CBSE Class 10 Biology Heredity And Evolution Set D

Multiple Choice Questions (MCQs)

Question. Exchange of genetic material takes place in
(a) Vegetative reproduction
(b) Asexual reproduction
(c) Sexual reproduction
(d) budding
Answer : C

Question. What will be the number of chromosomes present in each gamete produced by the plants if the palisade cells of a species of plant contain 28 chromosomes in all?
(a) 56
(b) 28
(c) 14
(d) 4
Answer : C

Question. The following results were obtained by a scientist who crossed the F1 generation of pure-breeding parents for round and wrinkled seeds.
Dominants trait - Round seeds
Recessive trait - Wrinkled seeds
No. of F₂ offspring - 7524
From these results, it can be concluded that the actual number of round seeds he obtained was:
(a) 1881
(b) 22572
(c) 2508
(d) 5643
Answer : D

Question. A Mendelian experiment consisted of breeding of tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers but almost half of it were short.
This suggests that the genetic make up of tall parent can be depicted as:
(a) TTVV
(b) TTvv
(c) TtVV
(d) TtVv
Answer : C

Question. In peas, a pure tall plant (TT) is crossed with a pure short plant (tt). The ratio of pure tall plants to pure short plants in F₂ generation will be:
(a) 1 : 3
(b) 3 : 1
(c) 1 : 1
(d) 2 : 1
Answer : C

Question. The process where characteristics are transmitted from parent to offspring is called
(a) Variation
(b) Heredity
(c) Gene
(d) Allele
Answer : B

Question. When two parents are crossed, the offspring are referred to as :
(a) recessives
(b) test cross
(c) F1 generation
(d) F2 generation

Answer : C

Question. A cross between two individuals results in a ratio of 9 : 3 : 3 : 1 for four possible phenotypes of progeny.This is an example of a :
(a) dihybrid cross
(b) monohybrid cross
(c) test cross
(d) none of these

Answer : A

Question. For his experiments on heredity, Mendel used :
(a) papaya plants
(b) potato plants
(c) pea plants
(d) pear plants

Answer : C

Question. The human animal which has an XY pair of chromosomes is called :
(a) male
(b) hybrid
(c) female
(d) doomed

Answer : A

Question. The science of heredity is known as :
(a) biology
(b) embryology
(c) genetics
(d) biochemistry

Answer : D

Question. A gene is a :
(a) hybrid
(b) heritable trait
(c) pure breed
(d) part of a chromosome that transmits a trait

Answer : D

Question. A normal cell of human body contains 23 pairs of chromosomes. The number of chromosomes in a sex cell (sperm or ovum) of a human being is most likely to be :
(a) 46
(b) 23
(c) 21
(d) 42

Answer : B

Question. In order to ensure that he had pure-breeding plants for his experiments, Mendel :
(a) cross-fertilised each variety with each other
(b) let each variety self fertilise for several generations
(c) removed the female parts of the plants
(d) removed the male parts of the plants.

Answer : B

Question. In the human blood grouping, the four basic blood types are type A, type B, type AB, and type O. The blood proteins A and B are :
(a) simple dominant and recessive traits
(b) incomplete dominant traits
(c) codominant traits
(d) sex-linked traits

Answer : C

Question. A plant with two ‘small’ genes breeds with a plant with two ‘tall’ genes to produce :
(a) small plants and tall plants in the ratio 1 : 3
(b) all small plants
(c) all tall plants
(d) tall plants and small plants in the ratio 3 : 1

Answer : C

Question. A pregnant woman has an equal chance of her baby being blood group A or blood group AB. Which one of the following shows the possible genotypes of the woman and the father of her child ?
(a) IA IA and IB IO
(b) IA IB and IB IO
(c) IA IO and IB IO
(d) IA IB and IA IO

Answer : A

Question : An example of homologous organs is:
(a) Our arm and a dog’s fore-leg.
(b) Our teeth and an elephant’s tusks.
(c) Potato and runners of grass.
(d) All of the above ✓

Answer : D

Question : In evolutionary terms, we have more in common with:
(a) A Chinese school-boy ✓
(b) A chimpanzee
(c) A spider
(d) A bacterium.

