Previous Year Questions: Heredity - Science Class 10

Very Short Answer Type Questions

Q1. What is heredity? (2021 C, AI 2014)

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

Q2. How many pairs of chromosomes are present in human beings? (2020 C)

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.

Q3. A Mendelian experiment consisted of breeding pea plants bearing violet flowers with pea plants bearing white flowers. W hat will be the result in F₁ progeny? (2018)

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).

Q4. All the variations in a species do not have equal chances of survival. Why? (NCERT, Foreign 2014)

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.

Q5. Name the information source for making proteins in the cells. (Delhi 2014)

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

Q6. What is a gene?   (At 2014)

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.

Short Answer Questions

Q1. (a) Name the two types of gametes produced by men
(b) Does a male child inherit X chromosome from his father? Justify
(c) How many types of gametes are produced by a human female? [2022]

(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.

Q2. Justify the statement "Sex of the children will be determined by what they inherit from their father". (Term II, 2021-22)

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.

Q3. "Sex chromosomes in human males and females are X Y and X X respectively. Statistical probability of getting either a male or a female child is 50%. Justify this statement giving reason. (Term II, 2021-22 C)

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.

Q4. What is variation? List two main reasons that may lead to variation in a population. (Term II, 2021-22)

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.

Q5. (i) In a cross between violet flowered plants and white flowered plants, state the characteristics of the plants obtained in the F₁ progeny. 
(ii) If the plants of F₁ progeny are self-pollinated, then what would be observed in the plants of F₁ progeny? 
(iii) If 100 plants are produced in F₁ progeny, then how many plants will show the recessive trait? (Term II, 2021-22)

(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.

Q6. (a) Why did Mendel carry out an experiment to study inheritance of two traits in garden pea?
(b) What were his findings with respect to inheritance of traits in F1 and F2 generation?
(c) State the ratio obtained in the F2 generation in the above mentioned experiment. [2020]

(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

Q7. Assertion (A) : The sex of a child in human beings will be determined by the type of chromosome he/she inherits from the father.
Reason (R) : A child who inherits ‘X’ chromosome from his father would be a girl (XX), while a child who inherits a ‘Y’ chromosome from the father would be a boy (XY).
(a) Both (A) and (R) are true and (R) is the correct explanation of the assertion (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation of the assertion (A).
(c) (A) is true, but (R) is, false.
(d) (A) is false, but (R) is true. (2020)

The correct answer is Option (a) 
(a) Both (A) and (R) are true and (R) is the correct explanation of the assertion (A). 

Q8. Define genetics. Why is a decrease in the number of surviving tigers a cause of concern from the point of view of genetics? Explain briefly. [AI 2019]

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.

Q9. Name the plant Mendel used for his experiment. What type of progeny was obtained by Mendel in the F₁ and F₂ generations when he crossed the tall and short plants? Write the ratio he obtained in F₂ generation plants. [Delhi 2019]

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.

Q10. Define "artificial selection." Comment on the purpose why farmers selected the following vegetables to cultivate:   [CBSE 2011, CBSE Sample Paper 2018-19]
(a) Cabbage
(b) Broccoli
(c) Cauliflower
(d) Kohlrabi through artificial selection

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

Q11. Sometimes, accidently a dead body or its parts get buried under depositing sediments and are preserved. These are fossils. How can the estimation of the age of fossils be done?    [CBSE 2013, 2017-18, C, 2018-19]

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.

Q12. List in tabular form the distinguishing features between acquired traits and inherited traits, with one example of each.    [DoE, Delhi 2017, 2016, 2015; AI 2012]
Or
List two differences between acquired traits and inherited traits by giving an example of each. [Delhi 2019]

Q13. What are acquired traits? Why are these traits generally not inherited over generations? Explain.    [CBSE 2017-18 C]

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.

Q14. What is speciation? List four factors responsible for speciation.    [CBSE 2017-18 C]

 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.

Q15. Briefly explain the role of natural selection and genetic drift in speciation by citing an example.    [CBSE 2011, 2017-18C]
Or
With the help of an example, explain how new species are produced.    [CBSE 2012]

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.

Q16. What are chromosomes? Explain how, in sexually reproducing organisms, the number of chromosomes in the progeny is maintained.    [Delhi 2011C]

Chromosomes are the structures that bear the DNA or genes; they can't the DNA or genes to the progeny cells.
1. There are special lineages of cells in the sexually- reproducing organisms.
2. These, cells undergo a special type of cell division, called meiosis; consequently, the germ cells formed have only half the number of chromosomes as the parents cell.
3. When two such germ cells (with half the number of chromosomes) fuse, a zygote/new individual is formed with the same number of chromosomes as the parent organism.

Q17. What are homologous organs? Give one example. Can the wings of a butterfly and the wings of a bat be regarded as homologous? Give reasons in support of your answer.    [AI 2017C]

 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.

Q18. “Evolution and classification of organisms are interlinked.” Give reasons to justify this statement.    [ AI 2017]

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.

