CBSE Class 10 Biology Heredity And Evolution Worksheet Set A

Heredity And Evolution Notes Class 10 Science

Accumulation of Variations

Genetics: Branch of science that deals with heredity and variation.

Heredity: It means the transmission of features/ characters/ traits from one generation to the next generation.

Variation: The differences among the individuals of a species/ population are called variations.

• There are two types of variations

Somatic variation: Takes place in the body Example: boring of pinna, cutting of tails in dogs

Genetic variation: Takes place in the gametes/cells. (Reproductive cells)
Example: human height, skin colour.

Traits: Trait is a specific characteristic of an individual . Traits can be determined by genes, environmental factors or by a combination of both. Traits are essentially physical characteristics. These include length of the body, body shape, colour pattern, eyesight.
Traits are of two types

Acquired: Those characters which are obtained during the lifetime by any organism. For example - dancing, swimming, cycling, knowledge etc.

Inherited : Those characters which are present since birth and can be transferred from one generation to another.
For example- eye colour, hair colour, height, complexion etc.
Accumulation of Variation during Reproduction

• Variations appear during reproduction whether organisms multiply asexually or sexually.
Asexual Reproduction
Variations are fewer.
Occurs due to small, inaccuracies in DNA copying

(mutation)
Sexual Reproduction
Variations are large.
Occurs due to crossing over and separation of chromosomes, (mutation).

Importance of Variation
• Depending upon the nature of variations different individuals would have different kinds of advantage. For Example- bacteria that can withstand heat will survive better in a heat wave.
• It increases the chances of its survival in a changing environment. Free ear lobes and attached ear lobes are two variants found in human populations.
• There will be only minor differences between them, generated due to small inaccuracy in copying of DNA.
• If sexual reproduction is involved, greater diversity will be generated.

Gene: It is a section of DNA that provides information for one protein in a cell.
Genes control the trait. It is a unit of inheritance.

Chromosome: Each gene set is present as separate independent pieces called chromosomes. Each cell will have two copies of each chromosome, one from each parent. The total number of chromosomes in the offspring will be 46, 23 from mother and 23 from father
Mendel and His Work on Inheritance

Gregor Johann Mendel (1822-1884): Started his experiments on plant breeding and hybridization

• Mendel was known as the Father of Genetics
Plant selected by Mendel: Pisum Sativum (garden pea). Mendel used a number of contrasting characters for garden pea.

Characters: Seven pairs of contrasting characters in garden pea .
Flower colour
Flower position
Seed colour
Seed shape
Pod shape
Pod colour
Height of plant
Genetic Terminology

Gene: Mendel used the term factor for a gene. A gene is the unit of DNA responsible for the inheritance of character.

Allele: A pair of genes that control the two alternatives of the same character e.g., TT/tt.

Heterozygous: The organism in which both the genes of a character are unlike e.g., Tt.

Homozygous: The organism in which both the genes of a character are similar e.g., TT, tt.

Dominant: The gene which expresses itself in F1 generation is known as dominant gene.

Recessive: The gene which is unable to express itself in presence of the dominant gene.

Genotype: It is the genetic constitution (genetic makeup) of an organism which determines the characters.

Phenotype: It is the physical appearance of an individual. (Tall or short)
Mendel’s Experiments: Mendel conducted a series of experiments in which he crossed the pollinated plants to study
one character (at a time)
TT, tt ------ Pure or homozygous condition
Tt ------ Heterozygous condition or Hybrid
Phenotypic ratio: Ratio among numbers of individuals with different external features, e.g., 3:1
Genotype → Genetic makeup [TT, Tt or tt]
Genotypic ratio: Ratio among numbers of individuals with different genetic makeup, e.g., 1:2:1

Monohybrid Cross:
• Cross between two pea plants with one pair (monohybrid cross) contrasting characters,
Example: Tall or Short Plants
Dihybrid Cross
• A cross made between two plants having two pairs of contrasting characters is called dihybrid cross.
Example- round green seed and wrinkle yellow seed Mendel’s laws of inheritance

Law of dominance
• Some alleles are dominant while others are recessive. An organism with at least one dominant allele will display the effect of the dominant allele.

Law of segregation
• During gamete formation, the alleles for each gene segregate from each other so that each gamete carries only one allele for each gene.

