Biology Practice set Paper I

 

Biology Practice set Paper I

How do Organism Reproduce?

1.        Name the life process of an organism which helps in the growth of its population.

Ans. Reproduction

 

2.      Where is DNA found in the cell?

Ans. DNA is found in the nucleus.

 

3.      What is DNA?

Ans. DNA means deoxyribo nucleic acid. It lies in the cell nucleus, which is the source of information for making proteins and different proteins lead to different body designs.

 

4.      Name the information source of making proteins in the cell. State two basic events in reproduction.

Ans. DNA in the nucleus of a cell is the source of information for making proteins. The two basic events in reproduction are: (i) creation of copy of DNA (ii) creation of an additional cellular apparatus by cell then the DNA copies separate, each with its own cellular apparatus.

 

5.      What is the effect of DNA copying which is not perfectly accurate in the reproduction process?

Ans. DNA copying will lead to variation in populations which helps in evolution of the species.

 

6.      The mode of reproduction depends on which feature of the organism?

Ans. The mode of reproduction depends upon the body design of organism. Those with simple body design may reproduce asexually. Those with complex body design may form gametes and may undergo sexual reproduction.

 

7.      Organisms have a varied body design. Name the property which gives the basic difference in body design.

Ans. Errors in DNA copying

 

8.      Explain how do organisms create an exact copy of themselves.

Ans. The cell uses biochemical reactions to make exact copies of DNA or genetic material.

 

9.      What is the importance of DNA copying in reproduction? OR Why is DNA copying an essential part of the process of reproduction?

Ans. Reproduction involves producing same kind of species from parents. The genetic information is passed to the offspring by DNA present in each cell. This DNA, which is replicated (copied) is responsible for the resemblance of parents with offsprings with same traits. New traits are passed on due to difference in copying, new variations may occur and new species may evolve.

 

10.    Why is variation beneficial to the species but not necessary for the individual?

Ans. If environmental conditions undergo drastic change, some member of a species having variations of tolerating changed temperature may be able to survive, e.g. suppose population of bacteria is living in temperate water. If temperature of water increases due to global warming, most of the bacteria will die. However, if few variants are resistant to heat, then they would survive and grow further. Thus variation is useful for the survival of species over time, but it is not necessary for the individual.

 

11.    How does reproduction help in providing stability to the populations of species?

Ans. . It is because reproduction is a process by which species increase their population. If the rate of birth is more than death, the size of population will increase and will be more stable.

12.    When a cell reproduces, what happens to its DNA?

Ans. During the process of reproduction, transmission of DNA from parents to offspring takes place. Before reproduction, DNA is replicated, which means two copies of DNA are produced. When the cell divides into two, these two copies are distributed equally between the two daughter cells so that similar amount and type of DNA is transferred from the parent cell to the daughter cells. It maintains the consistency in the amount and type of DNA in the living organism of a particular species.

13.  How does binary fission differ from multiple fission?

Ans. In binary fission cell divides into two cells whereas in multiple fission a cell divide into many daughter cells.

 

14.  Name the part of Bryophyllum where the buds are produced for vegetative propagation.

Ans. Buds arise from the notches in the leaf margins of Bryophyllum

 

15.  What happens when a Planaria gets cut into two pieces?

Ans. Each piece regenerates into new organism, planaria.

 

16.  What happens when a mature Spirogyra filament attains considerable length?

Ans. A mature spirogyra filament breaks into smaller fragments and each fragment grows into a new plant. This process of reproduction is called fragmentation.

 

17.  Name the method by which hydra reproduces. Is this method sexual or asexual?

Ans. Hydra reproduces by budding. It is an asexual mode of reproduction.

 

18.  Name two simple organisms which have ability of regeneration.

Ans. Planaria and Hydra

 

19.  Name the causative agent of the disease ‘kala-azar’ and its mode of asexual reproduction.

Ans. Leishmania causes ‘kala-azar’. It reproduces by binary fission in a definite orientation.

 

20.  Select two plants from the following which are grown by vegetative propagation process: Banana, Wheat, Mustard, Jasmine, Gram.

