Get Heredity and Evolution Class 10 Notes PDF which is based on the latest pattern of CBSE and NCERT. It involves every one of the points given in NCERT class 10 Science book. You can easily download this Class 10 Heredity and Evolution Notes PDF given below.

Download CBSE Class 10 Science Chapter 9 notes in PDF free of charge. It will help you to make your preparation better to score higher marks in exams. This Class 10 Heredity and Evolution Notes is prepared by our expert teachers.

This Class 10 Science Chapter 9 Notes assist you with revising the complete chapter in minutes. One of the best tips suggested by teachers is revising the notes during exam time.

Students should read Chapter 9 Heredity And Evolution Class 10 Science Notes provided below. These notes have been prepared based on the latest syllabus and books issued by NCERT, CBSE and KVS. These important revision notes will be really useful for students to understand the important topics given in the chapter Heredity And Evolution in Class 10 Science. We have provided class 10 science notes for all chapters.

Revision Notes Chapter 9 Heredity And Evolution Class 10 Science

Chapter 9 Heredity And Evolution is an important chapter in Class 10 Science. The following notes will help you to understand and easily learn all important points to help you score more marks.

Important Terms & Concepts

1. Variation: It refers to the differences in the characters or traits among the individuals of a species.
For ex Free earlobes and attached earlobes are the two variation found in human population

Note: The significance of a variation shows up only if it continues to be inherited by the offspring for several generations.

2. Importance of Variations
I) It increases the chances of survival in a changing environment.
II) Variations form the basis of heredity.
III) They form raw materials for evolution and development of new species
IV) The variation is necessity for organic evolution.

3. Accumulation of Variation
• If one bacterium divides, then resultant two bacteria divide again, the four individual bacteria generated would be similar with minor differences due to small inaccuracies in DNA copying.
• Depending on the nature of variations, different individuals would have different advantages like bacteria, which can withstand heat will survive heat wave better.
• Selection of variants by environmental factors forms the basis of evolutionary processes.

4. Gene: Genes are actually unit of heredity which transfer characteristics from parents to their offspring during reproduction.
Genes works in pair.
Genes for controlling the same characteristics of an organisms can be of two types :Dominant or Recessive .
5. Heredity:
• It refers to the transmission of characters or traits from the parent to their offspring.
• In simple terms heredity means continuity of features from one generation to the next .
• Hereditary information is present in the sex cells of the parents .Thus ,gamete constitute the link between one generation and the next and pass on the paternal and maternal
characters or traits to the offspring

6. Heredity and Variation in Asexual Reproduction
• In asexual reproduction, organisms raised are the exact copies of their parents.
•They tend to preserve the similarities among all the individuals belonging to a given line of descent.
• They exhibit very little variations due to some environmental factors or mutations which are sudden changes in genes. Out of these two factors, only mutations are heritable.

7. Heredity and Variation in Sexual Reproduction
• In sexual reproduction, two parents are involved and there is formation and fusion of gametes.
• The offspring’s show variations from their parents due to crossing over and exchange of gene segments.
• They are not carbon copies of their parents, due to recombination of parental genes. So, variations which occur are heritable.

8. Gregor Johann Mendel was an Austrian Geneticist and regarded as the ‘Father of Genetics’.
Mendel was the first scientist to make a systematic study of patterns of inheritance which involved the transfer of characteristics from parents to progeny.
• Mendel chose pea plant for studying inheritance because pea plant had a number of clear cut differences.
• His experiments with garden peas (Pisum sativum) and the inferences together with his interpretations constitute the foundation of modern genetics.
• Mendel conducted his hybridization experiments in the garden behind the monastery.
• Mendel blended his knowledge of Science and Mathematics and was the first one to keep count of individuals exhibiting a particular trait in each generation.
• Mendel’s experiments give us the mechanism for the inheritance of traits from one generation to the next.

9. Reason to choose pea plant by Mendel:
• They show variety of contrasting characters
• They were self-pollinating which enabled them to produce next generation of plants easily .
• Many generation of pea plants can be produced in a comparatively short time span.

