CBSE Class 10 Science HOTs Heredity Set 02

Accumulation of Variation During Reproduction

Question. Which of the following is an example of genetic variation?
(a) One person has a scar, but his friend does not.
(b) One person is older than another.
(c) Reeta eats meat, but her sister Geeta is a vegetarian.
(d) Two children have different eye colours.
Answer: (d) Two children have different eye colours.

Question. Differences between organisms in a species are described as variation. Which of the following would you describe as continuous variation?
(a) Hair colour
(b) Eye colour
(c) Weight
(d) Sex
Answer: (c) Weight

Question. Why is variation beneficial for the species, but not necessarily for the individual? 
Answer: Accumulation of variation in a species enables them to adapt according to the changes and the new needs. This provides survival advantage to the species. But an individual does not get any advantage due to variation that takes place on him. Thus, variation is beneficial for a species, but not necessary for the individual.

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. Variation is beneficial to the species because:

• It enable the organisms to adapt themselves in changing environment.
• It forms the basis of heredity.
• It forms the raw materials for evolution and development of new species.

Question. Describe briefly four ways in which individuals with a particular trait may increase in a population. 
Answer: The four ways in which individuals with a particular trait may increase in a population are as follows:

• Sexual reproduction results into increase in population.
• The individuals with special traits survive the attack of their predators and multiply while the other will perish.
• Genetic drift provides diversity without any adaptation.
• Variations in the species may lead to increased survival of the individuals with a particular trait.

Question. Mustard was growing in two fields-A and B. While Field A produced brown coloured seeds, field B produced yellow coloured seeds. It was observed that in field A, the offsprings showed only the parental trait for consecutive generations, whereas in field B, majority of the offsprings showed a variation in the progeny. What are the probable reasons for these? 
Answer: In field A, the reason for parental trait in consecutive generations of the offsprings is self-pollination. In field B, variation is seen to occur because of recombination of genes as cross-pollination is taking place.

Question. In an asexually reproducing species, if a trait X exists in 5% of a population and trait Y exists in 70% of the same population, which of the two trait is likely to have arisen earlier? Give reason. 
Answer: Trait Y which exists in 70% (larger fraction) of the population, is likely to have arisen earlier because in asexual reproduction, identical copies of DNA are produced and variations do not occur. New traits come in the population due to sudden mutation and then are inherited. 70 % of the population with trait Y is likely to have been replicating that trait for a longer period than 5 % of population with trait X.

Question. (a) All the variations in a species do not have equal chances of survival. Why? (b) Why variations are more in human beings?
Answer: (a) All the variations do not have equal chances of survival as some variations might not be beneficial and would ultimately be eliminated. (b) Because human being reproduce sexually and variation are more in sexually reproducing organisms.

Question. Explain with an example how variation took place due to inheritance.
Answer: Variation in Population: An example. Few red beetles live on a green leafy bush, grows by sexual reproduction and generate variation.

• Crows eat these beetles, leaving only fewer beetles available for reproduction.
• Due to colour variation during reproduction, only one green beetle evolved and therefore, all its progeny beetles become green.
• Crows cannot see green coloured beetles on green leaves and hence, cannot eat them, resulting in more green beetles than red ones in the beetle population.
• This type of variation gives a survival advantage.

Question. Name the scientific terms used to represent the following: (a) The branch of biology which studies heredity and variation. (b) The transmission of traits from parents to offspring. (c) Differences in a trait in human beings. (d) A recognisable feature of an organism.
Answer: (a) Genetics (b) Heredity (c) Variation (d) Character/Trait

Question. To study the natural phenomenon of inheritance, Mendel selected the pea plants. Which of the following properties were suitable for their studies? (i) Plants would easily self pollinate or cross pollinate in nature. (ii) Plants were easily grown in garden soil with a considerably shorter generation time. (iii) Pea plants do not require the true-breeding for hybridisation experiments. (iv) Many parts of the plant such as pod, seed, flower, cotyledons showed distinct phenotypes.
(a) (i), (ii) and (iii)
(b) (ii) and (iv)
(c) (i) and (ii)
(d) (ii), (iii) and (iv)

Answer: (c) (i) and (ii)

Question. If a tall pea plant is crossed with a pure dwarf pea plant then, what percentage of \( F_1 \) and \( F_2 \) generation respectively will be tall? 
Answer: In \( F_1 \) generation, 100% plants will be tall. In \( F_2 \) generation, 75% plants will be tall.