Ans. Banana and jasmine are grown by vegetative propagation, because they do not produce seeds.

 

21.  Give the respective scientific term used for studying. (i) The mechanism by which variation are created and inherited and (ii) The development of new type of organism from the existing ones.

Ans. (i) Heredity, (ii) Evolution.

 

22.  Name the type of cells which undergo regeneration.

Ans. Specialised cells called regenerative cells which can make large number of new cells.

 

23.  Regeneration is not possible in all types of animals. Why?

Ans. Regeneration is carried out by specialised cells which are present in few animals which can reproduce by regeneration.

 

24.  What is the advantage of reproduction through spores? OR How will organism be benefitted if it reproduces through spores?

Ans. Spore with a cell wall can survive even in adverse conditions. Large number of spores are produced in one sporangium. It is easily dispersed through wind as they are large in number and light in weight.

 

25.  Name the method by which spirogyra reproduces under favourable conditions. Is this method sexual or asexual?

Ans. Under favourable conditions, spirogyra reproduces by a process known as fragmentation. This is an asexual method of reproduction.

 

26.  How does Planaria reproduce? Is this method sexual or asexual?

Ans. Planaria reproduces by a process known as regeneration. It is a type of asexual reproduction in which if Planaria can be cut into any number of pieces, each piece grows into a new individual organism.

27.  How does Plasmodium reproduce? Is this method sexual or asexual?

Ans. Plasmodium is a single celled organism which reproduces by the process of multiple fission. Multiple fission (i.e. breaking up of a single cell into many daughter cells) is an asexual mode of reproduction.

28.  “Cell division is a type of reproduction in unicellular organism”. Justify.

Ans. Cell division in a unicellular organism results in the formation of two or more daughter cells which means it produces more individual.

 

29.  What is a clone? Why do offsprings formed by asexual reproduction exhibit remarkable similarity?

Ans. Clones are organisms which are exact genetics copies of their parents. Their DNA is exactly identical to their parents, which is the cause of remarkable similarity.

 

30.  Can you think of reasons why more complex organisms cannot give rise to new individuals through regeneration?

Ans. Those organisms which reproduce by regeneration have similar and non-differentiated cells throughout the body, therefore body parts of organism grows into new organism. In multicellular complex organisms, the cells get differentiated and perform different functions. In such organisms, body parts like skin, muscles, i.e. tissues can be regenerated, but the whole organism cannot be reproduced by regeneration.

 

31.  Why is vegetative propagation practised for growing some types of plants?

Ans. (i) Vegetative propagation is used to grow plants in which seeds are not formed or very few seeds are formed, e.g. banana, pineapple, orange.

(ii) It helps to grow plants in conditions where seed germination fails due to change in environment.

(iii) It is a faster, easier and cheaper process.

(iv) The plants produced are genetically similar and good quality or variety can be preserved easily

 

32.  List two advantages of growing grapes or banana plants through vegetative propagation.

Ans. (i) Characteristics of parent plants are preserved. (ii) Since these plants do not have viable seeds, therefore vegetative propagation is advantageous.

 

33.  List three conditions favourable for spores to germinate and grow.

Ans. (i) Availability of nutrition (food), (ii) moist place, (iii) warmth, (iv) dark place.

 

34.  Name the type of asexual reproduction in (a) Planaria (b) Rhizopus (c) Spirogyra (d) Hydra

Ans. (a) Regeneration (b) Spore formation (c) Fragmentation (d) Budding

 

35.  Leaves of bryophyllum fallen on the ground produces new plants, why?

Ans. Leaves of bryophyllum has adventitious buds or plantlets in the notches along the leaf margin. When buds fall on the soil, they develop into new plant under favourable conditions.

 

36.  Name the type of asexual reproduction in which two individuals are formed from a single parent and the parental identity is lost. Write the first step from where such a type of reproduction begins. Draw first two stages of this reproduction.

Ans. Binary fission, e.g. Fission in Amoeba Elongation of cell and its nucleus is the first step.

 

37.  What is multiple fission? How does it occur in an organism? Explain briefly. Name an organism which exhibits this type of reproduction.

