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Class 9 and 10 CBSE Sample papers are added for final exam 2018. Kindly practice it for getting good marks. All the best !! JSUNIL
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10th Maths SA2 Sample Paper 2017-18-6 and 10th Maths SA2 Sample Paper 2017-18-7 Download File      For more paper visit 10th Sample Question paper 2018
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Ans: Environmental Cue – Þ In some animals, the temperature at which fertilized eggs are kept determines whether the developing animal in egg is male or female eg lizard Þ In some animals like snail, individual can change sex. Genetically – A child who inherits an x chromosome from her father will be a girl and one who inherits a y chromosome from the father will be a boy. 2. a. Define the term Gene. b. 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. Ans: a. The functional unit of DNA which are made of nucleic acids and protein is called gene. b. Given Black hair – father Dominant [ BB ] and Red hair – Mother – Recessive [ bb] Parents Father BB/Bb X Mother Bb/bb F1 Bb (Black) 3 .a. Mention any two point of difference between acquired and inherited traits. b. If the tail of a mouse is cut for twenty one generations, will the tail occur in the twenty second generation of that mouse? Give reason to support your answer. c. Define the term – Natural Selection. Ans: a. Acquired Traits Inherited Traits Trait acquired during its lifetime Traits inherited from its parents cannot passed on to progeny passed on to progeny Not present in the genetic makeup Present in the genetic makeup b. The mouse continue to have information for presence of tail in its DNA So, mouse will continue to have tail, because it is an acquired trait. c. The process by which nature selects organisms which are more suitably adapted and possesses favourable variations is called Natural Selection 4. With regard to turnip, carrot, sweet potato and potato, three belong to the same category. Identify those three modifications and mention whether they are homologous or analogous. Also mention the reason why the fourth modifications do not belong to the same category. Ans: Turnip, Carrot, Sweet Potato – are modified root – Homologous organs but Potato – modified stem. 5. What are chromosomes ? Explain how in sexually reproducing organisms the number of chromosomes in the progeny is maintained Ans: Chromosomes are thread like structures made up of DNA found in the nucleus of cell. The original number of chromosomes becomes half during formation of gamete. Hence, when the gametes combine, the original number of chromosomes gets restored in the progeny. 6. Explain with an example for each, how the following provides evidences in favor of evolution in organisms: (a) Homologous organs (b) Analogous organs (c) Fossils Ans: Homologous organs – Study of homologous organs suggests that the organs having same structure but performing different functions have evolved from a common ancestor. Ex. - forelimbs of a frog, lizard, bird and man. Analogous organs – Study of Analogous organs suggests that these organs are having different origin and structural plan but perform same function . These are evolved from different ancestors and show adoption of organs for common use. Ex. – wings of butterfly and wings of bat. Fossils – provide the missing links between two species. Fossil Archaeopteryx possess features of reptiles as well as birds. This suggests that birds have evolved from reptiles 7. Explain the following: (a) Speciation (b) Natural Selection Ans: Speciation – The process by which new species develop from the existing species is known as speciation. Speciation takes place when variation is combined with geographical isolation. Natural selection – this is the change in frequency of some genes in a population. This give survival advantage to a species from elimination. Ex. – in a population of beetles, a new variation (green colour) get survival benefit / advantage to green beetles whereas other (red) perishes. 8. How do Mendel’s experiments show that the (a) traits may be dominant or recessive, (b) traits are inherited independently ? Ans: a) When Mendel cross pollinated pure tall pea plants with pure dwarf pea plants, only tall plants were obtained in F1 generation. On self pollinating the F1 progeny, both tall and dwarf plants appeared in F2 generation in the ratio 3:1 Appearance of tall character in both the F1 and F2 shows that it is a dominant character. The absence of dwarf character in F1 generation and its reappearance in F2 shows dwarfness is the recessive character. b) When Mendel conducted a dihybrid cross having two sets of characters, he obtained only one set of parental characters in F1 generation whereas in F2 generation he obtained both the set of parental characters now recombined in the ratio of 9:3:3:1. The appearance of new recombinants in the F2 generation along with parental type shows that traits are inherited independently. 9. What do you mean by Gene flow, Genetic drift . Ans: Gene flow is transfer of gene between population that are partly but not completely separated. Genetic drift: It is the random change in the frequency of alleles (gene pair) in a population over successive generations. Genetic drift takes place due to (a) Severe changes in the DNA (b) Change in number of chromosomes 10. ‘‘It is possible that a trait is inherited but may not be expressed.’’ Give a suitable example to justify this statement. Ans: Yes, it is possible. Example – When pure tall pea plants are crossed with pure dwarf pea plants, only tall pea plants are obtained in F1 generation. On selfing tall plants of F1, both tall and dwarf plants are obtained in F2 generation in the ratio 3:1. Reappearance of the dwarf character, a recessive trait in F2 generation shows that the dwarf trait/ character was present in individuals of F1 but it did not express (due to the present of tallness, a dominant trait / character). 11. ‘‘It is a matter of chance whether a couple will have a male or a female child.’’ Justify this statement by drawing a flow chart. Ans: A human male has XY sex-chromosomes and produces two types of sperms (heterogametic). Either with X-chromosome or with Y-chromosome. A human female has XX sex-chromosomes and produce ova of one type (homogametic) all with X-chromosomes. Parents Male Female Gametes X Y XX Progeny XX XY Female (50%) Male (50%) 12. What is speciation? List four factors that could lead to speciation. Which of these cannot be a major factor in the speciation of a self-pollinating plant species? Explain. Ans: Speciation is the process of the formation of new species from pre-existing ones. Factors – (i) Mutations (ii) Natural selection (iii) Genetic drift (iv) Geographical Isolation Geographical isolation cannot be a major factor in the speciation of a self pollinating plant species. This is because physical barrier cannot be created in self pollinating plants. 13. (a) Cite the evidence on the basis of which it is concluded that birds have evolved from reptiles. (b) Insects, Octopus, Planaria and Vertebrates also possess eyes. Can these animals be grouped together on the basis of the eyes they possess. Why or why not ? Give reason to justify your answer. Ans: a) i) Fossils showing imprints of feathers along with the bones in dinosaurs / reptiles found, ii) They could not fly and presumably using the feathers for insulation, iii) Later they developed / evolved and adapted feathers for flight. iv) Thus, they give evidence that birds have evolved from reptiles. b) No, the structure of the eye in each of these organisms is different / they have separate evolutionary origins. 14. How does sexual reproduction leads to similarities as well as variations ? Ans: Similarities and variations are dependent on the type of allele combination an offspring will get from their parents. If a child get dominant alleles from father then he will look quite similar to his father and quit different from his mother. If he got a combination of dominant alleles from both the parents then he will be quite different from both the parents. 15. What is hybrid? Give term for (i) externally exhibited traits (ii) Traits developed by gene - Ans: Organism having two or more mixed traits are called hybrid . Plants having two pairs of contrasting characters is called dihybrid. (i) Externally exhibited traits : Phenotype (ii) Traits developed by gene – Genotype Visit page dedicated Board exam Heredity and evolution Practice these paper will help you score high in 10th CBSE Board Exam 2017 
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