CBSE Class 10 Science Heredity Notes

This Class 10 Science exam notes page on Chapter 9, Heredity and Evolution, covers the accumulation of variation during reproduction, Mendel's monohybrid and dihybrid crosses with his two laws, sex determination in humans, the processes of evolution including natural selection, genetic drift and speciation, Darwin's theory, the Miller-Urey experiment, evolution and classification, homologous and analogous organs, fossils, human evolution and the evolutionary tree.

ACCUMULATION OF VARIATION DURING REPRODUCTION

By virtue of being the progeny of the parent, the progeny individual need not just be the replica of what its parents are. Inheritance of characters from the parents to the progeny (that is, heredity) ensures the passing of the parental characters to the progeny. The difference or change in the characteristics between individuals is called variation. Human population shows a great deal of variation.

Inheritance from the previous generation provides both a common basic body design, and subtle changes in it, for the next generation. The second generation will have differences that they inherit from the first generation, as well as newly created differences.

[Figure: Creation of diversity over succeeding generations. An original organism at the top gives rise to two individuals, similar in body design but with subtle differences. Each of them gives rise to two individuals in the next generation, so the bottom row has four individuals, each different from the others.]

Intext Questions, Page No. 143

Q1. 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?
Ans: Trait B. The percentage of any gene in a population increases from generation to generation.

Q2. How does the creation of variations in a species promote survival?
Ans: During reproduction (and also through inaccuracies in DNA replication), many variations occur in the offspring. Some individuals have more favourable variations than others, and such individuals survive and pass these variations on to their progeny. For example, in a population of beetles a colour arose during reproduction so that one beetle is green instead of red. This beetle can pass the colour on to its progeny. Crows cannot see the green beetles on green leaves, so the population of green beetles becomes larger than that of red beetles.

HEREDITY

The progeny produced through the reproductive process is similar to its parents in body design, function, etc. The rules of heredity determine the process by which traits and characteristics are inherited.

"The inheritance of characteristics through generations is called heredity."

The inheritable characteristics may be morphological, anatomical, physiological or reproductive, and are also known as traits.

Both father and mother contribute equal amounts of genetic material to the child. This means that each trait can be influenced by both paternal and maternal genetic material, that is, DNA.

RULES FOR THE INHERITANCE OF TRAITS: MENDEL'S CONTRIBUTIONS

Gregor Johann Mendel (1822-1884) carried out the first scientific experimental study on heredity. Mendel, an Austrian Augustinian monk, observed variations in the characteristics of the garden pea plant (Pisum sativum), which he cultivated in his monastery garden. He wanted to find out the results of crossing pea plants with variations in traits.

The visible contrasting characters that Mendel observed in garden pea plants were:
1. Seed shape: Round / Wrinkled
2. Seed colour: Yellow / Green
3. Flower colour: Violet / White
4. Pod shape: Full / Constricted
5. Pod colour: Green / Yellow
6. Flower position: Axillary / Terminal
7. Stem height: Tall / Dwarf

[Figure: Pictures of pea seeds, flowers, pods, flower positions and stems, each with its dominant and recessive trait labelled, for example round (dominant) and wrinkled (recessive) seed shape, yellow (dominant) and green (recessive) seed colour, violet (dominant) and white (recessive) flower colour.]

MENDEL'S MONOHYBRID CROSS

Mendel selected the garden pea plant, Pisum sativum, for his experiments. As pea plants produce seeds only by self-pollination, tall plants produced tall plants and dwarf plants produced dwarf plants generation after generation. Hence he termed them pure breeding varieties.

When a pure breeding tall plant was crossed with a pure breeding dwarf plant, all plants of the first filial generation (F1) were tall. There were no medium-height or dwarf plants. Only one parental trait was seen and not a mixture of the two. When this F1 tall plant was allowed to self-pollinate, both tall and dwarf plants appeared in the second filial generation (F2) in the ratio of 3:1. This shows that both tallness and dwarfness were inherited by the F1 plants, but only tallness was expressed.

[Figure: Diagrammatic representation of the monohybrid cross. Parental tall and dwarf plants give F1 tall plants. F1 plants self-pollinate to give F2 with tall and dwarf plants in the ratio 3:1.]

The first experiment of Mendel, considering the inheritance of a single trait (height of the plant, tall or dwarf), is called a monohybrid cross.

Expression of morphological characters, such as tall or dwarf plant and violet or white flower, is called the phenotype.
The expression of genes (or chromosomal make-up) of an individual for a particular trait is called the genotype.