Ans. The process in which parent cell divides into several small and equal sized daughter cells, which grows into a new organism is called multiple fission. The nucleus of a cell divides into large number of nuclei and cytoplasm separates, forming a membrane around it. All this occurs within a protective covering. Plasmodium exhibits multiple fission

 

38.  Explain the term ‘Regeneration’ as used in relation to reproduction of organisms. Describe briefly how regeneration is carried out in multicellular organisms like Hydra.

Ans. Regeneration is the ability to produce an organism from their body parts. Many fully differentiated organisms show ability to reproduce by regeneration. In hydra, if the body is cut into two or three pieces, the lower part will develop head while the upper part will develop rest of the body. If its cells are separated, hydra will reform its structure by regeneration.

39.  What happens when (i) Planaria gets cut into two pieces

(ii) A mature spirogyra filament attains considerable length

(iii) On maturation sporangia burst.

Ans. (i) Each piece will regenerate into complete organism. Cells at the location of the site of cut or wound, forms a small ball of cells that will differentiate into new tissues and regenerate the missing parts of the cut piece of the planaria.

(ii) When spirogyra attains considerable length, it breaks into smaller fragments and each fragment grows into a new plant.

(iii) On maturation, sporangial wall breaks and spores are dispersed to grow into new individual. These spores are very light and easily dispersed by wind.

 

40.  Explain budding in hydra with the help of labelled diagrams only.

Ans. A bud is an outgrowth from the body part of parent due to repeated cell division at a specific site. These buds develop into new individuals, which detach from the parent body on maturation. In hydra, the cells divide very fast at a specific site as an outgrowth called bud. These bud develops into a small individual, while remaining attached to the parent. When it matures, it detaches from the parent body to exist as an independent organism.

 

41.  What happens when: (a) Accidently, Planaria gets cut into many pieces? (b) Bryophyllum leaf falls on the wet soil? (c) On maturation sporangia of Rhizopus bursts?

Ans. (a) When Planaria gets cut into many pieces, it will undergo a process known as regeneration due to which each piece will grow into a new planaria organism.

(b) When bryophyllum leaf falls on the wet soil, the buds that are produced in the notches along the leaf will develop into new plants by the process known as vegetative propagation.

(c) When the sporangia of Rhizopus bursts upon maturation, the spores present inside it spread in the open environment. Then, with the help of different agents, they are carried to different places and when they land on a favourable surface, they start growing and produce new organism.

 

42.  What is vegetative propagation? State two advantages and two disadvantages of this method.

Ans. Vegetative propagation is a mode of asexual reproduction in which new plants are obtained from vegetative parts of the plants. It does not involve the production of seeds or spores for the propagation of new plants.

Two advantages of vegetative propagation are:

(i)                 Plants which do not produce seeds are propagated by this method, for example sugarcane, potato, etc.

(ii)               Vegetative propagation is a cheaper, easier, rapid method of propagation in plants than growing plants from their seeds. For example, lilies grow very slowly and take 4 to 7 years to develop flowers when grown through their seeds, but flowers are produced only after a year or two when grown vegetatively.

Two disadvantages of vegetative propagation are:

(i)                 As there is no genetic variation, there is no chance of development of new and better varieties.

(ii)               The vegetatively propagated plants are more prone to diseases that are specific to the species. This can result in the destruction of an entire crop.

 

43.  What is binary fission in organisms? With the help of suitable diagrams, describe the mode of reproduction in amoeba.

Ans. Binary fission is a fission in which two daughter cells are formed of nearly equal size after replicating the genetic material of a single cell. Single-celled organisms like amoeba undergoes binary fission.

 

44.  Identify the following methods and give one example of each

(a) Process in which reproduction takes place by breaking up of parent into several fragments.

(b) Process of dividing of organisms into many cells simultaneously.

(c) Process of reproduction by formation of bud on the parent body.

(d) Process of reproduction by formation of spores.

(e) Process used by multi-cellular organisms to reproduce by cutting into many pieces and each piece forms a new individual.