Question. Attached earlobes in humans is an inherited condition. The allele for attached earlobes is recessive. What are the chances of parents, both having attached earlobes, to have a child with attached earlobes? 
(a) 0%
(b) 25%
(c) 75%
(d) 100%
Answer: (d) 100%

Question. Humans have two different sex chromosomes, \( X \) and \( Y \). Based on Mendel’s laws; a male offspring will inherit which combination of chromosomes? 
(a) Both the \( X \) chromosomes from one of its parents.
(b) Both the \( Y \) chromosomes from one of its parents.
(c) Combination of \( X \) chromosomes from either of its parents.
(d) Combination of \( X \) and \( Y \) chromosome from either of its parents.
Answer: (d) Combination of \( X \) and \( Y \) chromosome from either of its parents.

Question. A monohybrid cross is conducted between one variety of pea plants having pods that are full \( (FF) \) and another having pods that are constricted \( (ff) \). What is the percentage of heterozygous offsprings in \( F_1 \) generation? 
(a) 100%
(b) 75%
(c) 50%
(d) 25%
Answer: (a) 100%

Question. A zygote which has an \( X \)-chromosome inherited from the father will develop into a 
(a) girl
(b) boy
(c) either boy or girl
(d) \( X \)-chromosome does not influence the sex of a child.
Answer: (a) girl

Question. A trait of an organism is influenced by
(a) paternal DNA only
(b) maternal DNA only
(c) both maternal and paternal DNA
(d) none of these
Answer: (c) both maternal and paternal DNA

Question. A cross between pea plant with white flowers \( (vv) \) and pea plant with violet flowers \( (VV) \) resulted in \( F_2 \) progeny in which ratio of violet \( (VV) \) and white \( (vv) \) flowers will be: [CBSE 2023]
(a) 1 : 1
(b) 2 : 1
(c) 3 : 1
(d) 1 : 3
Answer: (c) 3 : 1

Very Short Answer Type Questions 

Question. Sex determination in humans happens through sex chromosomes. Along with other parameters, such processes often help in forensic studies in crime investigation and / or identification of accidents and natural calamities, In order to determine whether an accident victim is male or female, which cells can be used and why? 
Answer: • Any cell of the body can be used.
• It is because every cell has the sex chromosomes as the 23rd pair.

Question. Why did Mendel select pea plants for conducting his experiments on inheritance?

Answer: Mendel selected pea plant for his experiment because-
(i) Many varieties of pea plants are available with observable contrasting traits.
(ii) Peas are normally self pollinating and the flower structure is also suitable for cross-pollination.

Question. What are ‘chromosomes’? Where are they located in the cell? 
Answer: ‘Chromosomes’ are long thread-like structures which contain hereditary information of the individual and are therefore the carriers of genes.
Chromosomes are located in the nucleus of a cell.

Question. “The sex of the children is determined by what they inherit from their father and not their mother.” Justify. 
Answer: It is because a child who inherits an \( X \) chromosome from father will be a girl and one who inherits a \( Y \) chromosome from father will be a boy. But all children will inherit an \( X \) chromosome from their mother regardless of whether they are boys or girls.

Question. What is a sex chromosome? 
Answer: Sex chromosome is a chromosome that operates in the sex-determining mechanism of a species. Many animals have two different types of sex chromosomes. For example, in humans there is a large \( X \) chromosome and a much smaller \( Y \) chromosome.