Answer : A

Question : A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were 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

Question. The palisade cells of a species of plant contain 28 chromosomes. How many chromosomes will there be in each gamete produced by the plant ?
(a) 56
(b) 28
(c) 14
(d) 4

Answer : C

Question. Which of the following may be used to obtain an F2 generation ?
(a) allowing flowers on a parent plant to be self-pollinated
(b) allowing flowers on an F1 plant to be self-pollinated
(c) cross-pollinating an F1 plant with a parent plant
(d) cross-pollinating two parent plants

Answer : B

Question. The following results were obtained by a scientist who crossed the F1 generation of pure-breeding parents for round and wrinkled seeds.
Dominant trait Recessive trait No. of F2 offspring
Round seeds Wrinkled seeds 7524
From these results, it can be concluded that the actual number of round seeds he obtained was :
(a) 1881
(b) 22572
(c) 2508
(d) 5643

Answer : D

Question. The visible characteristic in an organism is known as :
(a) prototype
(b) stereotype
(c) phenotype
(d) genotype

Answer : C

Question. The exchange of genetic material takes place in :
(a) vegetative reproduction
(b) asexual reproduction
(c) sexual reproduction
(d) budding

Answer : C

Question. A cross between a tall plant (TT) and short plant (tt) resulted in progeny that were all tall plants because :
(a) tallness is the dominant trait
(b) shortness is the dominant trait
(c) tallness is the recessive trait
(d) height of plant is not governed by gene T or t

Answer : A

Question. The number of pair(s) of sex chromosomes in the zygote of humans is :
(a) one
(b) two
(c) three
(d) four

Answer : A

Question. In peas, a pure tall plant (TT) is crossed with a pure short plant (tt). The ratio of pure tall plants to pure short plants in F2 generation will be :
(a) 1 : 3
(b) 3 : 1
(c) 1 : 1
(d) 2 : 1

Answer : C

Question. The two versions of a trait (character) which are brought in by the male and female gametes are situated on :
(a) copies of the same chromosome
(b) sex chromosomes
(c) two different chromosomes
(d) any chromosomes

Answer : A

Question. Select the statements that describe characteristics of genes :
(i) genes are specific sequence of bases in a DNA molecule
(ii) a gene does not code for proteins
(iii) in individuals of a given species, a specific gene is located on a particular chromosome
(iv) each chromosome has only one gene
(a) (i) and (ii)
(b) (i) and (iii)
(c) (i) and (iv)
(d) (ii) and (iv)

Answer : B

Question. Select the group which shares the maximum number of common characters :
(a) two individuals of a species
(b) two species of a genus
(c) two genera of a family
(d) two genera of two families

Answer : A

Question. A trait in an organism is influenced by :
(a) paternal DNA only
(b) maternal DNA only
(c) both maternal and paternal DNA
(d) neither by paternal nor by maternal DNA.

Answer : C

Question. In human males all the chromosomes are paired perfectly except one. This/these unpaired chromosomes is/are :
(i) large chromosome (ii) small chromosome (iii) Y chromosome (iv) X chromosome
(a) (i) and (ii)
(b) (iii) only
(c) (iii) and (iv)
(d) (ii) and (iv)

Answer : C

Question. The sex of a child is determined by which of the following ?
(a) the length of the mother’s pregnancy
(b) the length of time between ovulation and copulation
(c) the presence of an X chromosome in an ovum
(d) the presence of a Y chromosome in a sperm

Answer : D

Question. A zygote which has inherited an X chromosome from the father will develop into :
(a) baby boy
(b) baby girl
(c) adult
(d) either boy or girl

Answer : B

Question. Which of the following statement is incorrect ?
(a) for every hormone there is a gene
(b) for every protein there is a gene
(c) for production of every enzyme there is a gene
(d) for every type of fat there is a gene

Answer : D

Question. If the ratio of each phenotype of the seeds of pea plants in the F2 generation is 9 : 3 : 3 : 1, it is known as :
(a) tetrahybrid ratio
(b) monohybrid ratio
(c) dihybrid ratio
(d) trihybrid ratio.

Answer : C

Questions Based on High Order Thinking Skills (HOTS)

Question. In humans, if gene B gives brown eyes and gene b gives blue eyes. What will be the colour of eyes of the persons having the following combination of genes ?
(a) Bb
(b) bb
(c) BB

Answer : A

Assertion (A) and Reason (R) type questions.