Points to Remember:
1. Heredity: The passing of traits from the parents to offspring is called heredity.
2. Genotype: The complete set of genes in an organism's genome is called the genotype.
3. Phenotype: The observable characters in an organism make the phenotype. The phenotype is a result of the genotype's interaction with the environment, so many phenotypes are not inheritable.
4. Acquired traits: Traits acquired due to interaction with the environment are called acquired traits. Acquired traits are not inheritable.
5. Inheritable traits: Traits which can be expressed in subsequent generations are called inheritable traits. Such traits bring a change in the genotype of the organism and hence become inheritable.

Mendel's First Law (Law of Segregation): Every individual possesses a pair of alleles for a particular trait. During gamete formation, a gamete receives only one of the alleles. A particular trait can be dominant or recessive.

DIHYBRID CROSS

The cross in which two pairs of characters are studied is called a dihybrid cross. In his second experiment, Mendel used a dihybrid cross. He took plants with round and green seeds (RRyy) and plants with wrinkled and yellow seeds (rrYY). In the F1 generation all plants produced round and yellow seeds, which means that wrinkled texture and green colour were recessive characters. When F1 plants self-pollinated, most of the F2 plants produced round and yellow seeds. Some produced round green seeds, some wrinkled yellow seeds and some wrinkled green seeds. The ratio was 9:3:3:1.

[Figure: Dihybrid cross of round green (RRyy) and wrinkled yellow (rrYY) seeds. F1 seeds are all round yellow. The F2 generation shows round yellow, round green, wrinkled yellow and wrinkled green seeds in the ratio 9:3:3:1.]

Mendel's Second Law (Law of Independent Assortment): Alleles of different characters separate independently of each other during gamete formation. In the above example, the alleles for seed texture assorted independently of those for seed colour.

SEX DETERMINATION IN HUMANS

Somatic cells in human beings contain 23 pairs of chromosomes. The 23rd pair consists of the X and Y chromosomes. In a male, the 23rd pair contains one X and one Y chromosome. In a female, the 23rd pair contains two X chromosomes. All eggs have an X chromosome as the 23rd chromosome, while a sperm may have either an X or a Y chromosome. When a sperm with an X chromosome fertilises the egg, the zygote develops into a female child. When a sperm with a Y chromosome fertilises the egg, the zygote develops into a male child.

[Figure: Sex determination diagram. Parents XX (female) and XY (male) produce gametes X and Y in the male and X in the female. The zygotes are XX (female) and XY (male), in equal proportion.]

Intext Questions, Page No. 147

Q1. How do Mendel's experiments show that traits may be dominant or recessive?
Ans: Mendel crossed a pure tall pea plant (TT) with a pure dwarf pea plant (tt). All the progeny were hybrid tall (Tt). Only one trait was expressed in the F1 generation, which is the dominant trait. The recessive trait remained suppressed. When F1 plants were self-crossed, one-fourth of the plants were dwarf and three-fourths were tall.

Q2. How do Mendel's experiments show that traits are inherited independently?
Ans: Refer to the section on the dihybrid cross above.

Q3. 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: Yes. The daughter has blood group O, so she received the O allele from both parents, and her mother must be OO. The father must carry one O allele along with his A allele, so his genotype is AO. Blood group A is therefore dominant over O, and blood group O is recessive, since it appears only when both alleles are O.

Q4. How is the sex of the child determined in human beings?
Ans: Somatic cells in human beings contain 23 pairs of chromosomes. The 23rd pair is composed of different types of chromosomes named X and Y. The 23rd pair contains one X and one Y chromosome in a male, while in a female it contains two X chromosomes. All the eggs have an X chromosome as the 23rd chromosome, while a sperm may have either an X or a Y chromosome. When a sperm with an X chromosome fertilizes the egg, the zygote develops into a female child. When a sperm with a Y chromosome fertilizes the egg, the zygote develops into a male child.

EVOLUTION

The change in inherited traits in a biological population over subsequent generations is called evolution. Scientists have shown that life evolved in the form of simple unicellular organisms on this earth, and that all the organisms present today have evolved from a common ancestor. The idea of evolution is based on the premise of common ancestry.

Consider a group of twelve beetles. They live in bushes on green leaves, and their population grows by sexual reproduction, which generates variations in the population. Assume crows eat the red beetles, so if crows eat more red beetles, their population slowly reduces.

Situation 1: A colour variation arises during reproduction, so one beetle is green instead of red. This green beetle passes its colour to its progeny, so all its progeny are green. Crows cannot see green beetles on green leaves and therefore cannot eat them, but they can see and eat red beetles. As a result, green beetles become more numerous than red ones. The variation gave green beetles a survival advantage over red beetles, so it was naturally selected. The crows exert natural selection, and natural selection directs evolution in the beetle population, resulting in adaptation to the environment.