10. Monohybrid cross: In monohybrid cross we will study the inheritance of one pair of contrasting characteristics ‘tallness’ and ‘dwarfness’ of the pea plant by their first generation and second generation progeny.

11. Dihybrid cross: In Dihybrid cross we will study the inheritance of two pair of contrasting characteristics of the pea plant such as round yellow and wrinkled –green seeds.

Note: A new form of plant resulting from a cross (or breeding ) of different varieties of plant is known as a Hybrid.
Mendel’s Experiments on Inheritance of Traits: Mendel used a number of contrasting visible characters of garden peas like round/wrinkled seeds, tall/short plants, white/violet flowers, etc.
One Visible Contrasting Character
• Mendel took pea plants with different characteristics such as a tall plant and a short plant.
• The first generation or F₁ progeny thus formed are all tall.
• Mendel then allowed the F₁ progeny plants for selfpollination.

• The second generation of F₂ progeny of the F₁ tall plants is not all tall, some are short. This indicates that both tallness and shortness traits were inherited in F₁ plants but only the tallness trait was expressed.
• Thus, two copies of the traits are inherited in each sexually reproducing organism.
• In the figure, both TT and Tt are tall plants, while only it is a short plant.
• A single copy of T is enough to make the plant tall while both copies have to be ‘t’ for the plant to be short.
Therefore in traits Tt ‘T’ is dominant traits while ‘t’ is recessive traits.

Two Visible Contrasting Characters
• Mendel took pea plants with two different characteristics such as a tall plant with round seeds and a short plant with wrinkled seeds.
• F1 progeny are all tall with round seeds. Thus, tallness and round seeds are dominant traits.
• Mendel then allowed F₁ progeny plants for self-pollination to get F₂ progeny.
• F₂ progeny will have tall plants with round seeds, some short plants with wrinkled seeds, and plants with, new mixtures like tall having wrinkled seeds and short having round seeds.
• The ratio of plants with above characteristics will be 9 : 3 : 3 : 1
• Therefore tall/short trait and round seed/wrinkled seed trait are independently inherited.

12. Inherited Traits: It is transmission of particular characteristics from parent to their offspring, generation to generation, which bear all basic features with great deal of variation. Inherited trait is therefore, a particular generically determined characteristic that distinguishes a person.

13. Inheritance : It is the transmission of genetically controlled characteristics from one generation to the next.
14. Control of Traits
• A section of DNA that provides information for one protein in cell is called gene.
• If the proteins work efficiently, the traits get expressed in a better way.
• For example, plant height depends on the amount of growth hormone released by protein. If the protein is efficient, more growth hormone will be released and the plant will be taller. But if the protein is inefficient, the plant will be shorter. Thus, genes control the traits.

15. Mechanism of Inheritance
• If both parents help to determine the trait in the progeny, both parents must be contributing a copy of the same gene.
• Thus, each pea plant must have two sets of all genes, one inherited from each parent, so each germ cell must have only one gene set.
• Each gene set is present, not as single long thread of DNA, but as separate independent pieces each called a chromosome and each cell will have two copies of each chromosome; one each from male and female parents.
• When two germ cells combine, they will restore the normal number of chromosomes in the progeny, ensuring the stability of DNA species.
• Therefore, such mechanism of inheritance explains the results of Mendel’s experiments and is used by all sexually and asexually reproducing organisms.

16. Chromosomes are long thread-like structures present in the nucleus of a cell which contain hereditary information of the cell.

17. Deoxyribonucleic Acid (DNA) is a chemical in the chromosomes which carries the hereditary characters or traits in a coded form from one generation to the next in all the organisms.

18. Sex Determination:
Sex determination is the process by which sex of a newborn individual can be determined.
The chromosomes which determine the sex of a chromosomes is called Sex chromosomes .
While rest of the chromosomes which aren’t involved in sex determination is called Autosomes.

Different species use different stratifies for sex determination.
• In some animals, environmental factors such as temperature at which fertilised eggs are kept determine whether the animals developing in the eggs will be male or female.
• In other animal such as snails, individuals can change sex, indicating that sex is not genetically determined. In human beings, the sex of the individual is genetically determined.
Sex determination in human being:
• Human beings have 22 paired chromosomes and one pair called sex chromosome which is not a perfect pair.
• A male has one X chromosome and one Y chromosome, i.e., half of the male gametes or sperms will have X chromosome and the other half will have Y chromosome.
• A female has two X chromosomes, i.e., all the female gametes or ova will have only X chromosomes.
• Sex of a child depends on what happens during fertilisation.

