CBSE Class 10 Biology Heredity and Evolution Worksheet Set A

Question. The given box diagram represents the ratio of females to males or the sex ratio in our country for 10 decades (1901 to 2001). Answer the following questions in the light of your knowledge of sex determination and the data presented in the box diagram.

(i) What does the bar diagram show?
(ii) As per scientific knowledge regarding sex determination, what should be the sex ratio or the male to female ratio at a given point of time?
(iii) Assign one reason to the trend showing deviation from the expected sex ratio.
(iv) Suggest a way by which such a trend can be stopped.
Answer: (i) Bar diagram shows the proportion of females in the population over a decade.
(ii) 1 : 1
(iii) Female foeticide
(iv) Banning sex tests of unborn baby; increasing awareness and education.

Question. A blue flower plant denoted by BB is crossed with that of white coloured flower plant denoted by bb.
(i) State the colour of flower you would expect in their F1 generation plants.
(ii) What must be the percentage of white flower plants in F2 generation if flower of F1 plants are self – pollinated?
(iii) State the expected ratio of the genotypes BB and bb in the F2 progeny?
Answer: (i) In the first generation i.e. in F1, generation, all the flower plants will be of blue colour. It can be shown as follows: 

(ii) When flowers of f1 generation are self-pollinated: 

So, in F2 generation, there will be three blue Flowers and one white flower. So the ratio of blue flower plant to ratio of white flower plant is
3 : 1 but we need to find the ratio percentage of white flower which is calculated as follows:
Percentage = no. of white flower plant / Total plants in F2 generation x 100
Percentage = 1/4 x 100
So, Ratio percentage of white flower plant is 25%
(iii) The expected ratio of the genotypes BB and Bb in the F2 progeny is as follows: 

Question. The sex of a new born child is a matter of chance and none of the parents may be considered responsible for it. Draw a flowchart showing determination of sex of a newborn to justify this statement.
Answer: In human beings, there are two types of sex chromosome X and Y; female have XX chromosome whereas male have XY chromosome. Females produce eggs which carry only X chromosomes but males contain half of the sperms with X chromosomes and other half with Y chromosomes. During fertilisation when X carrying sperms fuse with an egg which
contains X chromosome the offsprings will be a female (XX). But when Y bearing sperms fuses with an egg (X) the offspring will be male (XY). Thus the sex of a child is determined by the type of sex chromosome X or Y received by the male gamete. 

Question. Answer the following questions:
(i) What are dominant and recessive traits?
(ii) “Is it possible that a trait is inherited but may not be expressed in the next generation? Give a suitable example to justify this statement.
Answer: (i) The trait which can express its effect over contrasting trait is called dominant trait whereas the trait which cannot express its effect over contrasting trait or which gets suppressed by the contrasting trait is called recessive trait. The inherited trait which is not expressed will be a recessive trait.
(ii) In Mendel’s experiment, when pure tall pea plants were crossed with pure dwarf pea plants, only tall pea plants were obtained in
F1 generation. On selfing the pea plants of F1 generation both tall and dwarf pea plants were obtained in F2 generation.
Reappearance of the dwarf pea plants in F2 generation proves that the dwarf trait was inherited but not expressed in F1 generation. The recessive trait does not express itself in the presence of the dominant trait. So, it is possible that one trait may be inherited but may not be expressed in an organism.

Question. Observe the ears of all the students in the class. Prepare a list of students having free or attached earlobes and calculate the percentage of students having each (Fig. below). Find out about the earlobes of the parents of each student in the class. Correlate the earlobe type of each student with that of their parents. Based on this evidence, suggest a possible rule for the inheritance of earlobe types.
Answer: It is observed that earlobe is attached to the side of head in some of students and not in others. Majority of students have free ear lobes. After observing the earlobes of parents it is found that free earlobes always occur in those cases where one or both the parents have free ear lobes. However attached earlobes were observed in atleast one of the two parents of children having attached earlobes. Earlobe trait is inherited from parents. Suppose earlobe trait is controlled by two alleles L or l. Individuals with free ear lobe have alleles LL or Ll and attached ear lobes have alleles ll. So free ear lobe is dominant trait and attached earlobe is recessive trait.

Question. Give the basic features of the mechanism of inheritance?
Answer: The basic features of mechanism of inheritance are:
(a) Traits are controlled by genes.
(b) Genes are present on chromosomes.
(c) Each gene controls one character.
(d) There may be two or more forms of a gene.
(e) One form may be dominant over the other.
(f) An individual possesses two forms of genes which may be similar or dissimilar.
(g) The two forms of gene separate at the time of gamete formation.
(h) The two forms of gene are brought together in a zygote after fertilisation.

