, cauliflower, red cabbage, etc. by man.
b. Migration, gene flow, natural selection, new species.
c. Capable of interbreeding among themselves.

Question. a. “Evolution has occurred in stages.” Justify the statement.
b. Differentiate between eye and eyespots. Which animal possesses eyespots?
Answer: a. “Evolution has occurred in stages” it is said because the process of evolution has took place over a long period of time. The complex organs are created bit-by-bit over generations.
There was increasing complexity of the organs. For example, eye was present in the earliest organism as eyespot which got developed into a more complex and evolved eye.
Further there are some organs in the human body which are present in the reduced form and do not perform any function, e.g., nictitating membrane of the eye, vermiform appendix, third pair of molars etc.
Similarly, during the course of evolution, some structures changed their function. So we can see that evolution has occurred in stages over a long period of time.
b. Eye is a complex structure made-up of different tissues which can see image as well as light whereas eyespots are merely dot-like structure to sense light.

Question. What is speciation? List four factors that could lead to speciation. Which of these cannot be major factors in the speciation of a self pollinating plant species? Explain.
Answer: Speciation – formation of new species from pre-existing ones.
Factors – (1) Mutations (2) Natural selection (3) Genetic drift (4) Geographical Isolation Geographical isolation cannot be a major factor in the speciation of a self pollinating plant species.
Reason – physical barrier cannot be created in self pollinating plants.

Question. How are fossils formed? Describe, in brief, two methods of determining the age of fossils.
Answer: 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:
a. 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.
b. Radio-carbon dating method: By detecting the ratios of different isotopes of carbon in the fossils.
Fossils help in determining the connecting links between various groups and their origin from their primitive ones, e.g., Archaeoptyrx.

Question. Explain the term homologous organ with an example.
Answer: The organs which have similar basic structure and mode of origin but perform different functions in different animals are called homologous organs. E.g., Forelimbs of amphibians, reptiles, birds and mammals.
Homologous organs give an evidence of common ancestory and evolutionary relationship between apparently different species.

Question. How green beetles had colour advantage over red beetles? Explain.
Answer: Green beetle merge with green background of vegetation which give survival advantage to a species from elimination.
a. in the population of beetles, the new variation (green colour) get survival benefit / advantage to green beetles whereas other (red) perishes.
b. It is because of Natural selection — some variations are advantageous for an organism to adapt better in the prevailing conditions of habitat. It makes it easy to obtain food and mating partner by them.
In this manner they are able to propagate more, transmitting their genes to next generation and producing more individuals with similar genetic makeup and phenotype.

Question. “Only variations that confer an advantage of an individual organism will survive in a population”. Comment.
Answer: According to Theory of Natural Selection, some variations are advantageous for an organism to adapt better in the prevailing conditions of habitat.
It makes it easy to obtain food and mating partner by them. In this manner they are able to propagate more, transmitting their genes to next generation and producing more individuals with similar genetic makeup and phenotype.
Though according to Genetic Drift Theory, even those not having a variation that confers an advantage also get to 5 survive but only in a small population. If due to an accident most of the individual with other variation get removed, the organisms with leftover variation get to propagate and survive under such circumstances.
Hence it is wrong to say that only variations that confer an advantage of an individual organism will survive in a population.

Question. Give reasons for the following:
a. Traits acquired during lifetime of an individual are not inherited.
b. All the human beings belong to a single species.
c. Variations keep on accumulating during reproduction and do not disappear in next generation.
Answer: a. These are acquired traits that do not make any change to the DNA of an organism.
b. Homo sapiens originated in Africa and spread across the globe in stages. The humans with different colours in different regions have come into being as an accident of evolution, so that they could live their lives the best they could. So all belong to the same species.
c. Inheritance from the previous generation provides both a common basic body design, and subtle changes in it, for the next generation. So the changes keep on accumulating generations after generations.

Long Answer Type Questions :

Question. A blue colour flower plant denoted by BB is cross bred with that of white colour flower plant denoted by bb.
(a) State the colour of flower you would expect in their F1 generation plants.
(b) What must be the percentage of white flower plants in F₂ generation if flowers of F₁ plants are self-pollinated?
(c) State the expected ratio of the genotypes BB and Bb in the F₂ progeny. 
Answer: 

Question. Discuss the role of variations in evolution. What is natural selection and genetic drift? 
                  or
Explain the following:
(a) Speciation (b) Natural Selection
Answer: a. The members of a population have minor differences among them which is called variations.
b. Two sources of variations are sexual reproduction and environmental factors.
c. Sexual reproduction has greater chances of producing variation.
d. Variations with some advantage has greater chances of surviving, for example, long neck of giraffe.
e. Natural selection and genetic drift lead to formation of new species hence evolution.
Natural Selection: Some variations may have survival advantage hence they happen to gain over others so that they can propagate more than others. Ultimately such variations are selected and propagated among all members of the population.
This is called adaptation of the species which help them to cope well in their surroundings.
In course of time, it could lead to accumulation of adaptation. In geographically separated populations of a species and development of reproductive barrier among them may lead to the formation of a new species.
Genetic Drift: Some variation may not givesurvival advantage to members of population. But if by chance other variations are wiped out from populations, the remaining variations get propagated in next generations.

Question. Define evolution. How does it occur? Describe how fossils provide us evidences in support of evolution.
Answer: Evolution is the formation of more complex organisms from pre-existing simpler organisms over a certain period. Accumulation of variation in genetic material forms the basis of evolutionary processes.
Fossils provide a unique view into the history of life by showing the forms and features of life in the past. Fossils tell us how species have changed across long periods of the Earth’s history.
Importance of fossils in the evolutionary process:
a. Some invertebrates living on the sea bed died and were buried in the sand.
b. More sand was accumulated and formed , sandstone under pressure.
c. After millions of years, dinosaurs living in the area died and their bodies were buried in the mud.
d. The mud got compressed into the rock, just above the rock containing earlier invertebrate fossils.
e. Again millions of years later, the bodies of horselike creatures dying in the area were fossilised in the rocks above the earlier rocks.
f. Much later, because of erosion and water flow, some rocks wore out and exposed the horse-like fossils.