I) If a sperm carrying X chromosome fertilises an ovum which carries X chromosome, then the child born will be a girl.
II) If a sperm carrying Y chromosome fertilises an ovum which carries X chromosome, then the child born will be a boy.

• Thus, the sperm determines the sex of the child.

19. Acquired and Inherited trait: A trait of an organisms which is not inherited but develops in response to the environment is called the acquired trait while A trait of an organisms which is caused by a change in its genes is called an inherited trait.

20. Evolution is the sequence of gradual change which takes place in the primitive organisms over millions of years and new species are produced. Since the evolution is of living organisms, so it is called ‘Orgame Evolution’.

21. Variation in Population: An Example: 12 red beetles live in a green leafy bush grows by sexual reproduction and generate variation.
Situation-I
• Crows eat these beetles, leaving only fewer beetles available for reproduction.
• Due to colour variation during reproduction, one green beetle evolves and thereafter, all its progeny beetles become green.
• Crows cannot see green coloured beetles on green leaves and hence cannot eat them resulting more green beetles than red ones in the beetle population.
• This type of variation gives a survival advantage.

Situation-II
• Due to colour variation during reproduction, a blue colour beetle appears and all its progeny beetles become blue.
• Crows can see both red and blue beetles and therefore, eat them.
• Initially, there are less number of blue beetles and more of red beetles.
• Then an elephant stumps on the bushes and kills most of the beetles. By chance, few beetles that survived were mostly blue.
• Thus, the blue beetle population slowly expands.
• There is no survival advantage on this variation and provides diversity without adaptation.

Situation-III
• As the beetle population begins to expand, the bushes suffer from a disease and amount of leaf available for beetles have reduced.
• Thus, the beetles are poorly nourished and the average weight of an adult beetle has decreased.
• After few years, the plant disease is eliminated and enough food is available for the beetles. Thus, the beetles have come back to its normal size and weight.
• This change is not inherited over generation.

22. Charles Robert Darwin was a British naturalist who formulated his hypothesis that evolution took place due to natural selection. Darwin’s theory of evolution tells us how life evolved from simple to more complex forms.

23. Origin of Life on Earth: J.B.S. Haldane, a British scientist suggested in 1929 that life must have developed from the simple inorganic molecules which were present of earth to more complex organic molecules. Thus, the first primitive organisms would arise from further chemical synthesis.

24. Speciation: It is the evolution of reproductive isolation among once – interbreeding populations, i.e., the development of one or more species from an existing species.

25. Micro-evolution: It is the evolution on a relatively small scale, involving the emergence of new species or of new groups below the species level, such as races and subspecies which are significant. They also change the common characteristics of a particular species.

26. Gene flow: It is the exchange of genetic material by interbreeding between populations of the same species or between individuals within a population. Gene flow increases the variation in the genetic composition of a population.

27. Genetic drift: It is the random change in the frequency of alleles in a population over successive generations due to sampling error in the gametes.
• Each new generation differs from its parental generation with regard to allele frequencies simply because of random variation in the distribution of gametes.
• This process is more rapid in smaller populations, hence genetic drift can cause loss of genetic diversity if there are no counteracting factors.
• Over generations, genetic drift will accumulate different changes in sub-population.

28. Natural selection: It is the process, according to Darwin, which brings about the evolution of new species of animals and plants.
• It was noted that the size of any population tends to remain constant despite the fact that more offsprings are produced than are needed to maintain.
• Darwin found that variations existed between individuals of the population and concluded that disease, competition and other forces acting on the population eliminated those individuals which are less well adapted to their environment.
• The surviving population would pass the hereditary advantageous characteristics to their offsprings.
• But with time this process would give rise to organisms different from the original population and new species are formed.

