Class 10 Science Chapter 8 Previous Year Questions - Heredity and Evolution

Ans: (b)

The sex of a child is determined by the father, as the sperm can carry either an X or Y chromosome, while the mother's egg always carries an X chromosome (statement ii).

The sex of a child is determined at the time of fertilization when the male and female gametes (sperm and egg) fuse to form a zygote (statement iv).

Ans: (i)  

• In F₁ generation, all plants were tall / No short plants were observed
• No medium height plants / No halfway characteristics were observed / Only dominant parental traits were seen and not the mixture of the two.

(ii)
(iii) (A) Self-pollination / Self-fertilisation / Selfing of F₁ plants

Traits are inherited independently.
OR
(iii) (B) If pea plants with yellow seeds are crossed with plants of green seeds, it is found that in F₁ generation all the plants have yellow seeds. When F₁ plants are self-pollinated, it is found that in F₂ generation, plants with yellow seeds and plants with green seeds are obtained. This shows that both the traits are inherited but only one trait is visible in F₁ progeny while the other remains unexpressed. 

Ans: (b)
The answer is (b) because both the assertion and reason are true, but they explain different things. The human female has two X chromosomes (a perfect pair), and while it's true that the male's sperm decides the sex of the child, this fact is not directly explaining the assertion about the female's sex chromosomes.

Ans: (c)

The 9:3:3:1 ratio occurs when two traits are inherited independently, as in Mendel’s dihybrid cross experiment. It represents the combinations of dominant and recessive traits for both seed shape and color in the F₂ generation.

Ans: (a) 2 visible characters of garden pea plants are: 

• Tallness (dominant) , Dwarfness (recessive) 
• Yellow seeds (dominant) , Green seeds (recessive)

OR
(b)

Ans: Example: A population of bacteria living in temperate waters that can withstand heat due to the rise in temperature due to global warming will survive better in a heat wave than the non-variant bacteria having no capacity to tolerate heat wave. Thus, suitable variations promote survival.

Ans:
 75% yellow seeds

Ans: (a) All Plants Tall
Gene combination: Tt
(b) It is a recessive trait / it cannot be expressed in presence of dominant trait.
(c) Tall: Short 3 : 1
Conclusion: Tall trait is dominant and short trait is recessive. 

Ans: (c)

In the cross Tt (tall) and tt (short), 50% of the offspring inherit Tt (tall) and 50% inherit tt (short). This results in a 1:1 ratio of tall to short plants, explaining why option (c) is the correct answer.

Ans: (a)

The parental plants must be Tt (heterozygous tall) and Tt (also heterozygous tall) because this combination can produce offspring with both tall (TT and Tt) and dwarf (tt) plants. In this case, the expected ratio would be 3 tall to 1 dwarf, which explains the presence of dwarf plants among the offspring.

Ans: (a)

In this cross, the parent with violet flowers (VV) is homozygous dominant, while the white flower parent is homozygous recessive (ww). When they are crossed, all offspring in the F₁ generation will have violet flowers (Vw). If F₁ plants are crossed with each other, the F₂ generation will have a 1:1 ratio of violet (Vw) to white (ww) flowers, making option (a) the correct answer.

Ans: (d)
The assertion (A) is false because the progeny with the gene combinations Bb (black eyes) and bb (brown eyes) would not have black eyes; only BB would. However, the reason (R) is true because the black color is indeed a dominant trait. Therefore, the correct answer is (d), which states that (A) is false but (R) is true.

Ans: (c)
In the given statements, the trait that expresses itself in the F₁ generation is called a "dominant trait," while the trait that can be passed down through generations, even if it doesn't show up in every generation, is referred to as an "inherited trait." Therefore, the correct answer is (c) "Dominant trait; Inherited trait."

Ans: (c)
Sol: Variations get accumulated or discarded as combined effect of environmental factors and reproduction process.