Law of independent assortment
• Genes for different traits can segregate independently during the formation of gametes.
Mechanism of Heredity: 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 maybe enzymes, hormones, antibodies, and structural components of different types of tissues. In other words, DNA/ genes are responsible for structure and functions of a living body. Genotype of an individual controls its phenotype.

Sex Determination
• Phenomenon of decision or determination of sex of an offspring.
Factors Responsible for Sex Determination
Environmental: In some animals the temperature at which the fertilised eggs are kept decides the gender.
Example- in snails, some lizards and turtle.
In some animals like humans gender of individual is determined by a pair of chromosomes called sex chromosomes.

Sex Chromosomes: In human beings there are 23 pairs of chromosomes. Out of these 22 pairs of chromosomes are called autosomes and the last pair of chromosomes that help in deciding gender of that individual are called sex chromosomes. XX – female; XY – male.
• 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.
• This shows that all children will inherit an X chromosome from their mother regardless whether they are boys or girls.
• Thus sex of children will be determined by what they inherit from their father, and not from their mother.

Objective Questions

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 : B

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 

Very Short Answer Questions

Question : Why are the small numbers of surviving tigers a cause of worry from the point of view of genetics?
Answer: Small numbers of tigers means that fewer variations in terms of genes are available.This means that when these tigers reproduce, there are less chances of producing progeny with some useful variations. Hence, it is a cause of worry from the point of view of genetics.

Question : Why do all the gametes formed in human females have an X chromosome? 
Answer:  Women have a perfect pair of sex chromosomes, both called X. Women are XX, while men are XY. All children will inherit an X chromosome from their mother regardless of whether they are boys or girls. Thus, the sex of the children will be determined by what they inherit from their father. A child who inherits an X chromosome from her father will be a girl. During meiosis, one X chromosome enters each gamete. Hence, all the gametes formed in human females have an X chromosome.

Question : Do all variations in a species have equal chances of surviving in the environment in which they find themselves?
Answer: No, all variations in a species do not have equal chances of surviving in the environment in which they find themselves. Depending upon the nature of variations, each individual would have different advantages of survival.

Question : How is the normal number of chromosomes restored in the progeny of sexually reproducing organisms?
Answer: Each cell has two copies of each chromosome, one each from the male and female parents. Each germ cell or gamete takes one chromosome from each pair and when two germ cells combine, the original number of chromosomes is restored in the progeny.

Question : What is phylogeny and molecular phylogeny?
Answer: Phylogeny is the evolutionary history of an organism.
Molecular phylogeny traces the evolutionary relationships by comparing the differences in the DNA of different organisms.

Question : What is environmental selection?
Answer: It is the selection within a population resulting from the influence exerted by the environment. It leads to a change in the composition of genes within a population.

Question : How many pairs of chromosomes do human beings have, specify the types of chromosomes also?
Answer: Human beings have 23 pairs of chromosomes the first 22 pairs are called autosomes are similar in males and females. The 23rd pair is called the sex chromosome. In males it is XY and in females it is XX.

Question : What are analogous organs? Explain with an example.
Answer: Analogous organs have the same function but have different structural design and origin. For example, wings of birds and insects have the same function but have different structural design and origin.

Question : What is the significance of studying homologous and analogous organs?
Answer: Organisms that have homologous organs show relatedness and a common ancestory.

Question : Which of the following are homologous and analogous organs?
(a) Wings of birds and insects.
(b) Flippers of whale and fins of fish.
(c) Flippers of whale and wings of bat.
(d) Our teeth and elephants tusks.
(e) Potato and runners of grass.
Answer: (a) — Analogous organs
(b) — Analogous organs
(c) — Homologous organs
(d) — Homologous organs
(e) — Homologous organs.

Question : A true breeding tall plant is crossed with a true breeding short plant. All the offsprings of the F1 generation are tall. Of these two characters which one is dominant and which is
recessive.
Answer:

Question : How is the sex of the child determined in human beings?
Answer: Human beings have 23 pairs of chromosomes. 22 pairs are autosomes and the 23rd pair is the sex chromosome. The males have XY and the females have XX. All the gametes formed in the females are of one type i.e., X. In males there are two types of sperms that are formed –X and Y. If the sperms having X fertilizes with the egg the zygote formed is XX. This will form female child. If the sperm having Y fertilizes with the egg then the zygote formed is XY and the offspring will be male child. So basically it’s the male gametes that decide the sex of the unborn child.