29. Generation of New species: When a population of a species splits into two, it cannot reproduce with each other, and then a new species is generated, for example:
• A huge population of beetles feed on bushes spread a wide mountain range.
• Individual beetle however feed on nearby bushes and do not travel much.
• There is sub-population of beetles in a neighborhood and reproduction takes place within the sub-population.
Occasionally a migrant beetle enter a different sub-population and reproduce with them, thus genes of the migrant beetle enter in a new population.
• Over generation, genetic drift will accumulate different changes in each sub population.
• Changes due to genetic drift and natural selection will result in isolation of two sub-population which become more and more different from each other.
• Ultimately these two groups will be incapable of reproducing with each other and new species of beetles will form.

30. Hierarchy: It is a type of social organisation in which individuals are ranked according to their status or dominance relative to other members. Higher up in the hierarchy the similarities between members of a group become fewer.

31. Characteristics: It is the detailed of appearance or behavior of a particular form or a particular function. For example, the four limbs of human beings is a characteristic and that plants can perform photosynthesis is also a characteristic.

32. Classification: It is the arrangement of organisms into a series of groups based on physiological, biochemical, anatomical of other relationships.

33. Basic characteristics of organisms
• Cell is the basic unit of life in all organisms.
• Cells of some organisms have nucleus and some do not.
• Amongst organisms with nucleated cells some are unicellular and others are multicellular.
• Some multicellular organisms can perform photosynthesis and others cannot.
• Among multicellular organisms, some have skeleton inside the body and others around the body.

34. Evolutionary Relationship with Classification
• If two species have more common characteristics then they are more closely related.
• The more closer the species are, the more nearer they have a common ancestor.
• For example, a brother and a sister are closely related and they have a common ancestor, i.e., their parents in the first generation.
• A girl and her first cousin are closely related but less related than her brother. The cousins have a common ancestor, their grandparents in the second generation. Thus, evolutionary relationships are traced in the classification of organisms.

35. Evidences for evolution: The more characteristics two species have in common the more closely they will be related and the more closely related they are the more recently they will have had a common ancestor ,In such a way we can trace evidence for evolution.
• Homologous organs
• Analogous organ
• Fossils

36. Homologous organs are those organs which have the same basic structural design and developmental origin but have different functions and appearance.

Example: The forelimb of a frog, a lizard a bird and a man seem to be built from the same basic design of bones, but they perform different functions.

37. Analogous organs are those organs which have different basic structural design and developmental origin but have similar appearance and perform similar functions.

Example: The wings of birds and bats look similar but have different design in their structure. Wings of bats are skin folds stretched between elongated fingers but wings of birds are covered by feathers all along the arm.

38. Fossils: They are all the preserved traces or remains of living organisms of geological past.
• When organisms die, their dead bodies decompose and get lost.
• But some parts of the body may be in the environment that does not let it decompose.
• Example, a dead insect will not decompose in hot mud, it will harden and retain the impression of the insect body parts and thus preserved as fossils.
• Evolution can be worked out by the study of not only living species but also from fossils.

39. Kinds of Fossils: Fossils are of various kinds depending upon their differences in appearances and degrees of detail and preservation. They are – Fossils tree trunk, Fossil invertebrate (Ammonite), Fossil invertebrate (Trilobite) Fossil fish (Knightia) and Fossil dinosaur skull (Rajasaurus).

40. Fossil Dating: The age of fossils can be estimated by two ways—
I) If we dig into the earth and start finding fossils, it can be assumed that the fossils closer to the surface are more recent to those found in deeper layers.
II) By detecting the ratios of different isotopes of the same element in the fossil material.

41. Formation of Fossils: Fossils are formed by the formation of layer by layer in the earth’s crust.
• Suppose 100 million years ago invertebrates that were dead on the sea bed were buried in the sand, and with time more sands accumulate and sandstone are formed.
• After million years, the dinosaurs living in the area die and get buried in mud which are compressed into rock above the earlier invertebrate fossils.
• Eventually again million years later, the bodies of horse-like creature dies and their fossils are found in rocks above the earlier rocks.
• But much later, due to erosion, the water flow wears some of the rocks and the horse-like fossils are exposed. And as we dig into deeper layers older fossils are found.