Following questions consist of two statements – Assertion (A) and Reason (R). Answer these
questions selecting the appropriate option given below:
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.

Question. Assertion(A) : Variations are seen in offspring produced by sexual reproduction.
Reason (R) : DNA molecule generated by replication is not exactly identical to original DNA.
Answer : A

Question. Assertion (A) : When pea plants (pure line) having round yellow seeds are crossed with pure line plants having wrinkled green seeds, then all pea plants obtained in F₁, generation bear wrinkled green seeds.
Reason (R): Round and yellow seeds are dominant to wrinkled and green seeds.
Answer : D

Question. Assertion (A): Selfing of a plant for several generations helps plant breeders to obtain pure breeding varieties.
Reason (R): Pure breeding plants are heterozygous for many traits.
Answer : C

Question. Assertion(A): The sex of a child is determined by the mother.
Reason (R): Humans have two types of sex chromosomes: XX and XY.
Answer : D

Question. Assertion (A): Mendel chose a number of varieties of garden pea as plant material for his experiments.
Reason(R): Garden pea has well defined characters and is bisexual.
Answer : A

Very Short Answer Type Questions

Question. Which of the processes, sexual reproduction or asexual reproduction, brings about maximum variations in the offsprings ?
Answer : Sexual reproduction

Question. Name one variation in humans connected with ears.
Answer : Free earlobes and attached earlobes are an example of variation in human ears.

Question. What constitutes the link between one generation and the next ?
Answer : DNA is the bio-molecule which contains all the genetic information and passes it on to the next generation. Thus, DNA functions as the heredity material and acts a link between one generation and the next.

Question. If the 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 : B

Question. Mendel said that the characteristics or traits of organisms are carried from one generation to the next by internal factors which occur in pairs. What is the modern name for these factors ?
Answer : Genes

Question. Some plants occur in one of the two sizes : tall or dwarf. This characteristic is controlled by one pair of genes. Tallness is dominant to dwarfness. Choose suitable letters for this gene pair.
Answer : Tt

Question. What are the chromosomes XY and XX known as ?
Answer : XY and XX chromosomes are called as sex chromosomes or allosomes.

Question. Which of the two, sperm or ovum, decides the sex of the child ?
Answer : A baby’s sex is determined at the time of conception. When the baby is conceived, a chromosome from the sperm cell, either X or Y, fuses with the X chromosome in the egg cell, determining whether the baby will be female (XX) or male (XY). To be female, one needs to be (XX), whereas to be a male, (XY) is needed.

Question. State whether the following statement is true or false :
Answer : False

Question. A new born child has an XY pair of chromosmes. Will it be a baby boy or a baby girl ?
Answer : Baby boy

Question. Which of the following combinations of sex chromosomes produce a male child : XX or XY ?
Answer : Combination XY

Question. Name the first scientist who studied the inheritance of traits from one generation to the next.
Answer : Gregor Mendel

Question. What type of plants were used by Mendel for conducting his experiments on inheritance ?
Answer : Pea plants

Question. The gene for red hair is recessive to the gene for black hair. What will be the hair colour of a person if he inherits a gene for red hair from his mother and a gene for black hair from his father ?
Answer : Let the genotype of black hair be BB.
Let the genotype of red hair be bb (recessive gene given).
Therefore the genotype of the mother's hair is bb while the father's hair is BB.
Therefore the child will have black hair since it is dominant but will also contain the recessive red hair trait but will not be expressed i.e. Bb.

Question. What are the four blood groups in humans ?
Answer : The four blood groups in humans are :-
B, AB, and O.