[Figure: Beetles on green leaves. Crows eat the red beetles more easily than the green beetles, and the population gradually shifts towards more green beetles.]

Situation 2: A colour variation occurs in the progeny, and now the variant beetle is blue instead of red. Blue beetles pass on the colour to their progeny. Crows can see blue beetles on green leaves as well as red ones, so they eat both. There is no survival advantage for blue beetles as there was for green beetles. Suppose there are a few blue beetles and most are red. An elephant then stamps on the bushes, killing most of the beetles. By chance, the few survivors are mostly blue, so the population slowly increases with mostly blue beetles. Such accidents can change the frequency of genes in small populations. This is known as genetic drift, which provides diversity in the population.

[Figure: An elephant stamps on the bushes where the beetles live. Most red beetles are killed and the few surviving beetles are mostly blue.]

Situation 3: The beetle population is increasing, but a plant disease destroys the leaf material, so the beetles get less food and become poorly nourished. Their weight decreases, but their genetic material (DNA) does not change. After a few years the plant disease is eliminated and the bushes become healthy again.

[Figure: Poorly nourished beetles on damaged bushes, with smaller beetles than before.]

Acquired and Inherited Characters and Evolution
The germ cells of a sexually reproducing population are formed in specialised reproductive tissue. If the beetles lose weight because of starvation, the DNA of the germ cells does not change. Therefore low weight is not a trait that can be inherited, and a loss of weight over generations due to starvation is not an example of evolution. Changes in non-reproductive tissues cannot be passed on to the DNA of germ cells. Therefore the experiences of an individual during its lifetime cannot be passed on to its progeny and cannot direct evolution.

Darwin's Theory of Evolution
Charles Darwin wrote his famous book 'Origin of Species'. Some salient points of his theory are:

Organisms have unlimited capacity to reproduce: Organisms reproduce at a fast pace, which is necessary for survival, because a higher number of offspring ensures that at least some of them survive. Each organism has to struggle for its day-to-day survival. For example, a frog lays thousands of eggs at one go. Most eggs are washed away or eaten by predators. Some develop into tadpoles, and many tadpoles are also eaten, leaving a few to develop into adults. A large number of eggs is needed to ensure that at least some develop into adults.

Natural Selection: Different individuals of a species have different traits. Those with more suitable traits are selected by nature. Each organism needs particular traits for finding food and finding a mate. Those with better traits are able to pass them on to the next generation.

Survival of the Fittest: The fittest organisms survive while others perish. This is how many species become extinct, and how other species continue to evolve over time.

MOLECULAR ORIGIN OF LIFE

Stanley L. Miller and Harold C. Urey conducted the Miller-Urey experiment in 1953 to demonstrate how life could have originated on the earth. They created an environment in the laboratory which mimicked the environment of earth at the time of origin of life. Water, methane, ammonia and hydrogen were used. The liquid was heated to initiate evaporation, and electrodes were used to create electric discharge. At the end of two weeks, some organic molecules were formed, including some amino acids and sugars. This supported the hypothesis of J. B. S. Haldane that life originated from inorganic raw materials.

[Figure: Apparatus of the Miller-Urey experiment with a flask of water, methane, ammonia and hydrogen, a heating source, electrodes producing electric discharge and a condenser.]

Intext Questions, Page No. 150

Q1. What are the different ways in which individuals with a particular trait may increase in a population?
Ans: An individual with a particular trait may increase in a population in the following ways:
1. When a colour variation arises during reproduction, the individual can pass the colour on to its progeny, which may protect them from predators.
2. Accidents in small populations can change the frequency of some genes even if they give no survival advantage.
3. Poor nourishment may reduce the population, but when conditions improve the population increases again.
Thus, maturation, adaptation to the environment and natural selection may increase a particular trait in a population.

Q2. Why are traits acquired during the life-time of an individual not inherited?
Ans: Traits acquired during the life-time enable an individual to adapt to its surroundings, but they do not change the genetic make-up of the organism. Only variations that come into the germ cells are inherited.

Q3. Why are the small numbers of surviving tigers a cause of worry from the point of view of genetics?
Ans: Small numbers of surviving tigers are a cause of worry because their loss would cause a loss of the gene pool, that is, many genes would be eliminated from the gene pool.

SPECIATION

The process of origin of a new species is called speciation. A species is a group of organisms in which most characters are similar and whose members are able to breed among themselves. Speciation can happen if two groups of the same species are prevented from interbreeding for several generations. This can happen because of geographical segregation or because of genetic changes. The evolution of new species through geographical segregation is called geographical (allopatric) speciation. Genetic drift is a separate process, the random change in gene frequency in small populations.