42. Use of Fossils: Fossils help to analyse—
I) Racial history of plants and animals.
II) Pat climatic conditions of the earth and
III) To measure the geological time.

43. Artificial selection: It is the modification of species by selective breeding. Animals or plants with desirable characteristics or traits are interbreeding with the aim to produce new strain of organisms for a specific purpose.

44. Evolution by Stages
• Complex organs may have evolved because of the survival advantage of the intermediate stages.
• Complex organs like eyes are created bid by bid over generation which is a very popular adaptation.
The structure of eye in all organisms is different enough to have separate evolutionary origins.
• Organs or features may be adapted to new functions during the course of evolution. As for example, feathers are thought to have evolved for warmth and later adapted for flight.
• Some dissimilar looking structures also evolved from common ancestor. Example—Wild cabbage plant, from which different vegetables are generated by artificial selection like broccoli, cabbage, cauliflower, etc.
• The traces of evolutionary relationships depend of the original idea that ‘changes in DNA during reproduction are its basic events’.

45. Adaptation: It is any change in the structure or functioning of an organism that makes it better suited to its environment.

46. Phylogeny: It is the evolutionary history of an organism or group of related organisms.

47. Molecular Phylogeny: It is the idea that organisms those are more distantly related will accumulate a greater number of differences in their DNA. Such studies trace the evolutionary relationships.

48. Evolution versus Progress: Evolution cannot be said to ‘progress’ from lower forms to higher forms. It seems to have given rise to more complex body designs even while the simpler body designs continue to flourish. For example, human beings have not evolved from chimpanzees, but both have a common
ancestor. Thus, evolution is simply the generation of diversity and shaping of diversity by environmental selection.

49. Environmental selection: 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

50. Human Evolution: The study of human evolution indicates that all of us belong to a single species that evolved in Africa. The earliest member of the human species, Homo sapiens can be traced there.
• DNA sequences have been used for studying human evolution.
• Due to the diversified human forms and features, skin colour is the common way for identifying the races.
• Few thousand years ago some ancestors left Africa while others stayed back.
• The residents spread across Africa and the migrants spread across the planted from Africa to West Asia, Central Asia, Eurasia, South Asia, East Asia, Indonesia, Philippines, Australia and America.
• They went forward and backward with groups separating from one another, or sometime coming together.
• Like all other species, they were also living their lives to the best of their ability.

Important Questions Heredity And Evolution Class 10 Science

Very Short Answer Type Questions :

Question. No two individuals are absolutely alike in a population. Why?
Answer: Sexual reproduction involves combination of genes from two parents, the genetic material gets exchanged between pairs of chromosomes before formation of their gametes which cause changes.

Question. All the variations in the species do not have equal chances of survival. Why? 
Answer: All variations will not be able to survive in the environmental changes, which is an essential requirement for survival.

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

Question. Mendel observed a contrasting trait in relation to position of flowers. Mention that trait.
Answer: Terminal flowers have recessive trait whereas axial flowers have dominant trait.

Question. Name the genetic material that is responsible for the inheritance of traits.
Answer: Gene or DNA

Question. Do genetic combination of mothers play a significant role in determining the sex of a new born?
Answer: No, mother does not play an important role because mother has only ‘X’ chromosomes whereas father has both X and Y chromosomes. When offspring takes ‘X’ from mother and ‘Y’ from father, then baby boy is born.
If offspring takes ‘X’ chromosomes from mother as well as father, girl child is born. The probability of getting boy or girl is 50 : 50.

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

Question. Name the plant on which Mendel performed his experiments. 
Answer: Garden pea (Pisum Sativum)

Question. What type of traits are phenotype? 
Answer: The traits which are visible to us are called phenotype, e.g. tall or dwarf (short) are phenotype traits of the plants.

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

Question. If a trait ‘A’ exists in 10% of the population of asexually reproducing species and trait ‘B’ exists in 60% of the same population, which trait is likely to have arisen earlier? 
Answer: Trait ‘B’ is likely to have arisen earlier, because it has higher percentage in the population. In asexual reproduction, the traits which are present in parent generation are carried over to next generation with minimum variation.

Question. What is monohybrid cross? 
Answer: Monohybrid cross is the cross between two individuals with one pair of different traits.