Question. Name one reptile in each case where higher incubation temperature leads to the development of : (a) male progeny, (b) female progeny.
Answer :(a) male progeny, -Lizard (Agama agama)
(b) female progeny. - Turtle (Chrysema picta)

Fill in the following blanks with suitable words :
(a) Genes always work in ...................... .
(b) In pea plants, the gene for dwarfness is ...................whereas that for tallness is ...................... .
(c) Most people have .................earlobes but some have ..............earlobes.
(d) A human gamete contains................... chromosomes whereas a normal body cell has ................... chromosomes in it.
(e) All races of man have.............blood groups.
(f) The.................chromosomes for a ...............are XX whereas that for a ......................are XY.
Answer : (a) pairs (b) recessive ; dominant (c) free ; attached (d) 23; 46 (e) same (f) sex ; female ; male

Short Answer Type Questions

Question. Which of the following represent tall plants and which represent short plants (or dwarf plants) ?
(a) Tt (b) tt (c) TT
Give reason for your choice (The symbols have their usual meaning).
Answer : When we consider T as the dominant trait and t as a recessive trait, TT and Tt represent tall plants and tt represent short plants.
In TT, both the alleles are dominant.
In Tt, one allele is dominant and one allele is recessive. So, the dominant allele is expressed.
In tt, both alleles are recessive so the plant is short. 

Question. A man having blood group O marries a woman having blood group B and they have a daughter. What will be the blood group of the daughter ?
Answer : Equal chance of having blood group O or blood group B

Question. (a) Name the scientist who gave the laws of inheritance.
(b) Name an animal in which individuals can change sex. What does this indicate ?
Answer : (a) Gregor Mendel
(b) Snails. This indicates that sex is not determined genetically in some animals.

Question. Explain with an example, how genes control the characteristics (or traits).
Answer : The characteristics or traits in animals and plants are controlled by genes. For example the transmission of color of hair from the parents to the child. If a mother has black hair and the father has blonde hair and the child has black hair then the transmission of genes for hair colour from the mother and father to the child is as follows: Mother’s cell contains two genes HH for black hair.

Question. (a) State one advantage of variation to a species.
(b) What are sex chromosomes ? How many sex chromosomes are there ? Name them.
Answer : (a) Advantages:-
1. It make every individual different from each other so that each individual has its own identity and can be easily recognisable.
(b) Sex chromosomes are chromosomes that determine whether the individual is a male or female. Though these two chromosomes pair with each other during meiosis, there is usually a very minimal homology or recombination between them, primarily because of a large difference in their genetic content and size. Often one chromosome is smaller, and appears to retain only those genes that are necessary for sex determination.
These genes not only give rise to the external male or female characteristics, but are responsible for the development of their reproductive organs.
There are two types of sex chromosomes
XX and XY
In humans, each cell normally contains 23 pairs of chromosomes, for a total of 46. Twenty-two of these pairs, called autosomes, look the same in both males and females. The 23rd pair, the sex chromosomes, differ between males and females. Females have two copies of the X chromosome, while males have one X and one Y chromosome.
The 22 autosomes are numbered by size. The other two chromosomes, X and Y, are the sex chromosomes.
The spread of chromosomes in a human is listed below.

Question. Explain how, sex is determined in human babies.
Answer : Sex determination in humans takes place by XY system.
The sperm and ovum fuse to form a zygote.
Females give rise to an X chromosome in the gamete and males can give rise to either X or Y in the gamete.
Among the million sperms, the type of sperm that fuses with the ovum determines the sex in the human babies.
XX combination gives rise to the female sex and the XY combination gives rise to the male sex.

Question. What do the following symbols used in the topic on heredity represent ?
(a) TT (b) tt (c) XX (d) XY
Answer : (a) TT represents a tall plant. (b) tt represents a dwarf plant. (c) XX represents a female.

Question. (a) What will you get in the F1 and F2 generations in the following cross ? Pure tall pea plant × Pure dwarf pea plant
(b) Is it an example of monohybrid cross or dihybrid cross ?
Answer : (a) In the F1 generation, all the plants will be tall, while in the F2 generation, out of four plants, three will be tall and one dwarf.
(b) This cross is an example of a monohybrid cross as it involved only one characteristic, the height of the plant into consideration.

