CBSE Class 10 Science Heredity and Evolution Assignment Set E

Very Short Answer Type Questions :

Question. What is environmental selection?
Answer: It is the selection within a population resulting from the influence exerted by the environment. It leads to a change in the composition of genes within a population.

Question. What is genetic drift?
Answer: The change in the frequency of certain genes in a population over generations.

Question. What are fossils?
Answer: Preserved traces of living organisms.

Question. When a tall and dwarf pea plant were crossed, Fx showed only tall plants but in F₂ generation, some plants were dwarf. What does it mean?
Answer: It means reappearance of the dwarf character, a recessive trait in B’2 generation shows that the dwarf trait/ character was present in individuals of F* but it did not express (due to the present of tallness, a dominant trait / character)

Question. Why is the progeny always tall when a tall pea plant is crossed with a short pea plant?
Answer: The trait which represents the tallness in a pea plant is dominant over the another trait, shortness (dwarf).

Question. What are analogous organs?
Answer: The organs which have similar functions but different structures are called analogous organs.

Question. How many pairs of chromosomes do human beings have, specify the types of chromosomes also?
Answer: Human beings have 23 pairs of chromosomes the first 22 pairs are called autosomes are similar in males and females. The 23rd pair is called the sex chromosome. In males it is XY and in females it is XX.

Question. Define heredity.
Answer: Heredity deals with the inheritance of characters from one generation to the next.

Question. What are homologous organs? Explain with an example.
Answer: Homologous organs are those organs in different plants or animals which have the same basic structural design and origin but may have different functions. Example., hand of human and fore-limb of frog.

Question. “The chromosome number of the sexually producing parents and their offspring is the same”. Justify this statement.
Answer: In sexual reproduction, both the gametes (male and female) contain half the number of chromosomes (haploid or n) and by the fusion of these gametes, the zygote have full set (diploid 2n) chromosomes.

Question. How is the age of fossil determined?
Answer: The age of fossil fuels is determined through carbondating or time dating method.

Question. What is evolution?
Answer: Gradual unfolding of organisms from pre-existing organisms through change is called evolution.

Question. In a beetle population, the number of green beetles is more than blue and red beetles. Give a reason behind this situation.
Answer: Variation/Natural selection.

Question. The human hand, cat paw and the horse foot, when studied in detail show the same structure of bones and point towards a common origin.
a. What do you conclude from this?
b. What is the term given to such structures?
Answer: a. They have common origin/ancestory.
b. Homologous organs.

Question. What is the phenotypic ratio of a dihybrid cross in F2 generation?
Answer: 9 : 3 : 3 : 1.

Short Answer Type Questions :

Question. How is the sex of the child determined in human beings?
Answer: Human beings have 23 pairs of chromosomes. 22 pairs are autosomes and the 23rd pair is the sex chromosome. The males have XY and the females have XX. All the gametes formed in the females are of one type i.e., X. In males there are two types of sperms that are formed –X and Y. If the sperms having X fertilizes with the egg the zygote formed is XX. This will form female child. If the sperm having Y fertilizes with the egg then the zygote formed is XY and the offspring will be male child. So basically it’s the male gametes that decide the sex of the unborn child.

Question. What is meant by the term speciation? List four factors which could lead to speciation.
Answer: Speciation is the evolution of reproductive isolation among once interbreeding population.
Factors which can lead to speciation are:
1. Genetic drift: Over generations, genetic drift may accumulate which leads to speciation.
2. Natural selection: Natural selection may work differently in different location which may give rise to speciation.
3. Severe DNA change.
4. A variation may occur which does not allow sexual act between two groups.

