CBSE Class 10 Science HOTs Heredity Set 03

Question. Out of tallness and dwarfness of pea plants, tallness is more prominent among pea plants, because
(a) tallness is dominant over dwarfness.
(b) dwarfness is dominant over tallness.
(c) tallness is determined by one gene having many effects.
(d) tallness is determined by many genes having multiple effects.
Answer: (a) tallness is dominant over dwarfness.

Question. How many types of gametes will be produced in \( F_2 \) generation in a dihybrid cross?
(a) 3
(b) 4
(c) 8
(d) 16
Answer: (b) 4

Question. A pea plant with round green (RRyy) pea seed is crossed with another pea plant with wrinkled yellow (rrYY) seeds. What would be the nature of seeds in the first generation?
(a) Round Green
(b) Round Yellow
(c) Wrinkled Green
(d) Wrinkled Yellow
Answer: (b) Round Yellow

Question. The genotypic ratio of monohybrid cross in \( F_2 \) generation is
(a) 3 : 1
(b) 9 : 3 : 3 : 1
(c) 1 : 1
(d) 1 : 2 : 1
Answer: (d) 1 : 2 : 1

Question. Gametes contain
(a) haploid chromosomes
(b) diploid chromosomes
(c) homologous chromosomes
(d) no chromosomes
Answer: (a) haploid chromosomes

Question. “Genes and chromosomes have similar behaviour.” Justify.
Answer: Both genes and chromosomes occur in pairs in diploid cells. During gamete formation, both segregate so that a gamete receives only one of the pair. Both restore the diploid state upon fertilization.

Question. Explain Mendel’s experiment with peas on inheritance of traits considering two visible contrasting characters.
Answer: Mendel crossed pea plants having two contrasting traits such as round yellow seeds (RRYY) and wrinkled green seeds (rryy). All \( F_1 \) plants had round yellow seeds (RrYy). When \( F_1 \) were self-pollinated, the \( F_2 \) generation showed four types of plants: Round yellow, Round green, Wrinkled yellow, and Wrinkled green in the ratio \( 9 : 3 : 3 : 1 \). This showed that the two traits are inherited independently.

Question. On the basis of the possibilities of combination of the sex chromosomes, what percentage probability does a couple have of having a son or a daughter? Show the same by making a cross.
Answer: There is a 50% probability of having a son and a 50% probability of having a daughter. Cross: Father (XY) \( \times \) Mother (XX) Gametes: (X), (Y) from father; (X), (X) from mother. Combinations: XX (Daughter), XX (Daughter), XY (Son), XY (Son). Ratio \( 1 : 1 \).

Question. “It is a matter of chance whether a couple will have a male or a female child.” Justify this statement by drawing a flow chart.
Answer: Father produces two types of gametes (X and Y) in equal proportion. Mother produces only one type (X). The sex of the child depends on whether the egg (X) is fertilized by a sperm carrying X or Y. Since both chances are equal (50%), it is a matter of chance.

Question. With the help of a flow chart explain in brief how the sex of a newborn is genetically determined in human beings. Which of the two parents, the mother or the father, is responsible for determination of the sex of a child?
Answer: Sex determination is genetic. Females have XX and males have XY. All eggs are X, but sperms can be X or Y. If Sperm (X) + Egg (X) \( \to \) XX (Female). If Sperm (Y) + Egg (X) \( \to \) XY (Male). The father is responsible for sex determination because he produces two different types of gametes.

Question. Sameer’s father is a wrestler and has a well-built body. He was awarded as Mr. India when he was young. Sameer is his only son. As Sameer grew older, everyone expected him to have the same body build up as his father. But he is thin. His friends tease him and he feels depressed by it. (a) Is it true that a wrestler’s son should also have heavy muscles? (b) What type of character is it; acquired or inherited? (c) What are the values shown by Sameer’s friends?
Answer: (a) No, it is not true. (b) Muscular body is an acquired trait as it is developed through exercise and diet, not through genes. (c) Sameer’s friends show lack of empathy and scientific knowledge; they are being insensitive.