Question : Which is gene flow?
Answer: It is the exchange of genetic material by interbreeding between populations of the same species. Gene flow increases the variations in a population.

Question : How do we know how old fossils are?
Answer: We can find the relative of the fossil by the depth at which the fossil is found. The ones that are found near the surface are more recent than the fossils found in the deeper layer.
The second way of dating fossils in detecting the ratios of different isotopes of the same element in the fossils material.

Question. Match the terms given in column I with those given in column II :
       Column I                                   Column II
(i) Fossil                                 (a) A famous evolutionist
(ii) A theory of evolution          (b) Survival of the fittest
(iii) Probable ancestor of birds  (c) Petrified remains of prehistoric life
(iv) Charles Darwin                  (d) Father of genetics
(v) Gregor Mendel                    (e) Archaeopteryx
Answer : (i) c (ii) b (iii) e (iv) a (v) d

Question. What is meant by acquired and inherited traits ? Explain with one example each.
Answer : The traits of an organism that are not inherited, but develop in response to the environment, are called acquired traits. For example, if a mouse's tail gets cut, the cut tail is an acquired trait. The trait of an organism that is caused by a change in its genes is called an inherited trait.

Question. Why are the traits acquired during the lifetime of an individual not inherited ?
Answer : This happens because an acquired trait involves changes in non-reproductive tissues (somatic cells) which cannot be passed on to germ cells or the progeny. Therefore, these traits cannot be inherited.

Question. Can the wing of a butterfly and the wing of a bat be considered homologous organs ? Why or why not ?
Answer : No, wing of bat and wings of butterfly should not be considered as homologous organs because they have different structure and origin but have the same function of flying so they are analogous organs.

Question. Name two animals having homologous organs and two having analogous organs. Name these organs.
Answer : Lizards and frogs have homologous organs; their forelimbs are homologous. Insects and birds have analogous organs; their wings are analogous.

Question. What are fossils ? Giving one example, explain how fossils provide evidence for evolution.
Answer : The remains of animals or plants that lived in the past are known as fossils. The fossil bird Archaeopteryx looks like a bird, but it has many other features that are also present in reptiles. This is because Archaeopteryx had feathered wings like those of birds, but teeth and tail like those of reptiles. It gives the evidence of an evolutionary connection between birds and reptiles. Thus, the study of fossils provides evidence for evolution.

Question. Give an example of characteristics being used to determine how close two species are in evolutionary terms.
Answer : The relative closeness of two species can be determined using characters called the homologous structures. These are similar in different species, because of their inheritance from a common ancestor. For example, the limbs of human, frog, lizard and bird are similar in structure, though they perform different functions, suggesting their evolutionary relationship.

Question. In what way are homologous organs evidence for evolution ?
Answer : The similarities in structure and origin represents that all vertebrates are evolved from common ancestors. For example, forelimbs of humans, whale and bat shows similar structure but have different function. Hence, homologous organs give evidence for evolution.

Question : Explain the importance of fossils in deciding evolutionary relationships.
Answer: Fossils are the remains of the organism that once existed on earth. They represent the ancestors of the plants and animals that are alive today. They provide evidences of evolution by revealing the characteristics of the past organisms and the changes that have occurred in these organisms to give rise to the present organisms. Let us explain the importance of fossils in deciding evolutionary history with the help of the following example.
Around 100 million years ago, some invertebrates died and were buried in the soil in that area. More sediment accumulated on top of it turning it into sedimentary rock.
At the same place, millions of years later, some dinosaurs died and their bodies were buried on top of the sedimentary rock. The mud containing dinosaurs also turned into a rock.
Then, millions of years later, some horse-like creatures died in that area and got fossilized in rocks above the dinosaur fossils.Some time later, due to soil erosion or floods in that area, the rocks containing horselike fossils are exposed.
If that area is excavated deeper, then the dinosaur and invertebrates fossils can also be found. Thus, by digging that area, scientists can easily predict that horse-like animals evolved later than the dinosaurs and the invertebrates.
Thus, the above example suggests that the fossils found closer to the surface of the earth are more recent ones than the fossils present in deeper layers. 