Ans:  (i) In sexual reproduction offspring of human being is not a true copy of his parents because it inherits half of its genetic material from each parent. During the formation of gametes (sperm and egg cells), the genetic material undergoes recombination that leads to shuffling or mixing of genetic material from both parents, resulting in offspring with unique combination of genes.
(ii) F₁ generation is the first filial generation of the offspring from the parents whereas F₂ generation is the second filial generation of the offspring, generated through inbreeding of F₁ generation. F₁ generation can be distinctly different from the parental type whereas F₂ generation always exhibit some parental genotypes.
(iii) Variations allow genetic diversity that is important for the development of new traits and increasing adaptability towards changes in the environment. It makes the organisms better adapted for survival under changing conditions.

Ans: (a) Sex of child is determined by what it inherits from the father. A child who inherits X chromosome from her father will be a girl and one who inherits a Y chromosome from father will be a boy.
(b) Human beings have 22 pairs of autosomes and one pair of sex chromosomes. The females possess two homomorphic sex chromosomes, named XX while males contain two heteromorphic sex chromosomes, i.e., XY. The Y chromosome is shorter than X chromosome. Therefore a pair of sex chromosomes in human beings is called mismatched pair in terms of type and size.
(c) Gametes contain half the number of chromosomes of parent. But, when the two gametes (male and female) fuse to form the zygote, the normal diploid condition is restored. Hence, the formation of gametes by meiosis and fusion of male and female gamete help to maintain the number of chromosomes in organism.

OR

(c) Two animals in which sex is not determined genetically are turtle and crocodile. The type of sex ir them is determined by environmental factors. In turtles the temperature of egg incubation has a significant effect on the sex of developing embryos. Males are predominant below 28°C, females above 33°C and equa number of the two sexes between 28-33°C. In crocodiles high temperature induces maleness and low temperature femaleness.

Ans: The statement "Sex of the children will be determined by what they inherit from their father" is based on the principle of genetic inheritance. The determination of an individual's sex is influenced by the combination of sex chromosomes inherited from both parents.
In humans, females have two X chromosomes (XX) while males have one X and one Y chromosome (XY). The father contributes either an X or a Y chromosome to the offspring, while the mother always contributes an X chromosome. Therefore, it is the father's genetic contribution that ultimately determines the sex of the child.
During fertilization, if the father's sperm carries an X chromosome, the resulting combination of XX chromosomes will develop into a female child. On the other hand, if the father's sperm carries a Y chromosome, the combination of XY chromosomes will lead to the development of a male child.
It is important to note that the sex of the child is determined by the father's genetic contribution, but other factors such as environmental influences and chance also play a role in the development of an individual's sex characteristics. Therefore, while the father's genetic contribution is a significant factor, it is not the sole determinant of a child's sex.

Ans: Human female (XX) produces all gametes (ova) with X-chromosomes, while human male (XY) produces 50% gametes (sperms) with X-chromosome while 50% gametes with Y-chromosome.
If sperm having X chromosome fertilises the ovum with X chromosome then a female child will be produced, otherwise a male child will be produced.

Sex of the child (offspring) is determined by the type of sperm that fuses with ovum at the time of fertilisation. Therefore, there is 50% chance of a male child being born and a 50% chance of a female child being born.

Ans: Variation is the degree of differences in the progeny (off springs) and between the progeny and parents. Two main reasons of variations are mutations and genetic recombination during sexual reproduction.

Ans: (a) The two types of gametes produced by men are sperm cells (spermatozoa) and X or Y sex chromosomes. Sperm cells carry either an X or a Y sex chromosome, determining the sex of the offspring upon fertilization with a female's egg. 
(b) No, a male child does not inherit an X chromosome from his father. In humans, sex determination is based on the X and Y chromosomes. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). When a child is conceived, they inherit one sex chromosome from each of their parents. The mother always passes on an X chromosome, while the father can pass on either an X or Y chromosome. If the father passes on an X chromosome, the child will be female (XX), and if the father passes on a Y chromosome, the child will be male (XY).
(c) A human female typically produces one type of gamete, which is the egg or ovum.

Ans: (a) X chromosome is morphologically distinct from Y chromosome. Y chromosome is smaller than X chromosome. Hence, they are dissimilar or heteromorphic. Men have mismatched pair of sex chromosome in humans in which one is normal size X while other is a short one called Y.
(b) Human beings have 22 pair of autosomal chromosomes and one pair of sex chromosome. Women have a perfect pair of sex chromosomes both called X.
(c) (i) Sex of the individual is not always determined genetically. In some organisms, gender may be determined by environmental factors. For example snails, turtles and lizards sex is determined by the temperature at which fertilised egg are kept.