Question. In one of his experiments with pea plants Mendel observed that when a pure tall pea plant is crossed with a pure dwarf pea plant, in the first generation \( (F_1) \) only tall plants appear. (a) What happens to the trait of dwarfness in this case? (b) When the \( F_1 \) generation plants were self fertilised, he observed that in the plants of second generation, \( F_2 \) both tall plants and dwarf plants were present. Why it happened? Explain briefly.
Answer: (a) In the \( F_1 \) generation, the trait of dwarfness is recessive. (b) Both parents contribute equally in sexual reproduction. So each pea plant inherited genes of both tallness and dwarfness in the \( F_1 \) generation. But only the dominant trait, tallness got expressed. When \( F_1 \) plants are crossed, in the \( F_2 \) generation, there are some plants (25%) which carry only the dwarfness character and hence the same got expressed in the \( F_2 \) generation.

Question. How did Mendel explain that it is possible that a trait is inherited but not expressed in an organism?
Answer:

• Mendel crossed a tall pea plant with a short pea plant.
• All the plants produced in the \( F_1 \) generation were tall.
• When the \( F_1 \) tall plants were self-pollinated, the \( F_2 \) generation consisted of both tall and short plants.
• It explains that the dominant trait expresses itself in the \( F_1 \) plants, where the recessive trait (shortness) is hidden.
• The appearance of short plants in the \( F_2 \) indicates that the trait shortness has been inherited by the \( F_1 \) plants, but not expressed.

Question. (a) What is \( F_2 \) generation? (b) Why traits such as intelligence and knowledge cannot be passed on to the next generation? (c) “The sex of the children is determined by what they inherit from their father and not their mother.” Justify.
Answer: (a) The generation produced by the offspring to \( F_1 \) generation, i.e. first generation as parent is called \( F_2 \) generation. (b) Trait such as intelligence and knowledge are acquired traits wich do not bring any change in the DNA of the germ cell and therefore, cannot be passed to next generation. (c) It is because a child who inherits an X chromosome from father will be a girl and one who inherits a Y chromosome from father will be a boy. But all children will inherit an X chromosome from their mother regardless of whether they are boys or girls.

Question. “We cannot pass onto our progeny the experiences and qualifications earned during our life time”. Justify the statement giving reason and examples.
Answer: Experiences of life and qualifications we earn donot 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.

Question. Explain Mendel’s experiment with peas on inheritance of characters, considering only one visible contrasting character.
Answer: Mendel crossed tall pea plants with dwarf pea plants: Parents : (TT) \( \times \) (tt) Pure tall plant Pure short plant \( F_1 \) generation : (Tt) (Tt) (Tt) (Tt) Selfing of \( F_1 \) : (Tt) \( \times \) (Tt) \( F_2 \) generation : (TT) (Tt) (Tt) (tt) Observation: \( F_1 \) generation—No ‘medium-height’ plants were there. All plants were tall. Only one of the parental traits was seen, not some mixture of the two. \( F_2 \) progeny— Not all plants were tall. One quarter of them were short. This indicates that both the tallness and shortness traits were inherited in the \( F_1 \) plants, but only the tallness trait was expressed. Mendel proposed that something was being passed unchanged from generation to generation which we called ‘factors’. Factors contain and carry hereditary information.

Question. How do Mendel’s experimentshow that traits are inherited independently?
Answer: Mendel performed an experiment in which he took two different traits like tall and dwarf plant and round and wrinkled seeds. In second \( (F_2) \) 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.