Question. Some of the important fossils which have been studied are those of organisms X, Y and Z. X were marine arthropods which were common between 400 to 600 million years ago. Y were the invertebrate animals (molluscs) with a flat, coiled, spiral shell which lived in the sea about 180 million years ago. Z are the extinct carnivorous or herbivorous reptiles which appeared on the earth about 250 million years ago and became extinct about 65 million years ago. What are X, Y and Z ?
Answer : X : Trilobites ; Y : Ammonites ; Z : Dinosaurs

Question. The farmers have been cultivating a food plant X for over two thousand years and have produced as many as five entirely different looking vegetables A, B, C, D and E from it.
(a) What could the plant X be ?
(b) What are A, B, C, D and E ?
(c) What is the process of evolution involved in this example known as ?
Answer : (a) Wild cabbage (b) Cabbage, Broccoli, Cauliflower, Kohlrabi and Kale (c) Artificial selection

Question. There are five animals A, B, C, D and E. The animal A uses its modified forelimbs for flying. The animal B uses its forelimbs for running whereas the animal C uses its forelimbs for grasping. The animal D can live on land as well as in water and uses its forelimbs to prop up the front end of its body when at rest. The animal E which respires by using spiracles and tracheae uses wings for flying but its wings are analogous to the modified forelimbs of animal A.
(a) What could the animals A, B, C, D and E be ?
(b) Why are the forelimbs of animals A, B, C and D called homologous organs ?
(c) What does the existence of homologous organs in animals A, B, C and D tell us about their ancestors ?
(d) Why are the modified forelimbs of animal A and the wings of animal E called analogous organs ?
(e) State whether animals A and E have a common ancestor or not.
Answer : (a) A : Bird ; B : Lizard ; C : Human ; D : Frog ; E : Insect
(b) Because they have the same basic design but perform different functions
(c) They are derived from the same ancestor
(d) Because they have different basic design but perform similar functions (e) No

Question. X, Y, and Z are three animals. The animal X can fly but animal Y can only run on ground or walls. The forelimbs of animals X and Y have the same basic design but they are used for different purposes such as flying and running respectively. The animal Z became extinct a long time ago. The study of fossils of Z tells us that it had some features like those of X and some like those of Y. In fact, Z is said to form a connecting link in the evolutionary chain of X and Y.
(a) What could the animals X, Y and Z be ?
(b) What name is given to the forelimbs like those of X and Y which have the same basic design but different functions ?
(c) Name one feature in which Z resembled X.
(d) Name one feature in which Z resembled Y.
(e) Which is the correct evolutionary chain involving X, Y and Z : X → Z → Y or Y → Z → X ?
Answer : X : Bird ; Y : Lizard ; Z : Dinosaur (b) Homologous organs (c) Both Z and X had feathered wings like those of birds (d) Both Z and Y had tail like those of reptiles (e) Y → Z → X

Question. A population of red beetles lives in green bushes in a garden. Once during the process of breeding, a green beetle is produced.
(a) State whether the change in colour of beetle is a process of evolution or not.
(b) Can the new colour of green beetle be passed on to its next generations ?
(c) What will be the advantage (if any) of the green colour to the beetle ?
(d) State whether the production of green colour involved a change in genetic material or not.
Answer : (a) Yes (b) Yes (c) The green colour of beetle allows it to mix up with green bushes and helps in its survival (because then it cannot be seen easily by the predators) (d) Yes

Question. The organs P and Q of two animals have different structures but similar functions. On the other hand, the two organs R and S of two other animals have the same basic structure but different functions.
(a) What are the organs like P and Q known as ?
(b) Name the organs like P and Q. Also name the animals which have such organs.
(c) What are the organs like R and S called ?
(d) Name the organs like R and S. Also name the animals which have such organs.
Answer : (a) Analogous organs (b) Wings ; Insect and Bird (c) Homologous organs (d) Forelimbs; Lizard and Frog.

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 : The trait B which exists in 60% of the population is likely to have arisen earlier. This is because the traits (or variations) produced in an organism during successive generations get accumulated in the populations of the species.