Ans: (i) In the F₁ progeny, the plants obtained will have violet flowers.
(ii) If the plants of F₁ progeny are self-pollinated, the plants obtained in the F₂ progeny will show a phenotypic ratio of 3:1, with 75% of the plants having violet flowers and 25% having white flowers.
(iii) If 100 plants are produced in the F₁ progeny, approximately 25 plants will show the recessive trait of white flowers.

Ans: (a) F₁ progeny will have green stemmed tomato plants as green is dominant over purple stemmed tomato plants. 
(b) If F₁ plants are self pollinated, then the percentage of purple stemmed plant in F₂ progeny will be 25%.

(c) Ratio of GG and gg plant in F₂ generation will be 1:1.

Ans: (a) Human beings have 22 pairs of autosomes an one pair of sex chromosomes. The females possess tw homomorphic sex chromosomes, called XX while male have a mismatched pair in which one is a normal sized while the other is a short-one called Y.
(b) Gametes contain half the number of chromosome But, when the gametes fuse to form the zygote, the normal diploid condition is restored. Hence, the formation of gametes by meiosis helps to maintain the number of chromosomes in organism. (c) Two animals in which sex is not determined genetically are turtle and crocodile. The type of sex in them is determined by environmental factors. In turtles, the temperature of egg incubation has a significant effect on the sex of developing embryos. Males are predominant below 28°C, females above 33°C and equal number of the two sexes between 28-33°C. In crocodiles, high temperature induces maleness and low temperature induces femaleness.

OR

(c) Flow chart of sex determination in human beings is as follows

Ans: (a) The F₁ progeny will be tall plant with round seeds.
(b) Recessive traits are short plant and wrinkled seed.
(c) The new combinations will be tall plant with wrinkled seeds and short plant with round seeds. The ratio will be 9 : 3 : 3 : 1:: Tall plant with round seed : Tall plant with wrinkled seeds: Short plant with round seed : Short plant with wrinkled seed.

OR

(c) (i) The F₂ ratio will be 9: 3: 3:1:: Tall plant with round seed : Tall plant with wrinkled seeds : Short plant with round seed : Short plant with wrinkled seed.
Tall plant with round seeds = 9/16 x 1600 = 900
(ii) Short plant with wrinkled seed = 1/16 x 1600 = 100

Ans: The inheritance of characters (or traits) from the parents to their off springs is called heredity.

Ans: In human beings, 23 pairs of chromosomes are present in each cell. Out of 23 pairs, 22 pairs of chromosomes carry genes which control somatic traits, these chromosomes are called autosomes. The 23rd pair is called sex chromosomes.

Ans: (a) Mendel carried out crosses with two traits to see the interaction and basis of inheritance between them. In a dihybrid cross given by Mendel, it was observed that when two pairs of characters were considered each trait expressed independent of the other.

(b) For Example, a cross between round yellow and wrinkled green parents.

In F₁ generation, all plants are with round yellow seeds. But in F₂ generation, we find all types of plants : Round yellow, Round green, Wrinkled yellow, Wrinkled green.
F₂ generation ratio : Round-yellow = 9 : Round- green = 3 : Colour of stem in F₁ progeny Wrinkled- yellow = 3 : Wrinkled-green = 1

Ans: (a) (i) Colour of stem in F₁ progeny:

The colour in the F₁ progeny is green stemmed as green stem colour is dominant.

(ii) F₁ progeny on self pollination:

F₂ generation Green stemmed: Brown stemmed
14 or 25% of F₂ progeny are brown stemmed rose plant.

(iii) Ratio of GG and Gg in F₂ progeny:
Genotype of F₂ progeny – GG : Gg
1 : 2

(b) This is a monohybrid cross. This shows that out of two contrasting traits only one dominant trait appears in F₁ generation and the trait which does not express is recessive. On selfing the F₁ plants, both the traits appear in next generation but in a definite proportion.