Ans. (a) Fragmentation, Spirogyra (b) Multiple fission, Plasmodium (c) Budding, Yeast/Hydra (d) Spore formation, Rhizopus (e) Regeneration, Planaria

 

 

Heredity and Evolution

1.        If a trait ‘A’ exists in 10% of the population of asexually reproducing species and trait ‘B’ exists in 60% of the same population, which trait is likely to have arisen earlier?

Ans. Trait ‘B’ is likely to have arisen earlier, because it has higher percentage in the population. In asexual reproduction, the traits which are present in parent generation are carried over to next generation with minimum variation.

 

2.        How does the creation of variations in a species promote survival?

Ans. Variations lead to adaptation of an organism, which is then able to survive in difficult and unfavourable environmental conditions. It means variations promote survival.

 

3.        What is DNA?

Ans. DNA stands for deoxyribonucleic acid. It is self replicative, molecule present in all living organisms as the main constituent of chromosomes. It passes on genetic characteristics to offsprings.

 

4.        Where is DNA found in the cell?

Ans. DNA is found in the nucleus of the eukaryotic cells and cytoplasm of prokaryotic cells. It is also present in plastids and mitochondria.

 

5.        Give the respective scientific names used for studying: (i) the mechanism by which variations are created and inherited by organism from the previous generation. (ii) the development of new type of organisms from the existing ones.

Ans. . (i) Heredity (ii) Evolution

 

6.        What is a gene?

Ans. Gene is the unit of heredity present in chromosomes, DNA which directs specific protein synthesis, when it is passed from the parent to the offspring.

 

7.        Name the information source for making proteins in the cells.

Ans. DNA

 

8.        What is heredity?

Ans. Heredity is a process in which traits are passed onto the offspring from parents.

 

9.        No two individuals are absolutely alike in a population. Why?

Ans. Sexual reproduction involves combination of genes from two parents, the genetic material gets exchanged between pairs of chromosomes before formation of their gametes which cause changes.

 

10.    All the variations in the species do not have equal chances of survival. Why?

Ans. All variations will not be able to survive in the environmental changes, which is an essential requirement for survival.

 

11.    Why do mice whose tails were surgically removed, continue to produce mice with tails?

Ans. It is because cutting of tail is done surgically and it is not inherited.

12.  A tall plant is crossed with a dwarf plant. What will be the ratio of homozygous tall and heterozygous dwarf plant in F2 generation?

Ans. The ratio of homozygous tall and heterozygous dwarf plant in F2 generation is 3 : 1.

 

13.    Name the genetic material that is responsible for the inheritance of traits.

Ans. Gene or DNA

 

14.    Do genetic combination of mothers play a significant role in determining the sex of a new born?

Ans. No, mother does not play an important role because mother has only ‘X’ chromosomes whereas father has both X and Y chromosomes. When offspring takes ‘X’ from mother and ‘Y’ from father, then baby boy is born. If offspring takes ‘X’ chromosomes from mother as well as father, girl child is born. The probability of getting boy or girl is 50 : 50.

 

15.    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?

Ans. The information is not sufficient because information is confined upto two generation. We need information about third generations to predict the dominant trait.

 

16.    What type of traits are phenotype?

Ans. The traits which are visible to us are called phenotype, e.g. tall or dwarf (short) are phenotype traits of the plants.

 

17.    What is genotype?

Ans. Genotype shows the genetic constituent of an organism, i.e. it is the description of genes present in an organism. For example, TT, Tt or tt where T and t are the different forms of the same gene. The genotype of tall plant is TT and that of dwarf plant is ‘tt’.

 

18.    State Mendel’s first law of inheritance.

Ans. Mendel’s first law, also known as the law of segregation states that ‘during gamete formation, the alleles of the character segregate in such a way that each gamete carries only one allele for each gene’.

 

19.    State Mendel’s second law of inheritance.

Ans. Mendel’s second law, also known as the law of independent assortment states ‘that for different traits genes segregate independently during the formation of gametes’.

 

20.    What is variation?

Ans. Variation is the difference in characteristics or traits among the individuals in a particular species.

 

21.    Name the human traits which show variations.

Ans. (i) Colour of eye, (ii) Height, (iii) Colour of skin

 

22.    What is monohybrid cross?

Ans. Monohybrid cross is the cross between two individuals with one pair of different traits.