Question. How does the creation of variations in a species promote its survival ?
Answer : Due to the creation of variations, a species can adjust to the changing environment around it. And this promotes the survival of the species in the changing environment. For example, the accumulation of ‘heat resistant’ variation (or trait) in some bacteria will ensure its survival even when the temperature in its environment rises too much due to a heat wave or some other reasons. On the other hand, the bacteria which did not have this variation to withstand heat would not survive under these circumstances and die.

Question. How do Mendel’s experiments show that traits may be dominant or recessive ?
Answer : Mendel first crossed pure-bred tall pea plants with pure-bred dwarf pea plants and found that only tall pea plants were produced in the first generation or F1 generation (see Figure 1). No dwarf pea plants (or short pea plants) were obtained in the first generation of progeny. From this Mendel concluded that the first generation (or F1 cross) showed the traits of only one of the parent plants : tallness. The trait of other parent plant, dwarfness, did not show up in the progeny of first generation. Mendel then crossed the tall pea plants of the first generation (F1 generation) and found that tall plants and dwarf plants were obtained in the second generation (or F2 generation) in the ratio of 3 : 1. In other words, in the F2 generation, three-fourth plants were tall and one-fourth were dwarf (see Figure 2). Mendel noted that the dwarf trait of the parent pea plant which had seemingly disappeared in the first generation progeny, reappeared in the second generation. Mendel said that the trait of dwarfness of one of the parent pea plant had not been lost, it was merely concealed or supressed in the first generation to re-emerge in the second generation. Mendel called the repressed trait of ‘dwarfness’ as ‘recessive trait’ and the expressed trait of ‘tallness’ as the ‘dominant trait’. In this way, Mendel’s experiments with tall and dwarf pea plants showed that the traits may be dominant or recessive

Question. How do Mendel’s experiments show that traits are inherited independently ?
Answer : When Mendel crossed pure-bred tall pea plants with pure-bred dwarf pea plants, he found that only tall pea plants were produced in the F1 generation. Now, when Mendel further crossed the tall pea plants of the F1 generation, he found that tall plants and dwarf plants were obtained in the ratio 3 : 1 in the F2 generation. Mendel noted that all the pea plants produced in the F2 generation were either tall or dwarf. There were no plants with intermediate height (or medium height) in-between the tall and dwarf plants. In this way, Mendel’s experiment showed that the traits (like tallness and dwarfness) are inherited independently. This is because if the traits of tallness and dwarfness had blended (or mixed up), then medium sized pea plants would have been produced.

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 : No, this information is not enough to tell us which of the traits, blood group A or blood group O, is dominant. This is because : (i) if the blood group A is dominant trait and blood group O is recessive trait, the daughter can have blood group O, and (ii) even if the blood group A is recessive trait but blood group O is dominant trait, the daughter can still have blood group O. Let us discuss these two possibilities in detail. Possibility 1 : When blood group A is dominant trait but blood group O is recessive trait When father’s blood group A is dominant trait, it can have two genotypes : IAIA and IAIO. And when mother’s blood group O is recessive trait it can have only one genotype : IOIO (because it should have two recessive alleles). Now, if one recessive allele IO comes from father and one recessive allele IO comes from mother, then the daughter can also have the genotype IOIO which can give her blood group O. Possibility 2 : When blood group A is recessive trait but blood group O is dominant trait When father’s blood group A is recessive trait, it can have only one genotype: IA IA (because it should have two recessive alleles). And when mother’s blood group O is dominant trait, then it can have two genotypes : IOIO and IOIA. Now, if one dominant allele IO comes from the mother and one recessive allele IA comes from the father, the daughter will have the genotype IOIA which will again give her blood group O.

Question. How is the sex of the child determined in human beings ?
Answer : Genetics is involved in the determination of the sex of a child. This can be explained as follows : The chromosomes which determine the sex of a child are called sex chromosomes. There are two types of sex chromosomes, one is called X chromosome and the other is called Y chromosome. (i) A male (man or father) has one X chromosome and one Y chromosome. This means that half the male gametes or half the sperms will have X chromosomes and the other half will have Y chromosomes. (ii) A female (woman or mother) has two X chromosomes (but no Y chromosomes). This means that all the female gametes called ova (or eggs) will have only X chromosomes. The sex of a child depends on what happens at fertilisation : (a) If a sperm carrying X chromosome fertilises an ovum (or egg) which carries X chromosome, then the child born will be a girl (or female). This is because the child will have XX combination of sex chromosomes. (b) If a sperm carrying Y chromosome fertilises an ovum (or egg) which carries X chromosome, then the child born will be a boy (or male). This is because the child will have XY combination of sex chromosomes.