Question. In the F2 generation of a cross, progeny having different traits are produced in the ratio 3 : 1. State whether it is a monohybrid cross or a dihybrid cross ? Give one example of such a cross.
Answer : Monohybrid cross

Question. (a) What is the genotype of dwarf plants which always produced dwarf offspring ?
(b) What is the genotype of tall plants which always produced tall offspring ?
(c) What is the genotype of (i) dwarf plants, and (ii) tall plants, whose parental cross always produces tall offspring ?
Answer : (a) tt (b) TT (c) (i) tt (ii) TT

Question. (a) If a normal human cell has 46 chromosomes, how many chromosomes will be there in a human (i) sperm cell, and (ii) zygote ?
(b) What sizes of plants are produced if both parents have genes Tt ?
Answer : (a) (i) 23 (ii) 46 (b) 3 tall plants and 1 dwarf plant

Question. In a human, how many chromosmes are present in :
(a) a brain cell ?
(b) a sperm in the testes ?
(c) an egg which has just been produced by the ovary ?
(d) a skin cell ?
(e) a fertilised egg ?
Answer : (a) 46 (b) 23 (c) 23 (d) 46 (e) 46

Question. Gregor Mendel’s first law of genetics states “Of a pair of contrasted characters, only one can be represented in a gamete by its internal ‘factor’.
(a) Give the modern name for this ‘factor’.
(b) State where these factors are found in gametes.
Answer : (a) Genes (b) Chromosomes

Question. Does genetic combination of mother play a significant role in determining the sex of a new born baby ?
Answer : No, because mother has a pair of X chromosomes. All new born babies will inherit an X chromosome from mother regardless of whether they are baby boys or baby girls.

Question. Give the contrasting traits of the following characters in pea plant and mention which is dominant and which is recessive :
(a) Yellow seed (b) Round seed
Answer : (a) Green seed ; Yellow is dominant; Green is recessive (b) Wrinkled seed ; Round is dominant ; Wrinkled is recessive

Question. Why are the small numbers of surviving tigers a cause of worry from the point of view of genetics ?
Answer : Genetics is a process in which parents pass their genes to their children.
If surviving tigers are less in number, it increases the chance of loss of genetic variability.
When the tiger population decreases, then the genetic pool of the tigers will decrease and it will get extinct

Question. Will geographical isolation be a major factor in the speciation of an organism that reproduces asexually ? Give reason for your answer.
Answer : Geographical isolation:
Species separation by physical barriers.
Requires single parent.
Speciation:
Is an evolutionary process.
Leads to the formation of new species.
Asexual reproduction:
New offspring is produced by single parent.
Genetically and physically similar to each other.
Clones of their parents.
Occurs through mitosis.
Geographical isolation in asexually reproducing organisms does not lead to speciation because:
No variation in offspring occurs.
Absence of meiosis.
Lack of genetic recombination and selection.

Question. Name the various tools of tracing evolutionary relationships which have been used for studying human evolution.
Answer : The various tools of tracing evolutionary relationships that have been used for studying human evolution are excavation , carbon-dating, study of fossils and determination of DNA sequences.

Question. Out of bacteria, spider, fish and chimpanzee, which organism has a better body design in evolutionary terms ? Give reason for your answer.
Answer : Spiders, fish and chimpanzee have a better body design compared to bacteria, as they can adapt to the changes in the environment in a better way, due to their complex body structure; that increases their chances of survival.

Question. With the help of an example, explain how variation leads to evolution.
Answer : Variations which are beneficial to the organism is passed on to its progeny, leading to the evolution of species. For example, the green colour of a beetle is an inherited trait, that helps in its survival (as it can by mix with green bushes to hide).

Question. (a) What is meant by a species ? Give two examples of plant species and two of animals.
(b) State the various factors which could lead to the formation of new species.
Answer : (a) A species is a population of organisms consisting of similar individuals which can breed together and produce fertile offspring. Wheat and paddy are examples of plant species. Humans and cats are examples of animal species.
(b) The important factors which could lead to the formation of new species are:
(i) The geographical isolation of a population caused by various types of barriers
(ii) Genetic drift caused by drastic changes in the frequencies of particular genes, by chance alone.
(iii) Variations caused in individuals due to natural selection.

Question. What evidence do we have for the origin of life from inanimate matter (lifeless matter) ?
Answer : Miller-Urey experiment provided evidence for the origin of life from inanimate matter. In the experiment, they created an atmosphere containing molecules like ammonia, methane and hydrogen sulphide, but no oxygen. This atmosphere was similar to the atmosphere present during the primitive Earth. Temperature was maintained constant at 100
oC and sparks were passed through the mixture to stimulate lightning. After a particular time, certain amount of the carbon from methane had been converted to simple compounds of carbon like amino acids. Amino acids are the precursors of the proteins and proteins are the molecules that support the life in basic form.

