Heredity and evolution short answer question 2
What is DNA copying? State its importance.
[CBSE Delhi 2015]
A process where a DNA molecule produces two similar copies of itself in a reproducing cell is called DNA copying.
Its importance are:
It makes the transmission of characters from parents to the next generation possible.
It causes variation in the population.
“We cannot pass on to our progeny the experiences and qualifications earned during our life time”. Justify the statement giving reason and examples. [CBSE Delhi 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 (AI) 2017]
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.
‘Different species use different strategies to determine sex of a newborn individual. It can be environmental cues or genetically determined’. Explain the statement by giving example for each strategy.
[CBSE Sample Paper 2016]
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.
List the two types of reproduction. Which one of the two is responsible for bringing in more variations in its progeny and how?
[CBSE (AI) 2017]
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.
How do variations occur in an offspring?
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.
Why is variation beneficial to the species but not necessary for the individual?
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.
What is speciation? List four factors responsible for speciation. [CBSE Delhi 2015]
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
List in tabular form, two distinguishing features between the acquired traits and the inherited traits with one example of each.
[CBSE Delhi 2015]
OR
List three distinguishing features, in tabular form, between acquired traits and the inherited traits.
[CBSE Delhi 2015]
OR
Distinguish between the acquired traits and the inherited traits in tabular form, giving one example for each.
[CBSE Delhi 2017]
OR
Differentiate between inherited and acquired characters. Give one example for each type.
[NCERT Exemplar]
Acquired TraitsInherited Traits
1. Does not bring about change in the DNA of the germ cell.1. Brings about change in the DNA of the germ cell.
2. Cannot be passed on to the progeny.2. Can be passed on to the progeny.
3. Cannot direct evolution.3. Can direct evolution.
Examples: Acquiring knowledge, loss of weight, etc.Examples: Skin colour, colour of the eye, etc.
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.
[CBSE Delhi 2016]
What happens to the traits of the dwarf plants in this case?
The dwarf traits of the plants is not expressed in the presence of the dominant tall trait.
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.
In the F2 generation, both the tall and dwarf traits are present in the ratio of 3 : 1. 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.
What are chromosomes? Explain how in sexually reproducing organisms the number of chromosomes in the progeny is maintained. [CBSE (AI) 2015, Delhi (C) 2017]
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 Delhi 2017]
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.
“Two areas of study namely ‘evolution’ and ‘classification’ are interlinked”. Justify this statement.
[CBSE (AI) 2015]
OR
‘‘Evolution and classification of organisms are interlinked.’’ Give reasons to justify this statement.
[CBSE (AI) 2017]
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.
A pea plant with blue colour flower denoted by BB is cross-bred 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 F1 plants were selfed?
(c) State the expected ratio of the genotype BB and Bw in the F2 progeny. [CBSE (AI) 2015]
(a) Blue
(b) 25%
(c) BB : Bw = 1 : 2
Explain the following:
[CBSE (AI) 2015]
Speciation
The process by which new species develop from the existing species is known as speciation.
The factors which could lead to speciation are
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.
Genetic drift caused by drastic changes in the frequencies of particular genes by chance alone.
Variations caused in individuals because of natural selection.
Natural Selection
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.
“Natural selection and speciation leads to evolution.” Justify this statement. [CBSE Delhi 2017]
“Fossils are related to evolution”, justify this statement. Give the two ways by which age of the fossils can be estimated?
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.
Answer the following question :
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.
No we can not group them together because the structure of the eye in each of the organisms is different.
“Birds have evolved from reptiles”. State evidence to prove the statement.
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.
Mention three important features of fossils which help in the study of evolution.
[NCERT Exemplar]
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.
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.
Explain with the help of an example each, how the following provide evidences in favour of evolution:
[CBSE Delhi 2017]
Homologous organs
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.
Analogous organs
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.
Fossils
Fossils: They provide the missing link between the species, e.g., Fossils of dinosaurs with feathers or fossils of prehistoric horse, etc.
Describe the contribution of Lamarck.
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.
Name the organism Mendel used for his experiments. Explain about F1 and F2 progeny obtained by Mendel when he bred the tall and the short varieties of the organism he experimented with.
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 the F1 progeny. The progeny now obtained is F2. In F2 progeny, he found that 75% plants were tall and 25% were dwarf.
“It is possible that a trait is inherited but may not be expressed.” Give a suitable example to justify this statement.
[CBSE (F) 2015]
OR
With the help of an example justify the following statement: “A trait may be inherited, but may not be expressed”.
[CBSE (AI) 2016]
OR
How did Mendel explain that it is possible that a trait is inherited but not expressed in an organism?
[CBSE (AI) 2017]
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.
How did Mendel's experiments show that different traits are inherited independently? Explain.
[CBSE Delhi 2017]
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.
Name two homologous structures in vertebrates. Why are they so called? How do such organs help in understanding an evolutionary relationship?
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.
In a cross between plants with pink flowers and plants with white flowers the offsprings of F1 generation all had pink flowers. When the F1 generation was self-crossed, it was observed in the F2 generation that out of 100, 75 flowers were pink. Make a cross and answer the following:
What are the genotypes of the F1 progeny?
What is the ratio of Pink: White flowers in the F2 generation?
- Genotype of F1 progeny is Pp (Pink)
- Phenotypic ratio in F2 generation Pink : White
3 : 1
List three factors that provide evidences in favour of evolution in organisms and state the role of each in brief.
[CBSE (F) 2016]
Three factors that provide evidences are:
Analogous organs—organisms with similar looking organs may have different origin.
Homologous organs—organisms with apparently different looking organs may have similar origin.
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.
Does geographical isolation of individuals of a species lead to formation of a new species? Provide a suitable explanation.
[NCERT Exemplar]
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.
What is an organic evolution? It cannot be equated with progress. Explain with the help of a suitable example.
[CBSE (AI) 2017]
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.
Give an example of the characteristics being used to determine how close two species are in evolutionary terms.
[CBSE (F) 2017]
Study of homologous organs such as forelimbs of mammals, birds, reptiles and amphibian 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.
