NCERT Solutions for Class 10 Science Chapter 6 - Heredity and Evolution

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In asexually reproducing organism, trait B has originated earlier. The variations in a population are only due to inaccuracies of DNA copying.

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The useful variations in individuals of a species enable them to adapt according to the changes and new needs. Thus variations promote survival of species.

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1. Mendel conducted experiments on garden pea plant selecting seven visible contrasting characters.
2. He selected and crossed homozygous tall pea plant having the genotype TT with a homozygous dwarf pea plant having the genotype tt.
3. F1 generation consists only of tall plants, having genotype Tt.
4. Since they have an allele for dwarfness also, they are all hybrids. The expressed allele T for tallness dominated over the unexpressed allele t for dwarfness.
5. The fact that the allele for dwarfness is present in the F1 plants can be verified by self pollinating them when F2 progeny will consist of both tall and dwarf plants in the ratio of 3 : 1.
6. On this basis he proposed ‘‘Law of Dominance.’’

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Mendel proposed Law of Independent Assortment on the basis of a dihybrid cross between two homozygous parents. It states that the factors (genes) of different traits are independent of each other in their distribution into the gametes and in the progeny.

Mendel selected a dominant plant with round and yellow seeds and a recessive plant with wrinkled and green seeds, which yields F1 offsprings showing the dominant form of both traits, viz. round and yellow. F1 plants, on self-pollinating produce F2 progeny with two parental and two new recombinant phenotypes, which is round yellow: round green: wrinkled yellow: wrinkled green in the ratio of 9 : 3 : 3 : 1. This ratio is called Mendel’s dihybrid phenotypic ratio.

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Blood groups being a hereditary character, the knowledge of blood groups of parents can give information about the possible blood groups of children and vice versa.

However the data provided is insufficient. It is not mentioned father has homozygous or heterozygous A blood group. If it is homozygous A then 100 per cent of progeny will have A blood group as Gene I^A is dominant over Gene I^O.

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A particular population with specific traits will increase due to following reasons:
1. Sexual reproduction which results into variations.
2. Natural Selection i.e., individuals with special traits survive the attack of their predators and multiply while the others will perish.
3. Genetic drift, which provides diversity without any adaptation.

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Change in non-reproductive tissue (somatic cells) cannot be passed on to the DNA of germ cells. Thus the acquired trait will die with the death of individual. It is non-heritable and cannot be passed on to its progeny.

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As the population of tigers is decreasing, there is loss of genes from the gene pool. There cannot be recombination and variations. Hence no evolution occurs. If number falls suddenly they may become extinct.

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Factors leading to rise of new species
1. Genetic variations 2. Natural selection
3. Reproductive isolation 4. Origin of new species.

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No, in self-pollinating species geographical isolation will not play any role for speciation because the self-pollination occurs on the same plant.

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Geographical isolation of individuals of a species can lead to formation of new species: Darwin went on his world tour to study the flora and fauna of number of continents and islands. He observed Darwin finches (beaks of birds) in the Galapagos Islands on the west coast of South America .

Darwin finches have common ancestors present in South America. The island got separated from main land. The beaks of finches modified according to their food habits. Ancestor’s geographical isolation led to the formation of new varieties intermediates to new species.

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Homologous organs help to identify the relationship between organisms. These characteristics in different organisms would be similar because they are inherited from a common ancestor. Example: Fore limbs of mammals having the same basic structural plans in birds, reptiles and mammals. However the functions get modified in different species.

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Wings of insects and wings of birds have different basic structural plan and origin. They perform the same function. Thus they are analogous organs and not homologous organs.

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Fossils are preserved remains, tracks or traces of organisms that lived in the past. Fossils have been found linking all major groups of vertebrates.
Significance of fossils
1. Fossils are direct evidences in support of evolution.
2. Living forms with simple organisation appeared earlier than the complex forms. We can conclude this because fossils of lower layers of the Earth are simple as compared to fossils of the upper layers.
3. Several forms bearing intermediate characters indicate the transition from an earlier simple to a later complex form. Fossils of Archaeopteryx serves as a missing link between reptiles and birds. This bird has wings and unlike birds, it had teeth and a long tail.
4. On the basis of fossil records complete evolutionary history of present day horse has been studied.

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1. The DNA studies have shown that they belong to the same species.
2. Number of chromosomes is the same.
3. All have originated from a common ancestor.
4. They interbreed among themselves to produce fertile young ones of their own kind.

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Chimpanzees have the better body design as compared to other three mentioned. They are better adapted for locomotion, communication and thinking.

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(C) TtWW

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(d). All of the above

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(a) Chinese school-boy

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On this basis we cannot say that light eye colour is dominant or recessive until a cross is made between parent having light eye colour and another with dark eye colour is made. Only then it will be possible to predict the dominant or recessive nature of gene.

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Following points indicate that evolution and classification are interlinked:
1. Characteristics are shared by most of the organisms. The characteristic in the next level of classification will be shared by most of the organisms and not by all.
2. Cell designs also indicate this relationship.
3. Groups formed during classification are related to their similarities.

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Homologous organs: The organs of different classes have different forms because they have to perform different functions but their structures basically remain similar. Such organs are called homologous organs.
Examples: 1. Fore limbs of amphibians, birds and mammals have same fundamental structural plans but perform different functions.
2. In plants, the homologous organs may be a thorn of Bougainvillea or a tendril of Cucurbita, both arising in axillary position.
Analogous organs: The organs are quite different in their structure and origin but similar in function. Such organs are known as analogous organs. The presence of analogous organs proves that different structures can be modified to perform a similar function. Analogy indicates convergent evolution.
Example. The wings of insects and birds perform the same function.

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Make a chart on thermocol sheet showing the following monohybrid cross.

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On cutting a wire of resistance R into five equal parts, the resistance of each part is R/5. The total resistance R¢ of 5 resistors R/5 each in parallel is

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Urey and Miller provided experimental evidence regarding origin of life from inanimate matters. They assembled an atmosphere similar to that, thought to exist on early Earth.
In a spark flask they collected ammonia, methane and hydrogen sulphide, but no oxygen, over water at a temperature just below 100 °C and sparks were passed through the mixture of gases to stimulate lightning. At the end of a week they obtained simple compounds of carbon including amino acids.
Amino acids make up protein molecules. Thus they showed life originated from inanimate matter.

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Role of sexual reproduction in causing variation:

• Sexual reproduction is bi-parental whereas asexual reproduction is uni-parental.
• Sexual reproduction involves mixing of traits from the two parents in the progeny, which leads to new combinations of traits in the following generation.
• Due to gametes formation, there is crossing over and recombination in chromosomes so it will cause variation.
• Genetic variation is necessary for evolution. They are raw material of evolution.

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In sexual reproduction, both the parents contribute equal amount of genetic material (genes) to the offspring. This means that for each trait there will be two alternatives in the sexually reproducing organisms. Out of these two alternatives, one is called dominant trait and the other is called recessive trait. There will be some progeny with new combination. DNA controls the traits and is copied from one generation to the next generation. Inaccuracies do occur during DNA copying which is more prominent in sexual reproduction. These variations in DNA copying get inherited. Accumulation of variation generation after generation altogether leads to evolution of a new species.

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Yes. The organism with useful variations will adapt and survive. Moreover they leave behind more offsprings and populations with such genetic variations.