Question. Does geographical isolation of individuals of a species lead to the formation of a new species ? Provide a suitable explanation for your answer.
Answer : Geographical isolation of individuals of a species leads to the formation of a new species, as it interrupts the flow of genes between their isolated populations through the gametes.

Question. Bacteria have a simpler body plan when compared with human beings. Does it mean that human beings are more evolved than bacteria ? Explain your answer.
Answer : Bacteria:
If the body’s complexity is a criterion, humans are considerably superior to germs.
Bacteria are unicellular and prokaryotic organisms. They have a simple structure.
They are grouped under Kingdom Monera.
Whereas human beings are highly evolved multicellular and eukaryotic organisms.
They are grouped under Kingdom Animalia.
Bacteria have cellular-level organization and humans have an organ-level organization.
On the other hand, if we compare both based on the habitat, then, bacteria have developed faster than humans in terms of survival abilities.
Humans can exist in any environment as long as they are protected artificially.
Humans are unable to survive in harsher environments, whereas bacteria can be found anywhere on the planet.
They can withstand even the harshest environments, such as hydrothermal vents and sulfur springs.
Therefore it is not completely right to compare humans and bacteria in terms of the level of evolution.
Although the cellular structure and presence of developed organ systems may make humans more evolved than most other species yet this would always be debatable.

Long Answer Type Questions

Question. (a) Name the scientist who gave the theory of origin of life on earth. What is this theory ?
(b) How are those species which are now ‘extinct’ studied ?
Answer : (a) The theory of the origin of life on earth was given by J.B.S. Haldane in 1929. He suggested that life must have evolved from the simple inorganic molecules such as ammonia, methane and hydrogen sulphide that were present on earth when it was formed. He said that conditions on earth at that time could have converted simple inorganic molecules into complex organic molecules necessary for life. These complex organic molecules must have combined together to form the first living organisms.
(b) The species that are extinct are studied through their fossils that are found by digging the earth. Carbon-dating method is used to find the age of a fossil.

Question. What do you understand by the term ‘evolution’ ? State Darwin’s theory of evolution.
Answer : Evolution is the sequence of gradual changes which take place in the primitive organisms over millions of years, during which new species are produced.
Darwin's theory of evolution:
(a) Within any population, there is natural selection. Some individuals have more favourable variations than others.
(b) Even though all species produce a large number of offsprings, the population remains fairly constant naturally.
(c) This is due to the struggle between members of the same species and different species for food, space and mate.
(d) the struggle within populations eliminates the unfit individuals. The fit individuals possessing favourable variations survive and reproduce. This is called natural selection.
(e) The individuals having favourable variations pass on these variations to their progeny from generation to generation.
(f) These variations when accumulated over a long period of time lead to the origin of new species.

Question. (a) Explain the terms ‘analogous organs’ and ‘homologous organs’ with examples.
(b) In what way are analogous organs evidence for evolution ?
Answer : (a) Analogous organs are those organs which have a different basic structure but perform the same function. For example, the wing of an insect and a bird have completely different basic structure, but they perform the same function—of flying.
Homologous organs are those organs which have the same basic structure but perform different functions. For example, the forelimbs of humans, cheetahs, whales and bats have the same basic structural plan. But the forelimbs in humans are used for grasping, in cheetah for running, in whales for swimming and in bats for flying.
(b) The presence of analogous organs in different animals provide evidence for evolution by telling us that though they are not derived from common ancestors, they can still evolve to perform similar functions to survive, flourish and keep on evolving in the prevalent environment. Thus, the presence of analogous organs provides a mechanism for evolution.