Question. If we cross-bred tall (dominant pea plant with pure-breed dwarf (recessive) pea plant, we will get plants of \( F_1 \) generation. If we now self-cross the pea plant of \( F_1 \) generation, we obtain pea plants of \( F_2 \) generation. (a) What do the plants of \( F_1 \) generation look like? (b) State the ratio of tall plants to dwarf plants in \( F_2 \) generation. (c) State the type of plants not found in \( F_1 \) generation but appeared in \( F_2 \) generation. Write the reason for the same.
Answer: (a) The plants of \( F_1 \) generation will be tall like the dominant parent. (b) Tall plants 3 : Dwarf plants 1, i.e., \( 3 : 1 \). (c)

• Dwarf plants are not found in \( F_1 \) generation.
• It is because, when two copies of a gene (alleles) exist together in the \( F_1 \) plants, only the trait; tallness is expressed, i.e. it is dominant.
• The other trait dwarfness remains hidden as it is a recessive trait.

Question. List two differences in tabular form between dominant trait and recessive traits. What percentage/proportion of the plants in the \( F_2 \) generation/progeny were round, in Mendel’s cross between round and wrinkled pea plants?
Answer:

Dominant Trait

• When both dominant and recessive traits are inherited, the dominant trait gets expressed.
• A single copy of dominant trait is enough to get it expressed.

Recessive trait

• When both dominant and recessive traits are inherited, the recessive trait does not get expressed.
• Both the copies of a trait should be recessive to get it expressed.

75% of the plants in \( F_2 \) generation were round in Mendel’s cross between round and wrinkled pea plants.

Question. “It is possible that a trait is inherited but may not be expressed.” Give a suitable example to justify this statement.
Answer: 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 \( F_1 \) progeny were tall. Mendel then allowed the \( F_1 \) progeny plants for self-pollination. It was found that all the \( F_2 \) progeny plants are not tall, some are short. This indicates that both tallness and shortness traits were inherited separately in the \( F_1 \) progeny but shortness trait was not expressed in the \( F_1 \) progeny.

Question. List in tabular form distinguishing features between acquired traits and inherited traits, with one example of each.
Answer:

Acquired traits

• These traits are the characteristics which are developed during the lifetime of an individual.
• These traits are not passed on to the next generation.
• Their effect is only in the somatic cells, which does not get inherited to another generation. e.g. Acquired trait: Loss of body weight due to starvation.

Inherited traits

• These are the characteristics transmitted from parent to the offspring.
• Inherited trait is genetically determined characteristic that distinguishes a person.
• These have effects on the non-somatic cells which pass to the progeny. e.g. Inherited trait: Colour of hair and eye.

Question. What are acquired traits? Why are these traits generally not inherited over generations? Explain.
Answer: Acquired traits are those traits which an individual acquired after birth during its life-time.

• These are changes in the non-reproductive tissues.
• The DNA or gene of the germ cells is not influenced/changed by these characters; hence, they cannot be passed on to the next generation.

Question. What are chromosomes? Explain how in sexually reproducing organisms the number of chromosomes in the progeny is maintained.
Answer: Chromosomes are the structures that bear the DNA or genes; they carry the DNA or genes to the progeny cells.

• There are special type of cells in the sexually reproducing organisms.
• 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.
• 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 organsism.

Question. Two pea plants - one with round yellow seeds (RRYY) and another with wrinkled green (rryy) seeds produce \( F_1 \) progeny that have round, yellow (RrYy) seeds. When \( F_1 \) plants are self-pollinated, which new combination of characters is expected in \( F_2 \) progeny? How many seeds with these new combinations of characters will be produced when a total 160 seeds are produced in \( F_2 \) generation? Explain with reason.
Answer: Round green : 30 Wrinkled yellow : 30 New combinations are produced because of the independent inheritance of seed shape and seed colour trait.