Question. What are the different ways in which individuals with a particular trait may increase in a population ?
Answer : The various ways in which individuals with a particular trait may increase in a population are : (i) By the process of natural selection in which the 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 on. (ii) By the process of genetic drift caused by drastic changes in the frequencies of particular genes by chance alone.

Question. Why are the traits acquired during the life-time of an individual not inherited ?
Answer : For a trait of an organism to be inherited, it should bring about a change in the genes (or DNA) present in the reproductive cells or gametes of that organism. The traits acquired during the life-time of a person do not bring about a change in the genes (or DNA) present in its reproductive cells or gametes and hence they are not inherited by the offsprings.

Question. Why are the small number of surviving tigers a cause of worry from the point of view of genetics ?
Answer : Sometimes a species (a type of animal or plant) may completely die out. It may become extinct. Once a species is extinct, its genes are lost for ever. It cannot re-emerge at all. The small numbers of surviving tigers are a cause of worry from the point of view of genetics because if they all die out and become extinct, their genes will be lost for ever. Our coming generations will not be able to see tigers at all.

Question. What factors could lead to the rise of a new species ?
Answer : The important factors which could lead to the rise (or formation) of a new species are the following : (i) Geographical isolation of a population caused by various types of barriers (such as mountain ranges, rivers and sea). The geographical isolation leads to reproductive isolation due to which there is no flow of genes between separated groups of population. (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. Will geographical isolation be a major factor in the speciation of a self-pollinating plant species ? Why or why not ?
Answer : Geographical isolation will not be a major factor in the speciation of a self-pollinating plant because it does not depend on other plants for its process of reproduction to be carried out.

Question. Will geographical isolation be a major factor in the speciation of an organism that reproduces asexually ? Why or why not ?
Answer : Geographical isolation cannot be a major factor in the speciation of an asexually reproducing organism because it does not require any other organism to carry out reproduction.

Question. Give an example of the characteristics being used to determine how close two species are in evolutionary terms.
Answer : The changes in DNA during reproduction are mainly responsible for evolution. The changes which take place in the DNA of species go on accumulating from one generation to the next. So, if the changes in the DNA of any two species are less, then the two species are quite close to one another in evolutionary terms. But if the changes in the DNA of two species are much more, then the two species will be far apart from one another in evolutionary terms. Thus, it is the characteristic of the extent of change in the DNA which is being used to determine how close two species are in evolutionary terms.

Question. Can the wings of a butterfly and the wings of a bat be considered homologous organs ? Why or why not ?
Answer : The wings of a butterfly and the wings of a bat cannot be considered homologous organs because though the function of wings in both cases is the same (flying) but they have different basic design. The butterfly (which is an insect) has a fold of membranes as wings which are associated with a few muscles but no bones are present. On the other hand, a skeleton made of bones supports the bat’s wings.

Question. What are fossils ? What do they tell us about the process of evolution ?
Answer : The remains (or impressions) of dead animals or plants that lived in the remote past are known as fossils. The fossils provide evidence for evolution. For example, a fossil bird called Archaeopteryx looks like a bird but it has many other features which are found in reptiles. This is because Archaeopteryx has feathered wings like those of birds but teeth and tail like those of reptiles. Archaeopteryx is, therefore, a connecting link between the reptiles and birds, and hence suggests that the birds have evolved from the reptiles. Thus, fossils provide the evidence that the present animals (and plants) have originated from the previously existing ones through the process of continuous evolution.

Question. Why are human beings who look so different from each other in terms of size, colour and looks said to belong to the same species ?
Answer : The human beings who look so different from each other in terms of size, colour and looks are said to belong to the same species (Homo sapiens) because they can interbreed to produce fertile offsprings (sons and daughters).