Ans: Genetics is the branch of biology that deals with the study of heredity and variations. The term genetics’ was coined by William Bateson in 1906. When a population is small, the number and scope of variations is limited and hence diversity and traits are reduced. Small numbers of surviving tigers are a cause of worry from the point of genetics because of the following reasons:

• Their loss would cause a loss of gene pool, i.e., many genes will be eliminated from a gene pool.
• Tigers are surviving in limited numbers, so if some natural calamity kills these small population of tigers, they will suddenly become extinct as per genetic drift phenomenon,
• A disease may wipe out the leftover population, if the entire population is susceptible to the disease. This can cause sudden extinction of the tiger species and loss of their genes forever, thus, adversely affecting the diversity of nature.

Ans: Mendel selected garden pea (Pisum sativum) for his series of hybridization experiments.
He first selected two pure line plants (tall plant having gene TT and short plant having gene tt) and then crossed such plants having contrasting characters. In the F₁ generation, he observed that only one of the two contrasting character appeared, he called this character as dominant and the one which does not get expressed in F₁ was called as recessive. He later self pollinated the F₁ plants and observed that both the traits appear in next generation but in a definite proportion. This can be explained by the following cross :So, the plants of F₁ generation will be all tall plants and after self pollinating the ratio of tall and dwarf plants that Mendel obtained in F₂ generation plants is 3 : 1.

Ans:

Ans: (а) Homologous organs are those organs in different groups of organisms, which are similar in their basic structure/anatomy but are different in their functions. Such a similarity indicates that they are inherited from a common ancestor and the two species are closely related. For example, forelimbs of vertebrates like humans, wings of binds.
(b) Analogous organs are those organs/structures in different groups of organisms, that are similar in their function, but are dissimilar in their basic structural plan design and origin. Such organs do not indicate common ancestry of the species. For example, wings of birds and those of bats.
(c) Fossils are the preserved traces of organisms that lived in the past. Fossils indicate the time periods when the different groups of organisms lived in the earth. For example, Dinosaurs are reptilian: fossils, some of which show resemblance to birds in having feathers.

Ans: (a) Homologous organs are those organs in different groups of organism which are similar in their basic structure, but are modified to perform different functions.
e.g. forelimbs of mammals, and those of reptiles and amphibians.
(b) Sex of a child depends on what happens during fertilization:

(i) The female gamete, ova always contributes an X chromosome during fertilization.
(ii) The male gamete, sperm contributes either X or Y chromosome during fertilization. Whether sperm will contribute the X chromosome or Y chromosome is a matter of chance and the man does not have any control on it.
(iii) If a sperm carrying X chromosome fertilizes an egg which always carries a X chromosome, then the child born will be a girl. But if a sperm carrying Y chromosome fertilizes an egg which always carries X chromosome, then the child born will be a boy.
(iv) Thus, sex of a new born child is a matter of chance and none of the parents may be considered responsible for it.

Ans: According to the Mendelian experiment, violet colour (W) is a dominant trait while white colour (vv) is a recessive trait. Hence, the colour of the flower in F₁ progeny will be violet (Vv).

Ans: Human beings have artificially selected certain variants that arose in nature by chance. This led to evolution of different species.
For example, wild cabbage was cultivated and its variants were selected due to different advantages, by artificial selection.
(а) Short distances between leaves - led to formation of modem day cabbage.
(b) Arrested flower development - Broccoli.
(c) Sterile flowers - Cauliflower
(d) Swollen parts - Kohlrabi
(e) Large leaves - Kale

Ans: It can be done in two ways:
(а) The fossils found in upper layers are recent and the ones that are deeper are older.
(b) By radio-active carbon dating.

Ans:
(a) Variation refers to the differences in the characteristics among the individuals of a species.
1. Variation is created in a population by:

(i) Errors in DNA copying
(ii) Recombination during reproduction

2. In case of a drastic change in the environment of the niche of the population, at least some variants would have chances of survival.
(b)
1. There are special lineages of cells in specialized organs in multicellular organisms.
2. Such cells undergo a special type of cell division, called meiosis, and the germ cells (gametes) formed have only half the amount of chromosomes as the parent cell.
3. When two such germ cells (with half the number of chromosomes) fuse, a zygote/ new individual is formed with the reestablishment of the number of chromosomes as in parent organism.