Q19. Explain, with the help of an example each, how the following provide evidence in favour of evolution:    [Delhi 2017, AI 2019]
(а) Homologous organs
(b) Analogous organs
(c) Fossils

(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.

Q20. If we cross-bred a tall (dominant) pea plant with a pure-bred dwarf (recessive) pea plant, we will get plants of F₁ generation. If we now self-cross the pea plants of the F₁ generation, we obtain pea plants of the F₂ generation.    [A12017C, 13,12]
(i) What do the plants of the F1 generation look like?
(ii) State the ratio of tall plants to dwarf plants in the F₂ generation.
(iii) State the type of plants not found in the F₁ generation but that appeared in the F₂ generation. Write the reason for the same.

(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.

Q21. How did Mendel explain that it is possible that a trait is inherited but not expressed in an organism?    [AI 2017]

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.

Q22.  Define speciation. Mention the factors due to which this can happen.    [CBSE 2013, 2012, 2016-17C]

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.

Q23. List two differences in tabular form between dominant traits and recessive traits. What percentage/proportion of the plants in the F₂ generation/progeny were round, in Mendel’s cross between round and wrinkled pea plants?    [Foreign 2016]

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

Q24. How does Mendel’s experiment show that traits are inherited independently?    [Al 2016]

 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.

Q25. (a) If we cut the tail of a mouse, will the tail occur in the next generation of that mouse? Give reasons to support your answer.
(b) What are the features that Archaeopteryx had in common with the reptiles?    [CBSE 2013, 2016]

(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.

Q26. Name two homologous structures in vertebrates. Why are they named so? What is the significance of these structures in the study of evolution?    [CBSE 2012, 2016-17C]

(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.

Q27. Give two uses of fossils. How does the study of fossils provide evidence in favour of organic evolution?    [CBSE 2008, 2012, 2016-17C]

 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.

Q28. What is DNA?    [Delhi 2016]

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.

Q29.  “We cannot pass on to our progeny the experiences and qualifications earned during our lifetime." Justify the statement by giving reasons and examples.    [Delhi 2015; Foreign 2012] 

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.

Q30. A pea plant with a blue-coloured flower, denoted by BB, is cross-bred with a pea plant with a 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 flowers in F₂ generation, when the flowers of F₁ plants are self-pollinated?
(c) State the expected ratio of the genotypes BB and Bw in the F₂ progeny.    [AI 2015]

(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.

Q31. “It is possible that a trait is inherited but may not be expressed.” Give a suitable example to justify this statement.    [Foreign 2015]

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.

Q32. (i) We see eyes in Planaria, insects, octopuses, and vertebrates. Can eyes be grouped together in case of the above-mentioned animals to establish a common evolutionary origin? Why?
(ii) State one evidence to prove that birds have evolved from reptiles.    [Delhi 2015, 2013; Foreign 2015, 2012]

(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.

Q33. Explain the following:    [AI 2015]
(a) Speciation
(b) Natural Selection

(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.

Q34.  Define the following:    [CBSE 2012, 2015]
(a)  Natural selection
(b) Reproduction isolation.

(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.

Q35. (a) Give evidence that birds have evolved from reptiles.    [CBSE 2014, 2015]
(b) Insects, octopuses, planarians, and vertebrates possess eyes. Can we group these animals together on the basis of the eyes that they possess? Justify your answer by giving a reason.

(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.

Long Answer Questions

Q1. The most obvious outcome of the reproductive process is the generation of individuals of similar design, but in sexual reproduction they may not be exactly alike. The resemblances as well as differences are marked. The rules of heredity determine the process by which traits and characteristics are reliably inherited. Many experiments have been done to study the rules of inheritance.
(i) Why an offspring of human being is not a true copy of his parents in sexual reproduction?
(ii) While performing experiments on inheritance in plants, what is the difference between F1 and F2 generation?
(iii) Why do we say that variations are useful for the survival of a species over time?   (2023)

(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.

Q2. In some families, either rural or urban, females are tortured for giving birth to a female child. They do not seem to understand the scientific reason behind the birth of a boy or a girl. Infact the mother is not responsible for the sex of the child and it has been genetically proved that the sex of a newborn is determined by what the child inherits from the father. 
(a) State the basis on which the sex of a newborn baby is determined in humans. 
(b) Why is the pair of sex chromosomes called a mismatched pair in males? 
(c) How is the original number of chromosomes present in the parents restored in the progeny? (2023)
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.

Q3. Sex of an individual is determined by different factors in various species. Some animals rely entirely on the environmental cues, while in some other animals the individuals scan change their sex during their life time indicating that sex of some species is not genetically determined. However, in human beings, the sex of an individual is largely determined genetically.
(a) In what way are the sex chromosomes 'X' and 'Y different in size? Name the mismatched pair of sex chromosome in humans. 
(b) Write the number of pair/pairs of sex chromosomes present in human beings. In which one of the parent (male/female) perfect pair/pairs of sex chromosomes are present?
 (c) Citing two examples, justify the statement "Sex of an individual is not always determined genetically”.  (Term II, 2021-22)

(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.