Question. Why do mice whose tails were surgically removed, continue to produce mice with tails?
Answer: It is because cutting of tail is done surgically and it is not inherited.

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

Question. What are chromosomes? Where are they located? 
Answer: Chromosomes are long thread like structures which contain genetic information of an individual.
These are located in the nucleus of a cell.

Short Answer Type Questions :

Question. In a pea plant, the trait of flowers bearing purple colour (PP) is dominant over white colour (pp).
Explain the inheritance pattern of F1 and F2 generations with the help of a cross following the rules of inheritance of traits. State the visible characters of F1 and F2 progenies.
Answer: Let purple trait be represented by: PP, White trait be : pp 11
Visible characters of F1 progeny has all flowers purple coloured and in F2 progeny 3 are purple coloured and 1 is white coloured flower.

Question. (a) What is genetic constitution of human sperm?
(b) Mention the chromosome pair present in zygote determining the sex of male child.
Answer: (a) 22 + Y chromosome or 22 + X chromosomes.
(b) (22 + X + 22 + Y) = (44 + X + Y) chromosomes.

Question. Why are small number of surviving tigers a cause of worry from genetics point of view?
Answer: (i) Tigers may get extinct in near future due to natural calamities, lack of available food and due to their small number.
(ii) Cross breeding may be less possible. Variations will not occur, which is essential for the survival.
(iii) Adaptation due to changing environment, like cutting of forests, global warming becomes difficult.
(iv) Protection from enemies is easy if they are more in number.

Question. How can we say that change in genes can be brought by the change in DNA? 
Answer: A gene is present on a segment of DNA chromosome.
(i) It provides information of one protein to another and is responsible for its change.
(ii) It ensures stability of DNA.
It means change in gene segment of DNA will bring change in DNA.

Question. Sex determination in man depends upon 23rd pair of chromosomes, called the sex chromosomes. If it were a homologous pair-XX, it would be a female. If it were a heterologous pair – XY, it would be a male. Based on this statement, answer these questions:
(i) How many types of eggs/ova and how many type of sperms female and male human being will produce respectively ?
(ii) Which parent’s contribution of sex chromosomes determines sex of the child?
Answer: (i) Male produces two genetic types of sperms, half with X and other half with Y chromosome. The female produces only one genetic type of ova, all carrying X chromosome.
(ii) Sex of children will be determined by what they inherit from their father, and not from their mother.

Question. Explain the Mendel’s concept of heredity, by giving three points. 
Answer: (i) Mendel worked on pea plant.
(ii) He found that traits are controlled by genes which come in pairs.
(iii) Genes are inherited as separate units, one from each parent.

Question. An angiosperm plant having red flowers when crossed with the other having same colour flower
produced 40 progenies, out of which 30 were red coloured flowers, 10 plants were with white flowers.
Find out:
(i) What is the possible genotype of parent plants?
(ii) Which trait is dominant and recessive?
(iii) What is the cross called and what is phenotype ratio? 
Answer: (i) Rr and Rr
(ii) Red colour of flower is dominant trait while white colour is the recessive trait.
(iii) Monohybrid cross, phenotype ratio is 3 : 1.

Question. How did Mendel explain that it is possible that a trait is inherited but not expressed in an organism?
Answer: Some traits that are inherited may not express themselves. Such hidden traits are known as recessive traits. Mendel explained this phenomenon with the help of monohybrid cross. In a monohybrid cross performed by Mendel, a tall plant was crossed with a dwarf plant which produced all tall plants in Fl progeny.
However, when these F1 tall plants were crossed with each other, ‘dwarf’ trait, which was not observed in the Fl generation, reappeared in the F2 progeny.

By this, it is concluded that dwarfness is a trait of parent pea plant that was not lost. It was suppressed in the F1 generation by the tallness trait and reappeared in the F2 generation. So, we can say that a trait that is inherited may not be always expressed in an organism.

Question. The chromosomal number of the sexually producing parents and their offspring is the same. Justify this statement. 
Answer: Offspring always takes 23 chromosomes from father and 23 chromosomes from mother and in total has 46 chromosomes. It means that total number of chromosomes remain constant.