Ans: The traits that a person acquires during one’s life time and not by virtue of his/her genes, are known as acquired traits.
These traits are not present on our genes in reproductive cells. Hence they cannot be inherited.
Since these changes are in the non-reproductive cells and thus cannot be passed on to the germ-cells, hence are not inheritable by the future generations. An organism acquires them, for himself, in his life time, due to his environment/experiences.

Ans:  Speciation is the formation of new species from the pre-existing ones, due to accumulation of changes in such a way that two groups of individuals of one species can no longer interbreed. This results in formation of two new, independent species.
Factors responsible for speciation are:
(i) Natural selection - Nature selects due to survival advantage.
(ii) Genetic drift - Due to accumulation of changes over generations in sub-population.
(iii) Geographical isolation - In different geographical locations, the natural selection acts differently on different sub-population because of varying abiotic/biotic factors.
(iv) Mutation - Mutation is the change in the DNA. The large mutation in DNA can also result in speciation.

Ans: Let us study the example of beetles.  Let the original beetle population be red. If due to variation a green beetle was produced it would have survival advantage over red beetle. Thus green beetles would be naturally selected, and will grow in number.
If few of these green beetles move to some other area, adapt to changed environment and after few generations, would vary greatly from the original population of red beetles. These two populations may not be able to interbreed due to accumulation of variation.
Thus, due to Natural selection and Genetic Drift, a new species of beetles is formed.

Ans:  Homologous organs are those organs in different groups of organism which are similar in their basic structure, but are modified to perform different functions.

Example: Forelimbs of mammals, and those of Reptiles and Amphibians.
2. The wings of a butterfly and those of a bat cannot be considered as homologous, because they have a common function (flying), but their origin and basic structure are different.
3. They are analogous organs because they have a common function, though the basic structure is different.

Ans:
1. We group organisms into groups based on the similarities in their characteristics.
2. Certain basic characteristics are shared by most or all the organisms, like the cell is the basic unit of life in all organisms.
3. The characteristics at the next level of classification would be shared by most organisms, but not by all organisms.
4. By taking the fundamental design differences, a hierarchy is developed that allows making of classification groups.
5. We can work out the evolutionary relationships of the species by identifying the hierarchies of characteristics between them.
6. The more characteristics two species will have in common, the more closely related they are.
7. The more closely related the two species, they would have had a recent common ancestor.
8. Thus, classification of species is a reflection of their evolutionary relationship.

Ans:
(i) The plants of F₁ generation will be tall like the dominant parent.
(ii) Tall plants 3 : Dwarf plants 1, i.e., 3 : 1.
(iii) Dwarf plants are not found in F₁ generation.
It is because, when two copies of a gene (alleles) exist together in the F₁ plants, only the trait; tallness is expressed, i.e. it is dominant.
The other trait dwarfness remains hidden as it is a recessive trait.

Ans:
1.  Mendel crossed a tall pea plant with a short pea plant.
2. All the plants produced in the F₁ generation were tall.
3. When the F₁ tall plants were self-pollinated, the F₂ generation consisted of both tall and short plants.
4.  It explains that the dominant trait expresses itself in the F₁ plants, where the recessive trait (shortness) is hidden.
5.  The appearance of short plants in the F₂ indicates that the trait shortness has been inherited by the F₁ plants, but not expressed.

Ans: Any deviation from original trait in a species is variation.
Sometimes these variations may give added advantage of being of the nature so as to give better adaptability in changed conditions. Hence, it increases the chances of survival.
Smaller population of species would enforce inbreeding within the population which would result in fewer variations. This would also result in inbreeding depression and expression of recessive traits, thereby making it prone to diseases or extinction.
Hence, environmentalists are worried about small population size.

Ans: The typical characters present in all individuals of a species are said to be trait of that species.