Q4. Mendel blended his knowledge of Science and mathematics to keep the count of the individuals exhibiting a particular trait in each generation. He observed a number of contrasting visible characters controlled in pea plants in a field. He conducted many experiments to arrive at the laws of inheritance.
(a) If only one pair of contrasting characters like tall and short plants is taken, plants obtained in F₁ generation are not of medium height. Why?
(b) Name the recessive traits in above case.
(c) Mention the type of the new combinations of plants obtained in F₂ progeny along with their ratio, if F₁ progeny was allowed to self pollinate? [2022]

(a) Mendel's law of dominance states, "When parents with pure, contrasting traits are crossed together, only one form of trait appears in the next generation. The hybrid offsprings will exhibit only the dominant trait in the phenotype.” Thus, if only one pair of contrasting characters (let's say tall/dwarf) is taken into consideration, only the dominant trait will be expressed in F1 generation.
(b) The dominant trait in the above case is "Tall" (expressed in F1) and the recessive trait will be "Dwarf" (not expressed in F1).

(c) The new combinations of plants obtained in F2 progeny-

'

Q5. A green stemmed rose plant denoted by GG and a brown stemmed rose plant denoted by gg are allowed to undergo a cross with each other.
(a) List your observations regarding :
(i) Colour of stem in their F1 progeny
(ii) Percentage of brown stemmed plants in F2 progeny if plants are self pollinated.
(iii) Ratio of GG and Gg in the F2 progeny.
(b) Based on the findings of this cross, what conclusion can be drawn? [2020]

(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.

Q6. (a) What are homologous structures? Give an example.
(b) “The sex of a newborn child is a matter of chance and none of the parents may be considered responsible for it." Justify this statement with the help of a flow chart showing sex-determination in human beings.    [Allahabad 2019]

(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.

Q7. Define variation in a species. How does it increase the survival chance of a species? Why do environmentalists get worried due to the small population of a species?    [CBSE 2013, 2017-18C]

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.

Q8. (a) What is variation? How is variation created in a population? How does the creation of variation in a species promote survival?
(b) Explain how, offspring and parents of organisms reproducing sexually have the same number of chromosomes.    [CBSE 2018 C]

(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.

Q9. What is meant by the trait of a species ? Distinguish between acquired and inherited traits by giving an example of each.    [CBSE 2013,2016,2017]

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

Q10. What are the various evidences in favour of evolution?    [CBSE 2015,2017]

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.

 

Wing of a bird and an insect showing analogous 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.

Comparison of early Development stages (a) Fish (b) Salamander (c) Tortoise (d) Chicken (e) Human

Vermiform Appendix of a human being as a vestigal organ

(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.

Q11. “A trait may be inherited, but may not be expressed.” Justify this statement with the help of a suitable example.    [CBSE 2016-17 C]

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.

Q12. What are chromosomes? Explain how, in sexually reproducing organisms, the number of chromosomes in the progeny is maintained.    [CBSE 2016-17 C]

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.

Q13. If we cross-bred a tall (dominant) pea plant with a pure-bred dwarf (recessive) pea plant, we will get plants of F1 generation. If we now self-cross the pea plants of the F1 generation, we obtain pea plants of F2 generation.    [CBSE 2016-17 C]
(i) What do the plants of F1 generation look like?
(ii) State the ratio of tall plants to dwarf plants in F2 generation.
(iii) State the type of plants not found in F1 generation but appeared in F2 generation. Write the reason for the same.

(i) On crossing tall (dominant) i.e., TT with dwarf (recessive) tt, we get F1,
Hence, all plants of F1 will appear tall.

(ii) On self crossing F1F2 ratio : 3 tall; 1 dwarf

(iii) tt i.e., Dwarf plants are not found in F1. In FI generation, all plants are tall. This is because the gene £t’ even though present in F1 along with 'T’ is unable to express its e lf and hence all plants appear tall. It expresses itself in pure (homozygous) condition when any dominant gene is not present. This makes ‘t’ a recessive trait.

Q14. Define evolution. How does it occur? Explain how fossils provide evidences in support of evolution.    [CBSE 2016-17 C]

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.

Q15. (a) Can the wing of a butterfly and the wing of a bat be regarded as homologous? Why?
(b) What is speciation? State any two factors which could lead to speciation.
(c) Name the vegetables made from wild cabbage by artificial selection when farmers:
(i) opted for swollen stems
(ii) opted for sterile flowers.
(iii) opted for arrested flowers.
(iv) opted for large leaves.    [AI 2017C]

(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

Q16. How do Mendel’s experiments show that the    [Delhi 2017(C); DoE, AI 2015]

(a) traits may be dominant or recessive,

(b) traits are inherited independently?

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.

Q17. What is speciation? List four factors that could lead to speciation. Which of these cannot be a major factor in the speciation of a self-pollinating plant species? Explain.     [Delhi 2016; Foreign 2015]

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.

Q18. (a) What is meant by natural selection? Explain.
(b) Why are thorn of Bougainvillea plant and a tendril of Passiflora a plant considered homologous.    [CBSE 2013,2016]

(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.