Long Answer Type Questions

Question. Consider a pea plant that is recessive for plant height. Its ‘genotype’ is tt and ‘phenotype’ is dwarf. (a) Assuming that the gene for plant height obeys the Mendel’slaws of inheritance, indicate the genotypes and phenotypes of ALL the possible parent pairs that could have dwarf offspring. (b) Using any of the parent pairs mentioned by you in (a), perform a cross to show the genotypes of the offspring that might arise in the next generation.
Answer: (a) (i) \( Tt \times Tt \), \( Tt \times tt \) and \( tt \times tt \) (ii) Tt (tall) \( \times \) Tt (tall), Tt (tall) \( \times \) tt (dwarf) and tt(dwarf) \( \times \) tt (dwarf) (b) \( Tt \times Tt \)
\( \implies \) T t
\( \implies \) T TT Tt
\( \implies \) t Tt tt

Question. (a) With an example, explain how genes control the characteristics. (b) Which of the following traits can be passed on to the progeny and which cannot? (i) Hair type and colour. (ii) The cut tail of a mouse. (iii) Preferance for certain types of food. (iv) Red colour of beetles. (c) Define species. Give two examples of plant species and two of animals.
Answer: (a) Genes are the units of heredity. Each gene exercise its function by synthesizing specific protein which is responsible for the expression of characteristic. For example, consider the height as a characterstic of the plant. We know that plants have hormones that can trigger growth. So, height of plants depend upon the amount of particular hormone. However, if the gene has alteration that makes the enzyme less efficient, the amount of the hormone will be less and plant will be dwarf. This clearly indicates that characterstics are under the control of genes. (b) Traits which can be passed on to progeny are: (i) Hair type and colour. (ii) Red colour of beetles. Traits which cannot be passed on to progeny are: (iii) The cut tail of a mouse. (iv) Preferance for certain types of food. (c) It refers to a population of organism consisting of similar individuals which can breed together and produce fertile offsprings. Two examples of plants species are rose and lily. Two examples of animal species are lion and elephant.

Question. (a) What is the law of dominance of traits? Explain with an example. (b) Why are the traits acquired during the life time of an individual not inherited? Explain.
Answer: (a) Law of Dominance: Mendel took pea plant and carried two contrasting characters (tall and short) and cross pollination done among them . The traits which get expressed in \( F_1 \) generation are called dominant and which are unexpressed are called recessive which reappears in \( F_2 \) generation. This is called law of dominance. Traits may be dominant or recessive. • When Mendel crossed a tall pea plant with a dwarf pea plant, all the \( F_1 \) plants were tall. • When the \( F_1 \) plants were self-pollinated and an \( F_2 \) progeny raised, there were tall plants and dwarf plants. • The trait (tallness) which has appeared in the \( F_1 \) plants is called a dominant trait, while dwarfness that remained hidden in \( F_1 \) plants, but appeared in \( F_2 \) plants, is called a recessive trait. (b) Certain experiences and traits acquired by people during their lifetime are not passed on to their next generations because these traits do not change the gene/DNA of the germ cells. For example, (i) Losing weight due to starvation. (ii) Cutting the tails of mice for a few generation can not produce tailless mice. Such traits can only be passed on to the next generation when they alter/change the DNA of the germ cells.

Question. “Sex determination is an important developmental event in the life cycle of all sexually reproducing plants. Recent studies of sex determination in many plants species, from ferns to maize, have been fruitful in identifying the diversity of genetic and epigenetic factors that are involved in determining the sex of the flower or individual. ” The above is an excerpt from an article by two scientists Cristina Juarez and Jo Ann Banks. (a) What is the most likely genetic factor for sex determination in unisexual plants? (b) Epigenetic factors refer to factors external to the genetic component of an individual. Name evidence of ONE epigenetic condition that determines sexuality in animals. (c) State Mendel’slaw of segregation and explain how sex determination violates the law. (d) Which parent determines the sex of the offspring in human beings? Why?
Answer: (a) Sex chromosomes (b) In some reptiles temperature of the fertilised egg determines sex of the embryo. (c) The law of segregation states that a diploid organism passes a randomly selected allele for a trait to its offspring, such that the offspring receives one allele from each parent. If sex determination in plants is governed by genetic factors, the offspring will get one copy of a gene from each parent. Sex determination violates the law of segregation as the human female does not have any copy of the Y-chromosomal genes. (d) The father determines the sex of the offspring in human beings. It is because the father can pass either X or Y chromosome to the offspring.