Intext Questions, Page No. 151

Q1. What factors could lead to the rise of a new species?
Ans: Genetic drift and natural selection could lead to the rise of a new species.

Q2. Will geographical isolation be a major factor in the speciation of a self-pollinating plant species? Why or why not?
Ans: No. Self-pollinated plants receive pollen from the same flower or the same plant, so distance from other plants hardly affects their reproduction. Moreover, self-pollinated plants rarely show variations in characters.

Q3. Will geographical isolation be a major factor in the speciation of an organism that reproduces asexually? Why or why not?
Ans: No. Asexual reproduction involves a single parent, so geographical isolation does not affect its reproduction cycle. Moreover, asexually reproducing organisms rarely show variations in characters.

EVOLUTION AND CLASSIFICATION

The modern system of classification is based on evolutionary relationships, so it is also known as phylogenetic classification. The kingdom is the highest taxon, while the species is the lowest taxon. Members of a species share more common characters than members of a kingdom. For example, all human beings belong to the species Homo sapiens and can interbreed irrespective of race or skin colour. Human beings, monkeys, elephants and cows all belong to the class Mammalia, and they share common characters such as hair on the body and mammary glands in females. All animals are eukaryotes and their cells lack a cell wall. The degree of similarity or dissimilarity shows that all animals have evolved from a common ancestor.

Homologous Organs: Organs which have a common design but serve different functions in different animals are called homologous organs. For example, the forelimbs of all tetrapods are made of the humerus, radio-ulna, carpals and metacarpals. The forelimbs of frogs are adapted for jumping, those of birds for flying and those of humans for handling tools. This shows that frogs, birds and humans have evolved from a common ancestor.

[Figure: Forelimbs of a frog, a lizard, a bird and a human, showing the same basic bone pattern labelled as homologous organs.]

Analogous Organs: Organs which have a different design but serve a common function in different animals are called analogous organs. The wings of birds and the wings of bats are a good example. The wings of birds are made of the bones of the forelimb and are covered with feathers. The wings of bats are mainly made of digital bones and a thin membrane. Yet both are used for flying.

[Figure: Wing of a bat and wing of a bird, both labelled as analogous organs.]

Fossils
The preserved remains of animals, plants or other organisms from the distant past are called fossils. Fossils provide evidence that present animals have originated from previously existing ones through continuous evolution. Fossils can be used to reconstruct the evolutionary history of an organism. Ancient fossils in the bottom rocks are simple, while the most recent fossils in the upper strata are more highly evolved. Fossils therefore become more complex as we move from the earliest to the most recent rocks. They give an idea of the time when different species were formed or became extinct, and they indicate connecting links between two groups of organisms. For example, Archaeopteryx is a connecting link between reptiles and birds.

Intext Questions, Page No. 156

Q1. Give an example of characteristics being used to determine how close two species are in evolutionary terms.
Ans: Homologous organs, analogous organs, vestigial organs, etc.

Q2. Can the wing of a butterfly and the wing of a bat be considered homologous organs? Why or why not?
Ans: No. A butterfly belongs to phylum Arthropoda, which has a chitinous exoskeleton, while a bat belongs to class Mammalia, whose wings are skin folds. Both wings have a similar function but a different structure, so they are analogous, not homologous.

Q3. What are fossils? What do they tell us about the process of evolution?
Ans: Fossils are the preserved remains of animals, plants or other organisms from the distant past. Fossils provide the evidence that the present animals have originated from previously existing ones through the process of continuous evolution. Fossils can be used to reconstruct the evolutionary history of an organism. The distribution pattern of fossils shows that the ancient fossils present in the bottom rocks are simple, while the most recent fossils found in the upper strata are more highly evolved. It means fossils form and become more and more complex as we proceed from earliest to recent rocks. It gives us an idea of time in history when different species were formed or became extinct. Fossils also help to trace the evolutionary history of some animals. Fossils also indicate connecting links between two groups of organisms. For example, Archaeopteryx is a connecting link between reptiles and birds.

EVOLUTION BY STAGES

Evolution of complex organs, and therefore of complex organisms, has happened in stages. Take the evolution of the eye as an example. Planaria is the first animal to show an eye-like structure. The dark spots on planaria are light-sensitive spots, but planaria cannot distinguish between two different objects. The eyes of insects are compound eyes made up of thousands of optical units. The eyes of higher animals are simple eyes, composed of a single lens. Most animals cannot differentiate among colours, and depth perception is weak in many animals. Human eyes are the most advanced, because humans can recognise colours and have very good depth perception.

Most tetrapods use all four limbs for locomotion. Some apes can walk short distances using only the hind limbs. Humans have finally evolved bipedal walking.