 

23.    What is the scientific name of man and garden pea?

Ans. Man — Homo sapiens Garden pea — Pisum sativum

 

24.    What are sex chromosomes?

Ans. Those chromosomes which decide whether the offspring will be male or female are called sex chromosomes. Many animals have two types of chromosomes. Human beings have ‘X’ and ‘Y’ sex chromosomes.

 

25.    What are chromosomes? Where are they located?

  Ans. Chromosomes are long thread like structures which contain genetic information of an individual. These are located in the nucleus of a cell.

26.    What indication do we get by appearance of dwarf plant in F2 generation?

Ans. After obtaining F2 progeny from F1 generation in the dihybrid cross, Mendel concluded that when two pairs of traits are combined in a hybrid, one pair of character segregates independently from the other pair of character.

 

27.    Why is it that asexual reproduction produces exact copies but sometimes minor variations are seen in next progeny?

Ans. Bio-chemical reactions are not fully reliable, therefore, it may cause slight difference which causes these variations.

 

28.    Name the term used for traits that are exhibited externally

Ans. Phenotype

 

29.    Give an example where sex determination is regulated by environmental factors.

Ans. In snail, sex is determined by temperature i.e., environmental factor.

 

30.    Name the plant on which Mendel performed his experiments.

Ans. Garden pea (Pisum Sativum)

 

31.    Mendel observed a contrasting trait in relation to position of flowers. Mention that trait.

Ans. Terminal flowers have recessive trait whereas axial flowers have dominant trait.

 

32.    In order to ensure than Mendel gets pure breeding plants in his experiments, what did he do?

Ans. Mendel allowed each variety to self fertilise for several generations.

 

33.    A Mendelian experiment consisted of breeding pea plants bearing violet flowers with pea plants bearing white flowers. What will be the result in F1 progeny?

Ans. Violet flowers

 

34.    How can chromosomes be identified?

Ans. Individual chromosomes can be identified by their lengths, position of centromere and binding pattern of staining and shape.

 

35.    What is sex of the baby that inherits Y-chromosome from the father?

Ans. Male

 

36.    The chromosomal number of the sexually producing parents and their offspring is the same. Justify this statement.

Ans. 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.

 

37.    How do Mendel’s experiments show that traits may be dominant or recessive?

Ans. • 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.

 

38.    List two differences in tabular form between dominant traits and recessive traits.

Ans.

39.    Why are small number of surviving tigers a cause of worry from genetics point of view?

Ans. (i) Tigers may get extinct in near future due to natural calamities, lack of available food and due to their small number.

(iii)             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.

 

40.    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?

Ans. 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.

 

41.    (a) On what rules inheritance is based? (b) Is each trait influenced by both parental and maternal DNA?

Ans. (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.

 

42.    What is F2 generation?

Ans. The generation produced by the offsprings of F1 generation is called F2 or second generation.

 

43.    If YYRR is round yellow, what do the following represent? yyrr         yyRR.

Ans. yyrr – wrinkled, green seeds

yyRR – Round, green seeds

 

44.    In a monohybird cross between tall pea plants denoted by TT and short pea plant by tt, Sehaj Anant obtained only tall plants denoted by Tt in F1 generation. However in F2 generation she obtained both tall and short plants. Using the above information explain the law of dominance.

Ans. According to law of dominance, dominant trait is expressed in F1 generation although dominant as well as recessive traits are inherited. Single copy of dominant trait is sufficient for expression of tall dominant trait in F1 generation.

 

45.    How can we say that change in genes can be brought by the change in DNA?

Ans. A gene is present on a segment of DNA chromosome.

(i)                 It provides information of one protein to another and is responsible for its change.

(ii)               It ensures stability of DNA. It means change in gene segment of DNA will bring change in DNA.

 

46.    Where are genes located? What is the chemical nature of genes?

Ans. Genes are located on chromosomes at fixed positions. They are made of nucleic acid (DNA) therefore acidic in nature.