Question. When a tall plant is crossed with a dwarf plant what will be the ratio of tall to dwarf plants in \( F_1 \) generation? Show with a help of Mendel’s cross.
Answer: In \( F_1 \) generation, the ratio will be all tall plants (100% tall). Cross: TT (Tall) \( \times \) tt (Dwarf) Gametes: (T) and (t) \( F_1 \): Tt (All tall plants).

ASSERTION AND REASON QUESTIONS

Direction: In the following Questions, the Assertion and Reason have been put forward. Read the statements carefully and choose the correct alternative from the following:
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.

Question. Assertion: When pure breed tall plants are crossed with pure breed short plants, all the plants in \( F_1 \) progeny are tall. When the tall plants of \( F_1 \) progeny are crossed, short plants re-appear in \( F_2 \) progeny.
Reason: Traits are independently inherited.
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.
Answer: (b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.

Question. Assertion: Variation is high in sexually reproducing organisms compared to asexually reproducing organisms.
Reason: Inaccuracies during DNA copying give rise to variation.
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.
Answer: (b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.

Question. Assertion: Acquired trait cannot be passed on from one generation to next generation.
Reason: Inaccuracy during DNA copying of acquired trait is minimum.
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.
Answer: (c) Assertion is true but the Reason is false.

Question. Assertion: Height in pea plants is controlled by efficiency of enzymes and is thus genetically controlled.
Reason: Cellular DNA is the information source for making proteins in the cell. [CBSE Sample Paper 2023]
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.
Answer: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Question. Assertion: Variation is beneficial to species.
Reason: Variation enables a species for its survival.
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.
Answer: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Question. Assertion: In human beings, males produce similar gametes.
Reason: Males have a pair of sex chromosomes XY and 22 pairs of autosomes.
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.
Answer: (d) The statement of the Assertion is false but the Reason is true.

Question. Assertion: Each and every child has two versions of DNA—both paternal and maternal.
Reason: Both the father and the mother contribute equal amounts of genetic materials to their child.
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.
Answer: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Question. Assertion: When pure breed tall plants are crossed with pure breed short plants, all the plants in \( F_1 \) progeny are tall. When the tall plants of \( F_1 \) progeny are crossed, short plants re-appear in \( F_2 \) progeny.
Reason: Traits are independently inherited.
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.
Answer: (b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.

Question. Assertion: The sex of a new born individual is always genetically determined.
Reason: Snails can change their sex depending on the temperature at which the fertilized eggs are kept.
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.
Answer: (b) The Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.

QUESTIONS

Question. If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier?
Answer: In a population of asexually reproducing species, the chances of appearance of new traits (that is variation) is very low, so, the trait which is already present in the population in higher percentage has arisen earlier. As the trait B is present in 60% of the population, thus, this trait is likely to have arisen earlier.

Question. How does the creation of variations in a species promote survival?
Answer: Variations take place in response to the changes in the environment. Such variations enable a species to cope with the new changes in the environment. Thus, variations help a species to survive.
For example, if the temperature of water increases suddenly, most of the bacteria living in that water would die. Only few variants resistant to heat would be able to survive. If these variations were not there, then the entire species of bacteria would have been destroyed. Thus, these variations help in the survival of the species.

Question. How do Mendel’s experiments show that traits may be dominant or recessive? 
Answer: Mendel conducted experiments on garden pea plants, selecting several visible contrasting characters. He selected and crossed the pure-bred tall pea plants having the genotype TT with a pure-bred dwarf pea plant having the genotype tt. \( F_1 \) generation consists of only tall plants having the genotype Tt. Then, Mendel self-pollinated the \( F_1 \) generation plants and observed that all plants obtained in \( F_2 \) generation were not tall. Instead, one-fourth of the \( F_2 \) plants were short. Hence, Mendel concluded that \( F_1 \) tall plants were not true breeding. They were carrying traits of both short height and tall height. The trait of tallness is dominant character, while the trait of dwarfness is recessive character. Thus, Mendel’s experiments show that trait may be dominant or recessive.