Evolution Tree
An evolutionary tree is a branching diagram used to show the inferred evolutionary relationships among biological species, based on similarities and differences in their physical and genetic characters.

[Figure: A branching evolutionary tree showing the relationships among organisms, drawn from a common ancestor at the base.]

Evolutionary tree (contents of the figure):

• Base: simple unicellular organisms (amoeba-like and Paramecium-like forms). From these, three main lines arise:
  • Animal line (blue arrows): a small colonial ball-like organism gives rise to the animals:
    • Sea anemone (Cnidaria), flatworm (planarian), snail, centipede-like worm, prawn and grasshopper (invertebrates)
    • Fish, which gives rise to the frog (amphibian)
    • The frog line branches to the lizard (reptile), the bird and the ape
  • Fungal line: mushroom (fungi)
  • Plant line (red arrows): a small colonial ball-like organism gives rise to the plants:
    • Green alga and lichen-like forms
    • Moss and fern
    • Conifer tree
    • Flowering plants: grass, leafy herb, flowering plant with red flowers and a broad-leaved tree

Evolution vs Progress
Evolution does not always mean progress. Bacteria are among the simplest and oldest organisms on the earth, and their simple body design does not make them weak. They survive some of the harshest climates, such as the craters of volcanoes and sulphur springs. Many animals have features that hamper routine activities. For example, the branch-like horns of an antelope can get entangled in bushes while it runs, which may lead to its death. The colourful feathers of a male peacock attract females, but they also make it easy for predators to spot it, and its bulky feathers prevent it from flying away to safety.

HUMAN EVOLUTION

Modern humans are called Homo sapiens. Scientific investigations have shown that modern humans evolved in Africa. They migrated north and settled near the Mediterranean Sea. When the ice age ended, melting ice raised the water level. Humans then migrated in different directions. One branch went to western Asia, then to the Indian peninsula, and finally to Australia. From the Indian peninsula, one branch migrated towards China and then to North America, and from North America the humans migrated to South America. From the Mediterranean Sea, the second branch migrated towards Europe, where they are believed to have replaced the Neanderthals.

Human evolution is the evolutionary process leading up to the appearance of modern human beings. Early man-like forms appeared about 7,50,000 years ago. The first sure fossil of our own species, Homo sapiens, indicates that true man appeared on the earth about 2,50,000 years ago.

Evolution of man through ages:
1. Homo habilis lived between 1.6 and 2.5 million years ago.
2. Homo erectus lived between 1 and 1.8 million years ago.
3. Homo sapiens neanderthalensis lived between 2,30,000 and 3,00,000 years ago.
4. Homo sapiens (present man) appeared about 40,000 years ago.

[Figure: Human evolution, showing the progression of early human forms to present-day Homo sapiens.]

There is great diversity in human forms and features across the planet. For a long time, people spoke of human 'races', and skin colour was the commonest way of identifying the so-called races. Recent evidence has shown that there is no biological basis for the notion of human races. All humans are a single species with a common ancestor.

Intext Questions, Page No. 158

Q1. 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?
Ans: All human beings, despite differences in size, colour and looks, belong to the same species because they have similar DNA sequences, have descended from the same ancestors, and are capable of reproducing among themselves. These variations may have arisen due to environmental factors, mutation and the mixing of characters during reproduction.

Q2. In evolutionary terms, can we say which among bacteria, spiders, fish and chimpanzees have a 'better' body design? Why or why not?
Ans: Evolution is the generation of diversity through environmental selection, and more body designs have emerged over time. Among these organisms, the chimpanzee has the most complex body design, with different organs performing different vital functions, such as the respiratory, excretory, circulatory and nervous systems. Thus we can say that the chimpanzees have a better body design because of a more complex body design. Various organisms evolve in their own separate ways to give rise to the current forms and have a basic difference in their body design because of specialization of all types and tissues. The chimpanzees are best adapted to survive the present day conditions and have proper division of labour in their body.

Exercise Questions, Page No. 159

Q1. 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
Ans: (c) TtWW. The parent with genotype TtWW produces two types of gametes, TW and tW, while the short parent with genotype ttww produces only one type of gamete, tw.

[Figure: Genetic cross of TtWW with ttww showing gametes TW and tW from the tall parent and tw from the short parent, giving progeny TtWw and ttWw.]

Q2. 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.
Ans: (a) Our arm and a dog's fore-leg are examples of homologous organs.

Q3. In evolutionary terms, we have more in common with
(a) a Chinese school-boy.
(b) a chimpanzee.
(c) a spider.
(d) a bacterium.
Ans: (a) A Chinese school-boy, because both have almost similar DNA.