Question. In evolutionary terms can we say which among bacteria, spiders, fish and chimpanzees have a ‘better’ body design? Why or why not ?
Answer : In evolutionary terms, we can say that bacteria has a ‘better’ body design than spiders, fish, and chimpanzees. This is because though bacteria is one of the simplest and primitive life forms but it still inhabits and survives in some of the most inhospitable (most unfavourable) habitats such as hot springs, deep-sea thermal vents and ice in Antarctica. Most other organisms (including spider, fish and chimpanzees) cannot survive in such harsh environments.

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 Give reason for your choice.
Answer : (c) TtWW. Explanation : T is the gene for tallness, t is the gene for shortness (or dwarfness), W is the gene for violet colour and w is the gene for white colour. Now, in this case, all the progeny bore violet flowers, so the parent tall plant must contain only the dominant genes for colour (which is violet colour). That is, the parent plant should have the gene pair WW. Again, since almost half of progeny plants were short, this means that the parent tall plant should contain genes for tallness as well as shortness. That is, the parent plant should have genes Tt in it. Now, combining Tt and WW, the genetic make up of the parent plant becomes TtWW.

Question. An example of homologous organs is :
(a) Our arm and a dog’s foreleg
(b) Our teeth and an elephants’ tusks
(c) potato and runners of grass
(d) all of the above
Answer : (d) all of the above

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) a Chinese school boy.

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 : Just on the basis of the statement that children with light-coloured eyes are likely to have parents with lightcoloured eyes, we cannot say whether the light eye colour trait is dominant or recessive. This is because two copies of a trait (say eye colour) are inherited from both parents (one from father and the other from mother) and unless we know the nature of the two eye-colour traits, we cannot tell which is dominant and which is recessive. Recessive traits appear only when both the parents contribute recessive genes. So, from the statement given here we can only presume that both the parents are contributing recessive genes.

Question. How are the areas of study–evolution and classification–interlinked ?
Answer : The classification of organisms is a reflection of their evolutionary relationships. Classification is based on similarities and differences amongst organisms : (i) The more characteristics two organisms have in common, the more closely they are related. And the more closely they are related, the more recently they will have had a common ancestor in the evolutionary chain. (ii) The more different characteristics two organisms have, the more remotely they are related. And the more remotely they are related, they will have had a common ancestor in the more remote past.

Question. Explain the terms analogous and homologous organs with examples.
Answer : Those organs which have different basic structure (or different basic design) but have similar appearance and perform similar functions are called analogous organs The wings of an insect and the wings of a bird are analogous organsThe wings of an insect and a bird have different structures (the insects have a fold of membranes as wings which are associated with a few muscles whereas a skeleton of bones, flesh and feathers support bird’s wings) but they perform the same function of flying. Those organs which have the same basic structure (or same basic design) but different functions are called homologous organs: The forelimbs of humans (man) and a lizard are homologous organs Both these organs have the same basic design of bones but they perform different functions.The forelimbs of a human (man) are used for grasping whereas the forelimbs of a lizard are used for running.

Question. Outline a project which aims to find the dominant coat colour in dogs.
Answer : In order to find the dominant coat colour (or dominant hair colour) in dogs, we should first select pure-bred male and female dogs having black colour and pure-bred male and female dogs having brown colour. Then : (i) cross the pure-bred black male dog with pure-bred brown female dog (ii) also, cross the pure-bred brown male dog with pure-bred black female dog Observe the coat colour (or hair colour) of progeny (or puppies) produced. (a) If all the progeny (or puppies) are black in colour, then black will be the dominant coat colour in dogs. (b) If, however, all the progeny (or puppies) are brown in colour, then brown will be the dominant coat colour in dogs.

Question. Explain the importance of fossils in deciding evolutionary relationships.
Answer : The importance of fossils in deciding evolutionary relationships is that they provide evidence that the present animals (and plants) have originated from the previously existing animals (and plants) through the process of continuous evolution. For example, a fossil bird called Archaeopteryx looks like a bird but it has many other features which are found in reptiles. This is because Archaeopteryx has feathered wings like those of birds but teeth and tail like those of reptiles. Archaeopteryx is, therefore, a connecting link between the reptiles and birds, and hence suggests that the birds have evolved from the reptiles.