Ans: Evidences in favour of evolution are:
(i) Homologous organs: Such organs which perform different functions but have similar structure and origin are called homologous organs. For example, forelimbs of bird, forelimb of man and frog perform different functions, but have similar basic structure. Presence of such organs indicate that all these vertebrates had common ancestors.
Skeleton of forelimbs of (a) Human (b) Dog  (c) Bird (d) Whale showing homologous features.

 (ii) Analogous organs: Such organs which perform similar functions but are structurally different are called analogous organs. For example, wings of a bird and wing of an insect. Presence of such organs show that these organisms have different origin.

(iii) Evidences from embryology: Early embryos of different vertebrates show striking similarities such as presence of tail. This indicates common origin and ancestry of different vertebrates.

(iv) Vestigial organs : These are the organs which appear functionless in one organism and functional in some others. For example, Vermiform appendix of the large intestine is nonfunctional in human beings but functional in herbivorous, ruminant animals. Presence of such organs also show common ancestry.
(v) Evidences from fossils : Archaeopteryx a fossil that resembles reptiles but has some bird like features. This shows that birds have been evolved from reptiles.
The dinosaur skull fossil shown was found only a few years ago in the Narmada valley.

Ans: Homologous organs are those organs in different groups of organism which are similar in their basic structure, but are modified to perform different functions.

Example: Forelimbs of mammals, and those of Reptiles and Amphibians.
2. The wings of a butterfly and those of a bat cannot be considered as homologous, because they have a common function (flying), but their origin and basic structure are different.
3. They are analogous organs because they have a common function, though the basic structure is different.

Ans:
1. We group organisms into groups based on the similarities in their characteristics.
2. Certain basic characteristics are shared by most or all the organisms, like the cell is the basic unit of life in all organisms.
3. The characteristics at the next level of classification would be shared by most organisms, but not by all organisms.
4. By taking the fundamental design differences, a hierarchy is developed that allows making of classification groups.
5. We can work out the evolutionary relationships of the species by identifying the hierarchies of characteristics between them.
6. The more characteristics two species will have in common, the more closely related they are.
7. The more closely related the two species, they would have had a recent common ancestor.
8. Thus, classification of species is a reflection of their evolutionary relationship.

Ans:
(i) The plants of F₁ generation will be tall like the dominant parent.
(ii) Tall plants 3 : Dwarf plants 1, i.e., 3 : 1.
(iii) Dwarf plants are not found in F₁ generation.
It is because, when two copies of a gene (alleles) exist together in the F₁ plants, only the trait; tallness is expressed, i.e. it is dominant.
The other trait dwarfness remains hidden as it is a recessive trait.

Ans:
1.  Mendel crossed a tall pea plant with a short pea plant.
2. All the plants produced in the F₁ generation were tall.
3. When the F₁ tall plants were self-pollinated, the F₂ generation consisted of both tall and short plants.
4.  It explains that the dominant trait expresses itself in the F₁ plants, where the recessive trait (shortness) is hidden.
5.  The appearance of short plants in the F₂ indicates that the trait shortness has been inherited by the F₁ plants, but not expressed.

Ans:
(a) No, the wing of a butterfly and the wing of a bat cannot be considered homologous organs because they have a common function of flying but their origin and basic structural designs are not common. So, they are analogous organs.
(b) Speciation refers to the phenomenon in which new species are formed form the existing species.
The factors leading to speciation are: (i) genetic drift and (ii) natural selection
(c) (i) Kohlrabi
(ii) Cauliflower
(iii) Broccoli
(iv) Kale

Ans: Mendel’s Experiments on Inheritance of Traits. Mendel used a number of visible contrasting characters of garden pea like round/wrinkled seeds, tall/short plants, white/violet flowers, etc.

Independent inheritance of two separate traits, shape and colour of seeds(i) Traits may be dominant or recessive:

• Mendel used a number of visible contrasting pairs of characters in garden pea.
• He made crosses between pea plants with different characters; there were no halfway or intermediate characters.
• Only one of the parental traits appeared in the F₁ generation; it is called dominant trait and the trait which remains hidden, is called recessive trait.
• When the F₁ plants were self-pollinated, the F₂ progeny consists of plants with the dominant trait and recessive trait in the ratio of 3 : 1; it proves that traits may be dominant or recessive.