Q4. 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?
Ans: No. Both parents have light-coloured eyes, so all the children will show light-coloured eyes, though some variation may occur. This pattern cannot show whether the trait is dominant or recessive.

Q5. How are the areas of study, evolution and classification, interlinked?
Ans: The study of classification gives an idea of the evolutionary history of organisms. Organisms with certain similar characteristics are placed in one group, and those in one group may have evolved from common ancestors and share an evolutionary history.

Q6. Explain the terms analogous and homologous organs with examples.
Ans: Homologous organs have the same basic structure and origin but perform different functions. For example, the forelimbs of a frog, a bird and a man have the same basic bone design but perform different functions: jumping, flying and grasping. Analogous organs have a different basic structure and origin but similar appearance and function. For example, the wings of insects and the wings of birds differ in structure, since insect wings are folds of membrane while bird wings are modified forelimbs, but both are used for flying.

Q7. Outline a project which aims to find the dominant coat colour in dogs.
Ans: Coat colour T is dominant and white colour t is recessive. A cross between a pure-breeding coat-coloured dog (TT) and a white dog (tt) gives all coat-coloured progeny (Tt). Crossing the F1 progeny gives coat-coloured and white puppies in the ratio 3:1, which confirms the dominant colour.

[Figure: Genetic cross of coat-coloured (TT) and white (tt) dogs. The F1 progeny are all Tt, and the F2 progeny are TT, Tt, Tt and tt, in the ratio 3 coloured : 1 white.]

Q8. Explain the importance of fossils in deciding evolutionary relationships.
Ans: The preserved remains of animals or plants or other organisms from the distant past are called fossils. Fossils provide the evidence that the present animals have originated from previously existing ones through the process of continuous evolution. Fossils can be used to reconstruct the evolutionary history of an organism. The distribution pattern of fossils shows that the ancient fossils present in the bottom rocks are simple, while the most recent fossils found in the upper strata are more highly evolved. It means fossils form and become more and more complex as we proceed from earliest to recent rocks. It gives us an idea of time in history when different species were formed or became extinct. Fossils also help to trace the evolutionary history of some animals. Fossils also indicate connecting links between the two groups of organisms. For example, Archaeopteryx is a connecting link between reptiles and birds.

Q9. What evidence do we have for the origin of life from inanimate matter?
Ans: Life must have developed from simple inorganic molecules present on the earth soon after it was formed. Conditions on the early earth could have produced more complex organic molecules, and further chemical synthesis could have given rise to the first primitive organism. In the Miller-Urey experiment, simple gases over water, kept just below 100°C, were exposed to sparks to simulate lightning. Carbon was converted into simple compounds, including amino acids, which make up protein molecules. This shows that life could have originated from inorganic molecules.

Q10. 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?
Ans: Sexual reproduction involves the fusion of gametes. Offspring show variations from their parents due to crossing over and the exchange of gene segments, and due to recombination of parental genes. Favourable variations produced by the environment also appear. Because of these variations, sexually reproducing organisms show quicker evolution. In asexual reproduction, the offspring are exact copies of the parent and rarely show any variation.

Q11. How is the equal genetic contribution of male and female parents ensured in the progeny?
Ans: The male and female reproductive cells divide by meiosis to form haploid gametes, which contain equal genetic material. The zygote formed by the fusion of a male and a female gamete therefore has an equal genetic contribution from both parents, and the individual develops from the zygote.

Q12. Only variations that confer an advantage to an individual organism will survive in a population. Do you agree with this statement? Why or why not?
Ans: All the variations in a species do not have equal chances of surviving in the environment. Different variations give different advantages or disadvantages to individuals. Selection of variants by environmental factors forms the basis of evolution. Variations that confer disadvantages are less likely to survive because the environment cannot support them.

While some of these differences will be unique, others will be inherited from their respective parents, who were different from each other.

Question. When a red flowered homozygous pea plant is crossed with a white flowered plant what colour is produced in F₁ generation in mirabilis jalapa.
(A) Red
(B) White
(C) Pink
(D) Red and white
Answer : B

Question. Analogous organs have a :
(A) Common embryonic origin but perform different functions
(B) Different embryonic origin and perform different functions
(C) Common embryonic origin and perform similar functions
(D) Different embryonic origin but perform similar functions
Answer : D

Question. The evidence of evolution is based on
(A) paleontology
(B) embryology
(C) anatomically
(D) all of these
Answer : D

Question. Which of the following is a vestigial structure in python ?
(A) Teeth
(B) Scales
(C) Hind limbs
(D) Poison glands
Answer : C

Question. Which of the following can you call atavism ?
(A) Wings of bat
(B) Fins of fish
(C) Cervical fistula of man
(D) Flippers of whale
Answer : C