Question. What evidence do we have for the origin of life from inanimate matter (lifeless matter) ?
Answer : The British Scientist J.B.S. Haldane suggested in 1929 that life must have originated from inanimate matter (lifeless matter) consisting of simple inorganic molecules such as methane, ammonia and hydrogen sulphide, etc., which were present on the earth soon after it was formed. The evidence for the origin of life from inanimate matter was provided by the experiments conducted by Stanley L. Miller and Harold C. Urey in 1953. They assembled an apparatus to create an early earth atmosphere which was supposed to consist of gases like methane, ammonia and hydrogen sulphide, etc., (but no oxygen), over water. This was maintained at a temperature just below 100°C and electric sparks were then passed through the mixture of gases (to simulate lightning) for about one week. At the end of one week, it was found that about 15 per cent of carbon (from methane) had been converted into simple compounds of carbon including ‘amino acids’ which make up protein molecules found in living organisms. This experiment provides the evidence that the life originated from inanimate matter (or lifeless matter) like inorganic molecules.

Question. Explain how sexual reproduction gives rise to more viable variations than asexual reproduction. How does this affect the evolution of those organisms that reproduce sexually ?
Answer : (a) The asexual reproduction gives rise to small variations because in this process the DNA of only one parent is copied. Due to this, the offsprings produced look almost the same. For example, sugarcane reproduces by the process of asexual reproduction, so if we observe a field of sugarcane, we will find very little variations in various sugarcane plants. All the sugarcane plants look alike. On the other hand, sexual reproduction gives rise to large variations because in this process DNA from the gametes of two parents (male and female) is combined together. For example, it is due to the large variations produced by sexual reproduction that no two human beings look alike (except identical twins). (b) The large genetic variations produced during sexual reproduction lead to the continuous evolution of those organisms which reproduce sexually. In fact, sexual reproduction plays an important role in the origin of new species having different characteristics. All this is not possible in the case of asexual reproduction.

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 the special type of reproductive cells (called gametes) which have only half the amount of DNA as compared to other body cells (called non-reproductive cells). So, when the gametes from male and female parents combine during sexual reproduction to form a fertilised egg called zygote, they contribute equal amount of DNA (half each). For example, the normal body cells of human beings contain 46 chromosomes each (made of DNA). Now, the human sperm cell (or male gamete) has 23 chromosomes and the human egg cell (or female gamete) has also 23 chromosomes. So, the combination of 23 chromosomes from male and an equal number of 23 chromosomes from female during sexual reproduction ensures equal genetic contribution of male and female parents in the progeny (to give 23 + 23 = 46 chromosomes).

Question. Only variations that confer advantage to an individual organism will survive in a population. Do you agree with this statement ? Why or why not ?
Answer : Yes, only those variations that confer advantage to an individual organism will survive in a population. This will become clear from the following example. Suppose there is a population of red beetles in the green bushes and a colour variation arises during reproduction so that one beetle is now green in colour (instead of red). This variation offers advantage of survival because the green beetle can mix up with green bushes, it cannot be spotted and eaten up by a crow and hence its population will increase. If, however, the variation had produced a blue coloured beetle, then this colour could not offer any survival advantage because blue beetle in green bushes could be easily spotted by a crow and eaten by it.

More Question

Question : Name some tools for tracing evolutionary relationships.

Question : “ It is not as if one species is eliminated to give rise to a new one “ justify the statement.

Question : What do we understand by the term ‘ artificial selection ‘

Question : During evolution,a change that is useful for one property to start with can become useful later for quite a different function. Explain with the help of an example.

Question : How can we find out age of fossils.

Question : What is the relationship of classification & evolution?

Question : Explain genetic drift.

Question : Why do we find very little variations among sugarcane plants.

Question : Why did Mendel choose pea plant for his experiments.

Question : What is the difference between dominant & recessive gene.

Question : With the help of an example show that genes control traits.

Question : Give some contrasting characters seen in pea plant.

Question : Name an animal which can change its sex.

Question : Name the homologous organs of three animals.

Question : What name is given to the sequence of gradual changes over millions of years in which new species are formed.

Question : With the help of a diagram show how sex is determined in human beings.

Question : A mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bear violet flowers but almost half of them were shown. Suggest the genetic make up of the tall parent.

Question : What would be the genotypic ratio in F₂ generation of monohybrid cross.

Question : If a plant is heterozygous for tallness,the F₂ generation has both tall and dwarf plants. Which principle does it prove?