Question. How do Mendel’s experiments show that traits are inherited independently? 
Answer: In a dihybrid cross made by Mendel, it was observed that when two pairs of traits or characters were considered; each trait expressed itself independent of the other. Thus, Mendel was able to propose the law of independent assortment which says about independent inheritance of traits.

Question. A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits — blood group A or O — is dominant? Why or why not?
Answer: A person having blood group A may have two genotypes which are \( I^A I^A, I^A I^o \), so, this information is not sufficient to tell which of the traits — blood group A or O — is dominant.

Question. How is the sex of the child determined in human beings?
Answer: In human beings, somatic cells contain 23 pairs of chromosomes. Out of them the 23rd pair is the sex chromosome which is composed of different types of chromosomes which are named as X and Y chromosomes. This pair has one X and one Y chromosome in a male sperm. On the other hand, in a female egg, it has two X chromosomes. When a sperm with X chromosome fertilises the egg, the resulting zygote would develop into a female child. When a sperm with Y chromosome fertilises the egg, the resulting zygote would develop into a male child.

It has been established that in human beings, sex is genetically determined, mothers should not be blamed for giving birth to a girl child. Also both male and female babies have equal right to live and illegal absorption of female foetus is a crime. But in some of the animals, sex determination is also controlled by the environmental factors. For examples in a turtle (\(Chrysema \ picta\)), high incubation temperature leads to the development of female offsprings. On the other hand lizard (\(Agama \ agama\)), high incubation temperature results in male progeny.

Question. What are sex chromosomes? How many chromosomes are present in a sperm and an ovum?
Answer: Sex chromosomes are the chromosomes that determine the sex of an organism. In humans, these are the X and Y chromosomes. A sperm and an ovum each contain 23 chromosomes.

Question. Why do all the gametes formed is human females have an X chromosome?
Answer: Human females have two X chromosomes (\(XX\)). During meiosis at the time of gamete formation, one X chromosome enters each gamete, therefore all female gametes (ova) have an X chromosome.

Question. Explain how in sexually producing organisms the number of chromosomes in the progeny remains the same.
Answer: In sexually reproducing organisms, gametes are produced through meiosis, which reduces the number of chromosomes by half. When a male gamete (haploid) fuses with a female gamete (haploid) during fertilisation, the resulting zygote (diploid) regains the original full number of chromosomes, ensuring the chromosome number remains the same in the progeny.

Question. Or
A woman has only daughters. Analyse the situation genetically and explain.
Answer: Genetically, the sex of a child is determined by the father. A woman produces only one type of gamete (ova) containing an X chromosome. A man produces two types of gametes (sperms), 50% carrying an X chromosome and 50% carrying a Y chromosome. If an X-carrying sperm fertilises the ovum, a daughter is born. If a woman has only daughters, it means that in each instance of fertilisation, an X-carrying sperm from the father was involved.

EXERCISES

Question. A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. In the progeny all bore violet flowers, but almost half of them are short. This suggests that the genetic make-up of the tall parent can be depicted as
(a) TTWW
(b) TTww
(c) TtWW
(d) TtWw
Answer: (c) TtWW

Question. A study found that children with light-coloured eyes are likely to have parents with light-coloured eyes. On this basis, can we say anything about whether the light eye colour trait is dominant or recessive? Why or why not?
Answer: No, we cannot say whether light colour trait is a dominant trait or a recessive trait. However, since both parents and children have light coloured eye, light coloured eye may be recessive trait. Had light coloured eye been a dominant trait, heterozygus light eye parents might have some dark eye children of recessive trait.

Question. How is the equal genetic contribution of male and female parents ensured in the progeny?
Answer: The equal genetic contribution of male and female parents in a progeny is ensured through gametes which have only half the amount of DNA as compared to other body cells. The normal body cells of human being contain 46 chromosomes each. During sexual reproduction, a female gamete or egg cell fuse with a male gamete or sperm cell (which are haploid) to form zygote. Zygote is diploid which contains 23 chromosomes from mother and 23 from father in humans. In this way, an equal genetic contribution of male and female parents is ensured in the progeny.