Question. Define ‘evolution’. Describe Darwin’s theory of evolution.
Answer: Evolution is the sequence of gradual changes which take place in the primitive organisms over millions of years and new species are produced. Since, the evolution is of the living organisms, so it is called ‘Organic Evolution’.
Darwin’s theory of Evolution: Charles Robert Darwin gave the theory of evolution in his famous book, ‘The Origin of Species’. The theory of evolution proposed by Darwin is known as ‘The Theory of Natural Selection’. It is also called ‘Darwinism’.
According to Darwin’s theory of evolution:
1. There is natural variation within any population and some individuals have more favourable variations than others.
2. Population remains fairly constant even though all species produce a large number of off springs.
3. This is due to ‘competition’ or struggle for existence between same and different species.
4. The struggle for survival within population eliminates the unfit individuals and those with ‘favourable variations’ survive and pass on these variations to their progeny to continue. This is called natural selection.
5. The favourable variations are accumulated over a long time period leading to the origin of a new species.

Question. Define variation in relation to a species. Why is variation beneficial to the species? 
Answer: Variation refers to the differences in the characters or traits among the individuals of a species.
Variations are beneficial to the species because:
1. They enable the organisms to adapt themselves in changing environment.
2. Variations form the basis of heredity.
3. They form the raw materials for evolution and development of new species.

Question. Distinguish between homologous organs and analogous organs. In which category ; would you place wings of a bird and wings of a bat? Justify your answer giving a suitable reason.
Answer: 

Wings of a bird and wings of a bat are analogous organs as they have different basic structural design but have similar appearance and perform similar functions.

Question. Define the term ‘evolution’. “Evolution cannot be equated with progress”. Justify this statement.
Answer: Evolution is the sequence of gradual change which takes place in primitive organisms over millions of years and new organisms are formed. Evolution cannot be equated to progress from lower forms to higher forms. It seems to have given rise to more complex body designs even while the simpler body designs continue to flourish. For example, human beings have not evolved from chimpanzees, but both have common ancestor.

Question. Some of the traits are acquired while others are inherited. Explain. Which of two will not be passed on to next generation?
Answer: Acquired traits are those features which are present in an organism due to experiences during its life time, for example, swimming and knowledge of a language in human beings. Inherited traits are those features which are transferred from one generation to the next through DNA or genes present in the cells of a reproductive tissue. For example, colour of eyes and hair in human beings. Traits acquired during the life time of an individual can not be inherited because the information of this feature is not ingrained in the DNA of individual.

Question. How are fossils formed? Describe, in brief, two methods of determining the age of fossils.
Answer: When organisms die, their bodies decompose due to action of micro organisms. However, sometime the body or at least some parts of the body may be in such an environment that does not let it decompose completely. All such preserved traces of living organisms are called fossils.
The age of fossils can be estimated by the following two methods:
1. If we dig into the earth and start finding fossils, it can be assumed that the fossils closer to the surface are more recent to those found in deeper layers.
2. By detecting the ratios of different isotopes of the same element in the fossil material.

Question.”An individual cannot pass on to its progeny the experiences of its lifetime.” Justify the statement with the help of an example and also give reason for the same. 
Answer: Experience achieved during the lifetime of an individual does not make any change in the gene of the individual.
For example, if a person reads a book on birds, the knowledge he earns by reading the book does not make any change in the gene, hence, this knowledge will not get automatically transmitted to his next generation. Such a trait is called acquired trait.

Question. State the meaning of inherited traits and acquired traits. Which of the two is not passed on to the next generation? Explain with the help of an example.
Answer: Inherited traits are the characteristics transmitted from parents to their offspring.Acquired traits are characteristics which are developed during the lifetime of an individual.Acquired traits are not passed on to the next generation. For example, if we breed a group of mice, all their progeny will have tails. Now, if the tails of these mice are removed by surgery and allowed to breed, the next generation mice will also have tails. If these tails are also removed and allowed to breed, the progeny of mice will again have tails. Removal of tail by surgery is an acquired trait and do not change the genes of germ cells and hence, are not passed on to the next generation.

Question. How do Mendel’s experiments show that the a. traits may be dominant or recessive, b. traits are inherited independently?
Answer: a. 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.
b. When Mendel conducted a dihybrid cross having two sets of characters, he obtained only one set of parental characters in F1 generation whereas in F2 generation he obtained both the set of parental characters now recombined in the ratio of 9 : 3 : 3 : 1.
The appearance of new recombinants in the F2 generation along with parental type shows that traits are inherited independently.