Question. Mendel formulated the law of purity of gametes on the basis of
(A) dihybrid cross
(B) monohybrid cross
(C) back cross
(D) test cross
Answer : A

Question. Which of the following is a pair of homologous organs ?
(A) Lungs of rabbit and gills of rohu
(B) Wings of bat and wings of butterfly
(C) Pectoral fin of whale and fore limb of horse
(D) Wings of grasshopper and wings of crow
Answer : C

Question. The character that proves that frogs have evolved from fishes is :
(A) The ability to swim in water
(B) The tadpole larva in frogs which resembles the fishes in many characters
(C) Similarity in the shape of the head
(D) None of these
Answer : B

Question. The term for similarity in organ structure seen in great diversity is :
(A) Homology
(B) Identical
(C) Analogy
(D) Symmetrical
Answer : A

Question. The term evolution in biology means that
(A) fossils are old
(B) life began in Sea
(C) living things constantly change
(D) none of the above
Answer : C

Question. Which of the following is used for dating the bones ?
(A) C14
(B) I121
(C) K32
(D) All of these
Answer : A

Question. Analogous structures are :
(A) Structurally similar
(B) Functionally similar
(C) Structurally and functionally similar
(D) Normally non-functional
Answer : B

Question. Homologous structures have
(A) similar origin & dissimilar functions
(B) dissimilar origin but similar functions
(C) similar origin & similar functions
(D) dissimilar origin and dissimilar structures
Answer : A

Question. Fossils are
(A) fovea in the retina of vertebrate eye
(B) remains of organisms presents in the rocks
(C) the fossa present in the bones
(D) foramens through which nerves pass
Answer : B

Question. Connecting link between Reptiles and Birds is
(A) Dimetrodon
(B) Dodo
(C) Archaeopteryx
(D) Sphenodon
Answer : C

Question. In a dihybrid cross four phenotypes form in the ratio of 9 : 3 : 3 : 1, because of
(a) dominance of one phenotype in each pair of contrasting traits
(b) independent assortment of the genes of contrasting traits
(c) crossing over of genes
(d) mixed effect of dominance and independent assortment.
Answer. B

Question. Which of the following represents the characteristic of a pleiotropic gene?
(a) Controls sexual characters.
(b) Present only in prokaryotes.
(c) Controls one character in association with the other.
(d) Control more than one character.
Answer. D

Question. Fossils are dated now by
(A) stratigraphic position
(B) amount of calcium residue
(C) association with other animals
(D) radioactive carbon contents
Answer : D

Question. Evolution is the best defined by
(A) inheritance of acquired characters
(B) descent with modifications
(C) spontaneous generation
(D) struggle for existence
Answer : B

Question. Mendel conducted his famous breeding experiments by working on
(a) Drosophila
(b) Pisum sativum
(c) Escherichia coli
(d) all of these.
Answer. B

Question. The main reason for Mendel’s success in discovering the principles of inheritance was
(a) he considered each character separately
(b) he was lucky not to encounter with linkage problem
(c) the plant was pure breeding
(d) all of these.
Answer. D

Question. Why were pea plants more suitable than dogs for Mendel’s experiments ?
(a) There were no pedigree records of dogs.
(b) Pea plants can be self-fertilised.
(c) All pea plants have only two chromosomes.
(d) Dogs have many genetic traits.
Answer. B

Question. Which one is not a vestigial organ in man ?
(A) Vermiform appendix
(B) Plica semilunaris
(C) Ear muscles
(D) Epiglottis
Answer : D

Question. A segment of DNA providing information for a protein is called
(a) nucleus
(b) chromosomes
(c) trait
(d) gene.
Answer. D

Question. Which one of these is likely to have been absent in free form at the time of origin of life ?
(A) Oxygen
(B) Hydrogen
(C) Ammonia
(D) Methane
Answer : A

Question. An allele is said to be dominant if
(a) it is expressed in both homozygous and heterozygous conditions
(b) it is expressed only in second generation
(c) it is expressed only in heterozygous condition
(d) it is expressed only in homozygous condition.
Answer. A

CBSE Class 10 Science Chapter 9 Heredity and evolution Case Based MCQs

Case : Read the following and answer the following questions.

Refer to the schematic representation of the albinism that is an inherited condition caused by recessive allele (a). ‘A’ is the dominant allele for the normal condition. The inheritance of certain genetic traits for two or more generations is represented in a pedigree or family tree.
Study the given pedigree chart and answer the following questions.