 

47.    Sex determination in man depends upon 23rd pair of chromosomes, called the sex chromosomes. If it were a homologous pair-XX, it would be a female. If it were a heterologous pair – XY, it would be a male. Based on this statement, answer these questions: (i) How many types of eggs/ova and how many type of sperms female and male human being will produce respectively ? (ii) Which parent’s contribution of sex chromosomes determines sex of the child?

Ans. (i) Male produces two genetic types of sperms, half with X and other half with Y chromosome. The female produces only one genetic type of ova, all carrying X chromosome.

(iii)             Sex of children will be determined by what they inherit from their father, and not from their mother.

 

48.    (a) What is genetic constitution of human sperm? (b) Mention the chromosome pair present in zygote determining the sex of male child.

Ans. (a) 22 + Y chromosome or 22 + X chromosomes.

(b) (22 + X + 22 + Y) = (44 + X + Y) chromosomes.

 

49.    Explain the Mendel’s concept of heredity, by giving three points

Chemistry Clear Point

Ans. (i) Mendel worked on pea plant. (ii) He found that traits are controlled by genes which come in pairs. (iii) Genes are inherited as separate units, one from each parent.

50.    An angiosperm plant having red flowers when crossed with the other having same colour flower produced 40 progenies, out of which 30 were red coloured flowers, 10 plants were with white flowers. Find out: (i) What is the possible genotype of parent plants?

(iii)             Which trait is dominant and recessive?

(iv)             What is the cross called and what is phenotype ratio?

Ans. (i) Rr and Rr

(ii) Red colour of flower is dominant trait while white colour is the recessive trait.

(iii) Monohybrid cross, phenotype ratio is 3 : 1.

 

51.    DNA is the carrier of the genetic information’. Justify

Ans. Most of the characters or traits of an organism are controlled by the genes. Genes are actually segments of DNA guiding the formation of proteins by the cellular organelles. These proteins may be enzymes, hormones, antibodies, and structural components of different types of tissues. In other words, DNA (genes) is responsible for structure and functioning of a living body. Genotype of an individual controls its phenotype. In human beings there are 23 pairs of chromosome. We inherit half of our chromosomes (genes) from our two parents. This means half of DNA in each cell has been inherited from each parent, hence we resemble both of them or their side of family members.

 

52.    In humans, there is a 50% probability of the birth of a boy and 50 % probability that a girl will be born. Justify the statement on the basis of the mechanism of sex-determination in human beings.

Ans. Justification: Women produce only one type of ovum (carrying X chromosome) and males produce two types of sperms (carrying either X or Y chromosome) in equal proportions. So the sex of a child is a matter of chance depending upon the type of sperm fertilising the ovum.

 

53.    What do you understand by the following terms: (i) Phenotype (ii) Genotype (iii) Dominant trait (iv) Recessive trait (v) Factors or Genes.

Ans. (i) Phenotype: The set of characteristics observed in an individual as a result of interaction of its genotype with the environment.

(ii) Genotype: The genetic make up of the cell which determines the characteristics (phenotype) of an organism. (iii) Dominant trait: The trait of the parent that expresses itself in the offspring.

(iv) Recessive trait: The trait of the parent appearing in the offspring which cannot express itself in the presence of dominant trait.

(v) Factors or Genes: The units of inheritance responsible for a trait in an individual.

 

54.    Mention the total number of chromosomes along with the sex chromosomes that are present in a human female and a human male. Explain how in sexually producing organisms the number of chromosomes in the progeny remains the same as that of the parents.

Ans. Chromosomes are long thread like structures which contain heriditary information of individual and are carrier of genes. The total number of chromosomes present in both human male and female is 46. Out of these, two chromosomes are the sex chromosomes. In human males, the two sex chromosomes present are X and Y, while in human female, both sex chromosomes are X. During sexual reproduction, the new individual is formed by the fusion of gametes from both the parents. These gametes are haploid in nature, i.e. they contain only one set of the chromosomes. They are formed by the meiosis, a type of cell division which reduces chromosome number to half. When these haploid gametes fuse during fertilisation, the two nuclei of these gametes fuse and the chromosome number is then restored to normal. Hence, the progeny formed has the same number of chromosomes as that of the parents.

Concept Clear Point