Question. An angiosperm plant having red coloured flowers when crossed with the other having the same colour produced 40 progenies, out of which 30 plants were with red coloured flowers, 10 plants were with white coloured flowers. Find out:
a. What is the possible genotype of parent plants?
b. Which trait is dominant and recessive?
c. What is this cross called as and what is its phenotypic ratio?
Answer: a. Rr and Rr.
b. Red colour of flowers is the dominant trait while white colour is the recessive trait.
c. Monohybrid cross, phenotypic ratio is 3 : 1.

Question. What is the difference between homologous and analogous organs? Give one example each.
Answer: a. Homologous organs: Structures in animals share a common ancestry. The structure, which has similar basic structure and developmental origin but perform different functions.
For example: Fore limbs in vertebrates.
Homologous organs give an evidence of common ancestory and evolutionary relationship between apparently different species.
b. Analogous organs: Analogous organ are the organs in different organisms which have different basic structure but have similarity in shape and function. However, these organs do not provide an evidence in support with common ancestory, e.g., Wings of bat and bird.

Question. What is speciation? Discuss any two factors that lead to speciation.
Answer: Speciation means creation of new species from pre exiting ones.
The two main factors which could lead to the formation of a new species are natural selection leading to accumulation of adaptation in geographically separated populations of a species and development of reproductive barrier among them.
Complete separation of sub-populations of a spices (Genetic drift) and difference in way of Natural selection in them for many generations results in speciation i.e. formation of new species that cannot interbreed under natural conditions.

Question. What is DNA copying? State its importance.
Answer: A process where a DNA molecule produces two similar copies of itself in a reproducing cell.
Importance –
a. It makes possible the transmission of characters from parents to the next generation.
b. It causes variation in the population.

Question. (a) Which type of organs are shown in the figure below?

(b) Which type of origin and structure do these organs have?
Answer: a. These are organs which help in flying (wings of insects and wings of birds).
b. The structure and components of these wings are different. They look similar because they have a common use for flying, but their origins are not common, i.e., they are analogous.

Question. Give five points to show the significance of variations.
Answer: a. Variations help an organism to get adapted to the changing environment.
b. Variations lead to evolution through natural selection and adaptation.
c. During sudden change in the environment only those variants will make the population of that particular species which can withstand such changes.
d. Variations result in more genetic vigour, i.e., organism emerges as a strong species maintaining its large population.
e. Variations make an organism to be resistance to diseases and environmental fluctuations.

Question. (a) Identify the organism shown in the above figure.

(b) Name one incipient feature selected by the nature.
(c) Mention any other primitive feature of birds.
Answer: a. Planaria.
b. Eyes which were there only for detecting light.
c. Birds developed feathers for insulation.

Question. If we cross pure-bred tall (dominant) pea plant with pure-bred dwarf (recessive) pea plant we will get pea plants of F₁ generation. If we now self-cross the pea plant of F₂ generation, then we obtain pea plants of F₂ generation.
(a) What do the plants of F₂ generation look like?
(b) State the ratio of tall plants to dwarf plants in F2 generation.
(c) State the type of plants not found in F₂ generation but appeared in F₂ generation, mentioning the reason for the same.
Answer: (a) All plants of F₁ generation will be tall plants.
(b) 3:1
(c) Dwarf trait is recessive trait which was not expressed in the F₁ generation, the recessive trait gets expressed in the F₂ generation after self pollination.

Question. Give one example each of characters that are inherited and the ones that are acquired in humans. Mention the difference between the inherited and the acquired characters.
Answer: Eye colour or hair colour of a person is an example of inherited character whereas, body weight is an example of acquired character.
The basic difference between inherited and acquired character is that inherited character is passed on from parent to offspring and acquired characters are acquired by an individual during his lifetime depending upon his lifestyle.