Question. (a) Define ‘speciation’. Explain how speciation occurs.
(b) Will geographical isolation be a major factor in the speciation of a self-pollinating plant species ? Give reason for your answer.CQs)
Answer : (a) The process by which new species develop from the existing ones is known as speciation. Speciation occurs when the population of the same species splits into two separate groups, which then get isolated from each other geographically by barriers such as mountain ranges, rivers or the sea. The geographical isolation of the two groups of the population leads to their reproductive isolation due to which no genes are exchanged between them. However, breeding continues within the isolated populations producing more and more generations. Over the generations, the processes of genetic drift and natural selection operate in different ways in the two isolated groups of population and make them more and more different from each other. After thousands of years, the individuals of these isolated groups of the population become so different that they will be incapable of reproducing with each other, even if they happen to meet again. In this way, two new species have been formed.
(b) Geographical isolation cannot be a major factor in the speciation of self-pollinating plant species, as it does not have to look to other plants for its process of reproduction to be carried out.

Question. (a) Define ‘natural selection’.
(b) “Only variations that confer an advantage to an individual organism will survive in a population”. Do you agree with this statement ? Give reason for your answer.
Answer : (a) Natural selection is the process of evolution of a species in which characteristics that help individual organisms to survive and reproduce are passed on to their offsprings and those characteristics which do not help are not passed.
(b) Yes, we agree with the statement. Some changes always appear when animals produce their progeny by sexual reproduction. One of the progeny may be taller than the other. The advantage of long legs to the progeny is that when no food is available on the ground, those with long legs can reach the leaves on tall trees, eat them and survive. On the other hand, the progeny that are short cannot reach the leaves on tall trees. They will not get any food and eventually die from starvation. Now, since long legs are a beneficial variation, the long-legged animals will live long enough to produce their offsprings. So, all the future generations will be long-legged.

Case Study questions

Read the following and answer any four questions.

Pea plants can have smooth seeds or wrinkled seeds. One of the phenotypes is completely dominant over the other. A farmer decides to pollinate one flower of a plant with smooth seeds using pollen from plant with wrinkled seeds. The resulting pea pod has all smooth seeds.

Question : Which of the following conclusions can be drawn?
(i) The allele for smooth seeds is dominated over that of wrinkled seeds.
(ii) The plant with smooth seeds is heterozygous.
(iii) The plant with wrinkled seeds is homozygous.
a. 1 only
b. 1 and 2 only
c. 1 and 3 only
d. 1, 2 and 3
Answer : C

Question : Which of the following crosses will give smooth and wrinkled seeds in same proportion?
a. RR X rr
b. Rr X rr
c. RRX Rr
d. rr X rr
Answer : B

Question : On crossing of two heterozygous smooth seeded plants (Rr), a total of 1000 plants were obtained in F1 generation. What will be the respective number of smooth and wrinkled seeds obtained in F1 generation
a. 750, 250
b. 500, 500
c. 800, 200
d. 950, 50
Answer : A

Question : The characters which appear in the first filial generation are called
a. recessive characters
b. dominant characters
c. lethal character
d. non-mendelian characters.
Answer : B

Question : Which of the following crosses will give 100% smooth seeds ?
a. RR X rr
b. Rr X rr
c. RrX Rr
d. rr X rr
Answer : A

Read the passage and answer the given five questions

Pea plant is also tiny, easy to grow, and produces a big number of offspring. Pea plants can have tall plants or dwarf plants. One of the phenotypes is completely dominant over the other. A farmer decides to pollinate one flower of a tall plant with using pollen from plant with dwarf plant. The resulting pea pod has all tall plants.

Question : Which of the following conclusions can be drawn?
(1) The allele for tallness is dominated over that of dwarfness.
(2) The plant with tallness is heterozygous.
(3) The plant with dwarfness is homozygous.
A. 1 only
B.1 and 2 only
C. 1 and 3 only
D. 1, 2 and 3.
Answer : D

Question : Which of the following crosses will give tall and dwarf plants in same proportion?
A. TT x tt
B. Tt x tt
C. TT x Tt
D. tt x tt
Answer : B

Question : Which of the following cross can be used to determine the genotype of a plant with dominant phenotype?
A. TTXTT
B. Tt x Tt
C. Tt x TT
D. Tt x tt
Answer : D

Question : State the ratio of tall plant to dwarf plants in F2 genertion.
A. 2:2,
B.1:3
C.3:1,
D.4:0
Answer : C

Question : The characters which does not appear in the first filial generation are called
A. Recessive characters
B. Dominant characters
C. Lethal characters
D. Non-mendelian characters.
Answer : A