CBSE Class 10 Biology Heredity and Evolution Worksheet Set B

Short Answer Type Questions :

Question. Study carefully the given flowchart depicting cross between pea plant with yellow seeds and pea plant with green seeds and answer the following:
(i) What kind of cross it depicts?
(ii) In which proportions the characters will appear in F1 and F2 generation? 

Answer: (i) It depicts monohybrid cross.
(ii) In F1 generation all pea plants with yellow seeds will be produced whereas in F2 generation pea plants with yellow and green seeds in the ratio 3:1 will be produced.

Question. List two differences in tabular form between dominant traits and recessive traits.
Answer: Dominant traits
It expresses itself with or without the presence of recessive trait.
For example: tall plant, round seed, violet flower etc. are dominant traits in a pea plant.
Recessive traits
It is able to express itself only in the absence of dominant trait
Dwarf plant, wrinkled seed, white flower, etc. are recessive traits in a pea plant.

Question. (a) On what rules inheritance is based?
(b) Is each trait influenced by both parental and maternal DNA?
Answer: (a) (i) Law of dominance. (ii) Law of segregation (iii) Law of independent assortment
(b) Yes, each trait is influenced by DNA of both parents

Question. The chromosomal number of the sexually producing parents and their offspring is the same. Justify this statement.
Answer: Offspring always takes 23 chromosomes from father and 23 chromosomes from mother and in total has 46 chromosomes. It means that total number of chromosomes remain constant.

Question. If YYRR is round yellow, what do the following represent?
yyrr yyRR
Answer: yyrr – wrinkled, green seeds
yyRR – Round, green seeds.

Question. How do Mendel’s experiments show that traits may be dominant or recessive?
Answer: • Mendel selected pure breed of tall (TT) and short (tt) pea plants.
• He crossed these plants and carefully produced seeds. This is called F1 generation.
• All the plants in F1 generation were tall, showing that tallness is dominant trait.
• Now these plants were self pollinated.
• In the F2 generation, 3 out of 4 plants, i.e. 75% were tall and one out of four, i.e. 25% were short plants. It shows that shortness is the recessive trait.

Question. A cross was carried out between pure breed tall pea plant with pure dwarf pea plant and F₁ progeny was obtained. Later, F₁, progeny was selfed to obtain F₂ progeny. Answer the following questions:
(a) What is the phenotype of the F₁ progeny and why?
(b) Give the phenotypic ratio of the F₂ progeny.
(c) Why is F₂ progeny different from the F₁ progeny?
Answer: (a) Tall because it is a dominant trait whereas dwarf is a recessive trait.
(b) 3 : 1 is ratio between tall to dwarf.
(c) In F₂ generation, recessive genes are also expressed in homozygous condition

Question. What is F₂ generation?
Answer: The generation produced by the offsprings of F1 generation is called F2 or second generation.

Question. Why are small number of surviving tigers a cause of worry from genetics point of view?
Answer: (i) Tigers may get extinct in near future due to natural calamities, lack of available food and due to their small number.
(ii) Cross breeding may be less possible. Variations will not occur, which is essential for the survival.
(iii) Adaptation due to changing environment, like cutting of forests, global warming becomes difficult.
(iv) Protection from enemies is easy if they are more in number.

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 whether the light eye colour trait is dominant or recessive? Why or why not?
Answer: We cannot predict because information is not sufficient. We can decide dominant or recessive trait if we have the data for at least three generations. More information is required.

Question. How can variation occur in asexually reproducing organisms?
Answer: There would be only very minor differences in asexual reproduction, generated due to small inaccuracies in DNA copying.
Unlike sexual reproduction, asexual reproduction only introduces genetic variation into the population if a random mutation in the organism's DNA is passed on to the offspring.

Question. Answer the following:
(i) Who are more closely related— A brother and sister or two cousins? Why?
(ii) Why do all the gametes formed in human females have an Xchromosome ?
Answer: (i) A brother and sister are more closely related than two cousins as they have a common ancestor.
(ii) The sex chromosome in human female is homomorphic i.e., they contain same chromosome XX. During meiosis process at the time of gamete formation all egg cell will get one copy of X chromosome, hence all the gametes formed in human females have an X chromosome.

Question. How is the equal genetic contribution of male and female parents ensured in the Progeny? 
Answer: Human beings contain 23 pairs of chromosomes-22 pairs are autosomes and one pair sex chromosomes. During meiosis process gametes are formed in sex cells where the chromosome number is halved (n). At the time of fertilisation when male gamete fuses with female gamete the diploid number (2n) is restored back in zygote. Thus half of the chromosomes come from father and other half from mother. In this way meiosis process ensured equal genetic contribution of male and female parents in the progeny.

Question. Why do all the gametes formed in human females have an X chromosome?
Answer: The sex chromosome in human female is homomorphic i.e., they contain same chromo-some XX. During meiosis process at the time of gamete formation all egg cells will get one copy of X chromosome, hence all the gametes formed in human females have an X chromosome.

Question. If a trait A exists in 10% of a population of an asexually reproducing species and trait B exists in 60% of the same population, which trait is likely to have arisen earlier? 
Answer: Trait B would have arisen earlier than trait A because as species are asexually reproducing, there would be very minor differences generated due to small inaccuracies in DNA copying which results in variations in trait A. But as trait B occurs in more number in the population as compared to trait A so Trait B would have arisen earlier than trait A.

Question. Difference between Recessive and Dominant Traits
Answer: 

Question. Characteristics in an individual organism are caused by both genetic and environmental variation. Complete the table below stating whether the characteristic is genetic or environmental variation.

Answer: 

Question. In the given figure below, what experiment
would we do to confirm that the F2 generation did in fact have a 1 : 2 : 1 ratio of TT, Tt and trait combinations ? 


Answer: When a pure tall pea plant (TT) was crossed with pure short pea plant (tt) then in F1 generation all hybrid tall pea plants were produced. (Tt) Again pea plants of F1 were selfed where pure tall (TT), hybrid tall (Tt) and pure short (tt) pea plants were obtained in the ratio 1 : 2 : 1 were obtained. 

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 the light eye colour trait is dominant or recessive as the child inherits one copy of the trait from father and other from mother and here nature of the trait is not known. The nature can be known from at least three generations but here data is given only for two generations i.e., parents [P] and offsprings [F1].

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