Question. How do Mendel’s experiments show that the:
(i) Traits may be dominant or recessive
(ii) Traits are inherited independently.
Answer: (i) When Mendel crossed pure tall pea plants with pure dwarf pea plants in F1 generation only tall plants were produced. When these F1 plants were self-pollinated in F2 generation both tall and dwarf plants were produced in the ratio of 3: 1. In F1 generation only tall plants were found so it showed that tall is a dominant character. In F2 generation dwarf plants appeared which shows that dwarf is a recessive trait which is expressed only in recessive condition. 

(ii) When Mendel crossed pure pea plants with round, yellow seeds with pure plants with wrinkled, green seeds in F1 generation all pea plants with round and yellow seeds were produced. This shows that round and yellow are dominant characters whereas green and wrinkled are recessive characters. Again when these F1 plants were crossed round, yellow pea plants as well as green, wrinkled seeds pea plants were produced. But in addition to these two new characters were produced i.e., round and green, wrinkled and yellow seeds pea plants were produced. This shows that two pair of characters combine in F1 generation but they get separated and behave independently in F2 generation. 


Round-yellow, round-green, wrinkled-yellow, wrinkled-green – 9: 3: 3:1

Question. Mrs. Joshi, an eight months pregnant lady was suggested by her doctor to get an ultrasound done. She went to a radiologist with her husband and got the ultrasound done. When the ultrasound was done, her husband asked doctor about the sex of the baby in the womb.
(i) Is it ethical to determine the sex of the foetus? Why?
(ii) What is the chance of giving birth to a girl child in human beings?
(iii) What has government done to stop female foeticides?
Answer: (i) No, it is not ethical to determine the sex of the foetus because sex determination may lead to female foeticides which results in death of a girl child and thus the sex ratio become unbalanced in a society.
(ii) The chance of giving birth to a girl child in human beings is 50%.
(iii) Government has imposed a ban on sex determination techniques to stop female foeticides.

Question. Give the cross between RRYY x rryy for both F1 and F2 generation?
What are the combinations of characters produced in F2 generation and give reasons for appearance of new progeny?
Answer: 

Round-yellow, round-green, wrinkled-yellow, wrinkled-green in the ratio of 9: 3: 3: 1 are produced in F2 generations. New characters round-green and wrinkled-yellow are produced because when pure round-yellow pea plants are crossed with pure wrinkled-green pea plants the inheritance of one pair of character is independent of other pair i.e., the inheritance of character of round seed is not linked with yellow seed or the inheritance of wrinkled seed is not linked with green seed. The contrasting traits segregate and inherits independently and results in the formation of new combination of characters.
(b) The flowers of pea plants are hermaphrodite, i.e flowers have bisexual characteristics.
(c) Easy to obtain pure breed plant through self-fertilisation
(d) The generation time of pea plants is less.
(e) They have excellent disease resistance and have an optimal rate of survival.

Question. Acquiring characteristics or traits from one generation to the other is nothing but inheritance. Here, both the parents contribute equally to the inheritance of traits. It was Gregor Mendel, known as the Father of Genetics, who conducted immense research and studied this inheritance of traits. It was with his research on plant breeding that he came up with the laws of inheritance in living organisms. He conducted his experiments on pea plants to show the inheritance of traits in living organisms. He observed the pattern of inheritance from one generation to the other in these plants. 

(i) Name which Mendel’s experiments shows that traits are inherited indepe dently?
(ii) What is observed by Mendel in his experiment about the phenotype and genotype?
(iii) Define the term Genotype.
(iv) What do you understand by dominant alleles?
Answer: (i) The Dihybrid cross experiment that Mendel performed with the pea plants, shows that traits are inherited independently. In a cross between two plants with two pairs of contrasting characters, the expression of traits occurs independently.
(ii) In F2, the phenotype ratio is 9: 3: 3: 1. The genotype ratio is a very complex one.
(iii) Genotype: It is the complete heritable genetic identity of an organism. It is the actual set of alleles that are carried by the organism.
(iv) When an allele affects the phenotype of an organism, then it is a dominant allele. It is denoted by a capital letter. For example, ‘T’ to express tallness.

Question. A woman with blonde curly hair married a man with black soft hair. All of their children in first generation had black soft hair but in next generation children had different combinations in the ratio of 9 : 3 : 3 : 1. State the law that governs this expression.
Answer: Law of independent assortment governs this expression which states that inheritance of factors controlling a particular trait in an organism are independent of each other i.e., during the time of reproduction two pairs of factors of each of the two traits in a dihybrid cross segregate independently during gamete formation and randomly formed combinations in F2 generations.

Question. Given below is the experiment carried out by Mendel to study inheritance of two traits in garden pea. 