(ii) Traits are inherited independently:

• When a cross is made between a tall plant with round seeds, (when inheritance of two traits is considered), with a short plant with wrinkled seeds, the F1 progeny plants were all tall with round plants.
• When the F₁ plants are self-pollinated, the F₂ progeny consisted of some tall plants with round seeds and some short plants with wrinkled seeds; these two are the parental types of combinations of traits.
• There were also some new combinations like tall plants with wrinkled seeds and short plants with round seeds.
• Thus it is clear that the tall and short traits and round and wrinkled seed traits are inherited independently of each other.

Ans: Speciation - It is the process of formation of new species from an existing one. Factors that can lead to speciation are
(i) Natural selection.
(ii) Geographical isolation.
(iii) Migration - Genetic drift.

Ans:

75% of the plants in F₂ generation were round in Mendel’s cross between round and wrinkled pea plants.

Ans: Mendel performed an experiment in which he took two different traits like tall and dwarf plant and round and wrinkled seeds. In second (F₂) generation, some plants were tall with round seeds and some were dwarf with wrinkled seeds. There would also be dwarf plants having round seeds. Thus, the tall/short traits and round/wrinkled seed traits are independently inherited.

Ans: (a) Even after cutting tail of a mouse its progeny continues to have tail. This is because ‘no tail’ is an acquired trait. The mouse continues to have information for presence of tail in its DNA and hence the progeny will have tail.
(b) Archaeopteryx has reptilian features as presence of tail, vertebra, teeth etc.

Ans:
(i) Homologous structures in vertebrates are wings in birds and forelimbs of lizard.
(ii) They are so named as they have same structural design but different function.
(iii) Such structures give us idea about common ancestry.

Ans:  Two uses of fossils are:
(а) To help study evolution of plants and animals.
(b) To know past climatic conditions.
(c) To help calculate geological time etc. (Any two)
Evidence in favour of organic evolution:
(i) Fossils help to identify an evolutionary relationship between apparently different species.
(ii) The older fossils, present deeper, are simpler in body design, as compared to those present in upper layers which are more recent.
This clearly provides evidence in favour of organic evolution.

Ans: Deoxyribonucleic Acid (DNA) is a molecule which carry the hereditary characters or traits in a coded form from one generation to the next in all the organisms.

Ans: A trait may be inherited, but if it is recessive, it will not be expressed unless it is homozygous. Example:
Hence, genotype Tt with a recessive gene is expressed as phenotype of Tall. This shows that only dominant gene is expressed as the trait (T) while ‘t’ is not expressed.

Ans: Chromosomes are long DNA strands, presents in nucleus, carrying genes which code for a trait. Hence, they are the hereditary material.
In an organism, each cell has two copies of a chromosome, one each from a male and female parent. In sexually reproducing organisms, where fusion of gametes (germ-cells) takes place, one chromosome from each pair is taken up in formation of a germ-cell (these may be either maternal or paternal in origin) by a special cell division called meiosis. When two germ cells fuse, they restore the original number of chromosomes in the progeny.

Ans: The inbuilt tendency of variation, either due to errors in DNA copying or due to sexual reproduction, both result in some changes in the existing population of an organism. This continuous change ultimately leads to Evolution.
It occurs due to changes in DNA which keep accumulating over generations, ultimately giving rise to new species - Natural selection, genetic drift, mutation etc.
Fossils provide evidence for evolution as they are preserved traces of once living organisms. The fossils are formed when on death of an organism, the body doesn’t decompose, instead it gets trapped in the environment where it gets preserved. For example, an organism getting trapped in volcanic lava will not decompose. On cooling the lava will harden and retain the impression of the body parts of that organisms, as fossil.