Question. How do Mendel’s experiments show that traits may be dominant or recessive? 
Answer: • Mendel selected pure breed of tall (TT) and short (tt) pea plants.
• He crossed these plants and carefully produced seeds. This is called F1 generation.
• All the plants in F1 generation were tall, showing that tallness is dominant trait.
• Now these plants were self pollinated.
• In the F2 generation, 3 out of 4 plants, i.e. 75% were tall and one out of four, i.e. 25% were short plants. It shows that shortness is the recessive trait.

Long Answer Type Questions :

Question. (a) What is the law of dominance of traits? Explain with an example.
(b) Why are the traits acquired during the life time of an individual not inherited? Explain.
Answer: (a) Law of dominance: Some alleles are dominant while others are recessive. An organism with at least one dominant allele will display the effect of dominant trait e.g. If we carry out a cross between pure breed tall pea plant with pure dwarf pea plant then F1 progeny will be tall plant because it is dominant trait whereas dwarf is a recessive trait.
(b) Acquired traits even if cause genetic changes in non-reproductive tissues, can not be passed on to next generation.
Genetic changes in reproductive tissues can only be passed on to next generation.

Question. How do Mendel’s experiments show that traits are inherited independently?
Answer: Mendel carried out dihybrid crosses by crossing two pea plants differing in contrasting traits of two characters. For example, he crossed a pea plant having yellow colour and round seed characters with another pea plant bearing green colour and wrinkled seed characters. In the F2 generation, he obtained pea plants with two parental and two recombinant phenotypes as yellow round and green wrinkled (parental) and yellow wrinkled and green round (recombinant). This indicated that traits separated from their original parental combinations and got inherited independently.

Question. How do Mendel’s experiments show that
(a) traits may be dominant or recessive?
(b) inheritance of two traits is independent of each other? 
Answer: (a) Mendel demonstrated that traits can be either dominant or recessive through his monohybrid cross.
He crossed true-breeding tall (TT) and dwarf (tt) pea plants. The seeds formed after fertilisation were grown and the plants that were formed represent the first filial or F1 generation. All the F1 plants obtained were tall.
Then, Mendel self-pollinated the F1 plants and observed that all plants obtained in the F2 generation were not tall. Instead, one-fourth of the F2 plants were short.
(For diagram refer to Ans. 10, Short Answer Type Questions-II) From this experiment, Mendel concluded that the F1 tall plants were not true breeding; they were carrying traits
of both short height and tall height. They appeared tall only because the tallness trait was dominant over the dwarfness trait. This shows that traits may be dominant or recessive.
(b) Mendel demonstrated that traits are inherited independently through his dihybrid cross. He considered two traits at a time, seed colour and seed shape in which yellow colour (YY ) and round shape (RR) are dominant over green colour (yy) and wrinkled shape (rr), respectively.
Mendel cross bred the plants and observed that the F2 progeny of dihybrid cross had a phenotypic ratio of 9 : 3 : 3 : 1 and produced nine plants with round yellow seeds, three plants with round green seeds, three plants with wrinkled yellow seeds and one plant with wrinkled green seeds. 2
In this experiment, he found that round yellow and wrinkled green are parental combinations whereas round green and wrinkled yellow are the new combinations. In a dihybrid cross between two plants having round yellow (RRYY) and wrinkled green seeds (rryy), four types of gametes (RY, Ry, rY, ry ) are produced. Each of these gametes segregate independently of each other and each has a frequency of 50% of the total gametes produced.
From this experiment, he concluded that when two pairs of traits are combined together in a hybrid, one pair of character segregates independently of the other in terms of character. This is known as the law of independent assortment.

Students Who Practice these Heredity and Evolution Class 10 Notes will observe that each concept is depicted in a clear way including the equations, formula, diagram, important questions, and MCQs.

You can also download other study materials like Sample papers, MCQ Questions, NCERT Solutions, NCERT Book, Unseen Passage, and HOTs Questions for Class 10 Science on our website.

We hope these notes will help you to score good marks in your exam. If you have any doubt in regard to Heredity and Evolution Class 10 Notes, write a comment in the box given below.