(i) Fill the boxes.
(ii) Why did Mendel carry experiment with two traits?
(iii) What were his findings with respect to inheritance of traits in F1 and F2 generations?
Answer: (i) 

(ii) Mendel carried out experiment with two traits to study the independent assortment of characters during inheritance.
(iii) In F1 generation though both the traits were inherited but only dominant traits [i.e., round, yellow characters] are expressed, the recessive traits, [wrinkled, green] were not expressed. But in F2 generation both dominant and recessive traits were expressed in the ratio 9: 3: 3: 1.

Question. Guinea pig having black colour when crossed with a guinea pig having same colour produced 80 offspring, out of which 60 were black and 20 were white. Now find out:
(i) What is the possible genotype of the guinea pigs?
(ii) Which trait is dominant and which trait is recessive?
(iii) What is this cross called and what is its phenotypic ratio?
Answer: (i) The possible genotype of the guinea pigs is Bb × Bb.
(ii) Black colour is dominant and white colour is recessive.
(iii) This is an example of monohybrid cross and its phenotypic ratio is 3 : 1.

Creating Based Questions :
 

Question. With the help of two suitable examples, explain why certain experiences and traits earned by people during their lifetime are not passed on to their next generations. When can such traits be passed on?
Answer: Learning skills like swimming, dancing and low weight of beetle due to starvation earned by people during their lifetime are not passed on to their next generation. If a person knows swimming or dancing, it is not necessary that this trait is seen in the progeny because learning a new skill does not change the genes of the germ cells of humans. Also, low weight in a beetle is not a trait that can be inherited by the progeny of a starving beetle. These traits or experiences occur in the somatic cells which do not involve germ cells and genetic materials. Therefore, these traits are not passe on to their next generation. Traits can be passed only when they have some direct effect on the genes. For example, if a mutation is caused in the germ cells of a person due to exposure to some harmful radiations, it is highly likely that the mutation caused will be passed on to the subsequent generations.

Question. Study the table given below and answer the questions.
In a human, how many chromosomes are present in:
(i) a brain cell?
(ii) a sperm in the testes?
(iii) an egg which has just been produced by the ovary?
(iv) a skin cell?
(v) a fertilised egg?
Answer: (i) 46.
(ii) 23.
(iii) 23.
(iv) 46.
(v) 46.

Question. Study the table given below and answer the following questions. 

(i) What is sex determination?
(ii) What are the sex chromosomes in the males?
(iii) What are the sex chromosomes in the females?
(iv) s Is the father responsible for the sex of the child?
Answer: (i) The process by which the sex of a newborn organism is detected is called sex determination.
(ii) The males have two sex chromosomes which are X and Y.
(ii) The sex chromosomes in the females are X and X.
(iv) Yes, it is the father that gives either the X or Y chromosome to the child. In case, the child receives the X chromosome from the father, then it has XX chromosome, and it develops into a female child. However, if the father gives Y chromosome, then the child develops into a male as it gets XY chromosome.

Question. “A brother and sister are more related to each other in comparison to the case when any one of them is related with his or her cousin”.
Through this statement what will we get to know about their ancestors?
Answer: A brother and sister are more closely related to each other which means that they have common ancestors more recently as compared to the case when any one of them (brother or sister) is related to the cousin. A brother and sister have their “parents” in common while a brother or sister and cousin have “grandparents” in common.

Question. Ram met with an accident. John his school mate takes him to the hospital where Ram (AB blood group) needs blood transfusion. John also has AB blood group and is willing to donate his blood but Ram’s mother object by saying “John belongs to different community so has different type of blood”. Give your opinion about Ram’s mother views.
Answer: Blood group does not depend on community, same blood group is same for all communties. Blood group AB has two alleles A and B in the people of all communities.

Question. Why a mice whose tail has been removed by surgery does not produce tailless mice?
Answer: A mice whose tail has been removed by surgery does not produce tailless mice because tailless character is not present in germ cells of mice so, it is an acquired character which cannot be passed onto the next generation.

Question. A Blue Colour Flower Plant Denoted by Bb is crossbreed with that of White Colour Flower Plant Denoted by Bb.
(i) State the Colour of Flower You Would Expect in Their F1 Generation Plants.
(ii) What must be the percentage of white flower plants in F2 generation if flowers of f1 plants are self-pollinated?
(iii) State the expected ratio of the genotypes BB and Bb in the F2 Progeny.
Answer: (i) Blue colour flower plant: BB
(ii) White colour flower plant: bb
(iii) The cross involved is as follows: 

(a) In F1 generation, all plants will have blue flower.
(b) In the F2 generation, 25% of flowers are white in colour.
(c) The ratio of the genotypes BB and Bb in the F2 generation would be 1(BB): 2(Bb).