Ans: Speciation: Speciation is the evolution of reproductive isolation among once-interbreeding population. Factors which can lead to speciation are:
(i) Genetic drift: Over generation, genetic drift may lead to the accumulation of different changes which lead to speciation.
(ii) Natural selection: Natural selection may work differently in different location which may give rise to speciation.
(iii) Severe DNA change: Variations during of DNA copying often leads to speciation.
(iv) A variation may occur which does not allow sexual act between two groups.
Out of these variation, severe DNA change is not a major factor in the speciation of a self- pollinating plant species, because:
(i) Variation is the differences in the characters among the individuals of species. In self- pollinating species, pollen grains fall on the stigma of the same flower or another flower of the same plant. Since self pollination is taking place in the same plant, so changes among the flowers of the same plant is negligible and hence variation in self-pollinating plant do not have any major effect in speciation of a self- pollinating plant.
(ii) Due to severe DNA change, individuals may vary from each other. In case of self-pollinating plants, pollination take place within the same plant and hence severe DNA change among individual plants do not have any major impact.

Ans:
(a) Natural selection - It is selection of certain traits in nature in an individual in a population of a particular species.
This leads to survival advantage and hence variation leading ultimately to speciation.
(b) Thom of Bougainvillea and tendril of Passiflora both are modified stem, i.e., both have similar structure, but different function.
Thorn of Bougainvillea protects plants from being grazed while tendril of Passiflora helps the plants to climb up a support.
Hence, they are homologous organs.

Ans: Experiences of life and qualifications we earn do not make any change in the genes of the individual. Changes made in the gene are only passed on from one generation to the next. These qualities are acquired by an individual in his life, and are called acquired traits which cannot be passed on to future progeny. For example, if a person reads a book on birds, the knowledge he earns by reading the book does not make any change in his genes. Hence, this knowledge will not get automatically transmitted to his next generation.

Ans:
(a) All flowers in F₁ progeny will be blue in colour.
(b) When F₁ progeny are self pollinated, 25% of the flowers in F₂ progeny will be white.
(c) Expected ratio of the genotype BB and Bw will be 1:2.

Ans: The statement “It is possible that a trait is inherited but may not be expressed” can be explained with the help of Mendel’s experiment on pea plant with one visible contrasting character.
Mendel took pure breeding pea plant with one visible contrasting character viz, height of the plant (tall and short plant). The pure breed tall and short plant were crossed and it was found that all the plants in the F₁ progeny were tall. Mendel then allowed the F₁ progeny plants for self-pollination. It was found that all the F₂ progeny plants are not tall, some are short. This indicates that both tallness and shortness traits were inherited separately in the F₁ progeny but shortness trait was not expressed in the F₁ progeny.

Ans:
(i) Yes, eyes can be grouped together, which have evolved over generation from imperfect eyes in Planaria to perfect eyes in vertebrates.
(ii) Dinosaur is a type of reptile which has wings. Birds also have wings, so it can be opined that birds have evolved from reptiles.

Ans:
(a) Speciation: It is the evolution of reproductive isolation among once-interbreeding populations, i.e. the development of one or more species from an existing species.
(b) Natural Selection: It is the process, according to Darwin, which brings about the evolution of new species of animals and plants.

• It was noted that the size of any population tends to remain constant despite the fact that more off-springs are produced than are needed to maintain.
• Darwin found that variations existed between individuals of the population and concluded that disease, competition and other forces acting on the population eliminated those individuals which are less well-adapted to their environment.
• The surviving population would pass the hereditary advantageous characteristics to their off-springs.

Ans:
(a) Natural selection - Nature selects the best traits in a species, leading to survival of fittest and evolution of species. This phenomenon is known as natural selection.
(b) Reproduction isolation - It refers to the mechanism which checks the organisms of two different groups from interbreeding.

Ans: 
(a) Fossils are important evolutionary evidence to show what kind of organisms existed earlier.
Archaeopteryx is a fossil dinosaur with wings. This proves that it has features of both reptiles as well as birds. Hence we can say that birds evolved from reptiles.
(b) These organisms cannot be grouped together as the structure of eye in each is very different. This means that they have separate evolutionary origin.

Ans: All the variations do not have equal chances of survival in the environment in which they live. Depending on the nature of variations, different individuals would have different kinds of advantages. The organisms which are most adapted to the environment will survive.

Ans: Deoxyribonucleic acid (DNA) present in the chromosomes of cell nucleus is the information source for making proteins.

Ans: A gene is a unit of DNA on a chromosome which governs the synthesis of particular protein that controls specific characteristics (or traits) of an organism.