Question. Which of the following could be the genotypes of X and Y?
      X   Y
(a) AA AA
(b) AA Aa
(c) Aa Aa
(d) aa aa
Answer. C

Question. Which of the following could be the genotype of generation - 1 male and female?
 Male   Female
(a) AA aa
(b) aa AA
(c) Aa aa
(d) AA AA
Answer. B

Question. If X married an albino female, then what is the probability that their children would be albino?
(a) 0
(b) 0.125
(c) 0.25
(d) 0.5
Answer. D 

Question. If Y married a normal homozygous male, then what is the probability that their children would be albino?
(a) 0
(b) 0.125
(c) 0.25
(d) 0.5
Answer. A

Question. Which of the following could be the genotype of offsprings produced by cross of X and Y?
(a) AA, Aa, aa
(b) aa, aa
(c) Aa, Aa
(d) AA, AA
Answer. A

CBSE Class 10 Science Chapter 9 Heredity and evolution Very Short Answer Type Questions

Question. Name two human traits that show variations.
Answer. Colour of eyes and shape of external ears.

Question. What is a gene?
Answer. Gene is the unit of inheritance. It is a segment of the chromosome which controls hereditary characteristics.

Question. Give an example where sex determination is regulated by environmental factors.
Answer. In garden lizards, sex of the organism is determined by environmental factor such as temperature.

Question. How do genes control traits?
Answer. Genes carry information for the the production of proteins which, on the other hand, control the various body characteristics.

CBSE Class 10 Science Chapter 9 Heredity and evolution Short Answer Type Questions

Question. Differentiate between inherited and acquired traits.
Answer. Differences between inherited and acquired traits are as follows

Question. Why do all the gametes formed in human females have an X chromosome?
Answer. Genotype of human female is 44 + XX. Human female is homogametic. During meiosis, at the time of gamete formation, only one X chromosome enters in each gamete.
Hence, all female gametes have genotype (22 + X).

Question. An individual inherits different traits from his parents. On what basis classification of traits as dominant and recessive is done?
Answer. A trait which is able to express itself both in homozygous as well as heterozygous conditions is called a dominant trait,e.g., tallness is a dominant trait in pea plant. A trait which expresses itself only in homozygous condition, but remains suppressed in heterozygous condition is called recessive trait, e.g., dwarfness is the recessive trait in a pea plant.

Question. Which type of organisms will have more variations – sexually or asexually reproducing organisms? Justify.
Answer. Sexually reproducing organisms will show more variations as genetic material is exchanged between homologous pair of chromosomes during crossing over. However, during asexual reproduction, mutations are the only means of variations during DNA replication which are not very common and thus may lead to very little variation.

Question. “The chromosome number of the sexually reproducing parents and their offspring is the same.” Justify this statement.
Answer. Gametes formation involves meiosis or reduction division. The gamete mother cell is diploid (2n), i.e., it has two sets of chromosomes. This single diploid cell divides by meiosis to form 4 haploid (n) daughter cells. Each daughter cell becomes a gamete, either male or female. Each gamete possesses single set of chromosomes. Fusion of these gametes results in the formation of a zygote having a double set of chromosomes i.e., diploid (2n) (one set of paternal and the other set maternal). Thus the number of chromosomes in parents and offsprings of a particular species remains constant.

Question. Crossing of a pea plant with purple flower and pea plant with white flowers, produces 50 plants with only purple flowers. On selfing, the plants produced 470 plants with purple flowers and 160 with white flowers. Explain the genetic mechanism accounting for the above results.
Answer. In this breeding experiment, ratio of purple to white flowers is approximately 3 : 1 in F2 generation. So the ratio is according to Mendelian monohybrid cross. The cross further explains the following facts:
(i) F1 is represented only by dominant trait, i.e., purple flowered plants.
(ii) Both the traits, i.e., purple and white flower colour show segregation and thus appear in F2 generation.

Question. In which generation does the segregation of allelic phenotype takes place?
Answer. In F1 generation both alleles come together in hybrid but only dominant character is expressed. In F2 generation, on selfing of F1 hybrids, these alleles segregate. As a result,dominant (pure and hybrid) and recessive (pure) phenotypes are segregated.

Question. The gene for red hair is recessive to the gene for black hair. What will be the hair colour of a child if he inherits a gene for red colour from his mother and a gene for black hair from his father? Express with the help of flow chart.
Answer. The hair colour of child will be black. This can be illustrated as follows:

Question. Differentiate between genotype and phenotype.
Answer. Differences between genotype and phenotype are as follows :

Question. Explain the law of purity of gametes.
Answer. Principle of purity of gametes is also known as principle or law of segregation. According to this law, the two unit factors of a character which remains together in an individual do not get mixed up and keep their distinct identity. They separate during gamete formation so that each gamete receives only one factor or gene for each character and is always pure.