Heredity and Evolution

NCERT In Text Questions

Question: 1. If a trait A exists in 10% case of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier?

Answer: Trait B has appeared earlier.

Question: 2. How does the creation of variations in a species promote survival?

Answer: Variations are the key factor on which natural selection operates and selects the species or individuals that are fit to live in a particular environment. A species well- adjusted in one set of environment, may totally be eliminated by natural selection when environment changes. Variations of different nature present among the individuals of a species provide opportunity to be selected in the new conditions of environment. In this way, creation of variations in a species promotes survive.

Question:3. How do Mendel?s experiments show that traits may be dominant or recessive?

Answer: When Mendel's crossed two individuals with a pair of homozygous contrasting characters, in F₁ generation, only one character appeared, but in F₂ generation plants with both the characters appears in a ratio of 3:1. He concluded that when the two units of a factor are dissimilar, only one shows its appearance and is called dominant while the factor which remains hidden is called recessive.

Question:4. How do Mendel's experiments show that traits are inherited independently?

Answer:In a dihybrid cross when two pairs of contrasting characters were considered simultaneously by Mendel, in F₁ generation only two dominant characters showed their appearance but in F₂ generation phenotypically four different types of individuals appeared with parental as well as recombinant characters. Mendel concluded that each character is independent and it is due to their independent assortment that individuals with four different combinations are produced.

Question:5 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 not?

Answer: The blood group 'O' appears only when the gene for O remains in homozygous condition. The daughter has blood group O so her genotype must be 'OO'. She has inherited one O gene from her mother and one from her father. Since father has blood group A, it means genotypically he is heterozygous (AO). It shows that A is Dominant over O and that is why father with AO genotype has A blood group.

Question:6 . How is the sex of a child determined in human beings?

Answer: In human beings, sex of the child is determined at the time of fertilization. Human female is homogametic, i.e., all the eggs she produces contain 22A+X chromosome. On the other hand, males are heterogametic, i.e, they produce two types of sperms, 50% of them contain 22A+X chromosome and another 50% of them contains 22A+y chromosome. If the sperm containing X- chromosome fertilizes the eggs, the zygote becomes a female (44A+ XX) and if the sperm containing Y-chromosome fertilizes the egg, the zyote becomes a male (44A+XY).

Question: 7 . What are the different ways in which individuals with a particular trait may increase in a population?

Answer: Individuals with a particular trait may increase in a population when the trait it is carrying has a survival value and is being selected by natural selection generation after generation.

Question: 8 . Why are traits acquired during the lifetime of an individual nit inherited?

Answer: Acquired characters bring about changes in somatic organs. Somatoplasm does not take part in the process of reproduction, i.e, gametogenesis. It is the germplasm which is responsible for the formation of gametes for reproduction. Unless a change occurs in germ cells, it cannot be inherited.

Question:9 .Why are small numbers of surviving tigers a cause of worry from the point of view of genetics?

Answer: The small number of tigers indicates that they do not possess such genetic variations which permit the natural selection to select them and to adapt them in the changing environmental situations. Small number of tigers reduces the chances of mating between male and female, which is gradually eliminating gene pool.

Question: 10 . What factors could lead to the rise of a new species?

Answer: for the rise or origin of a new species, useful genetic variations with survival value, natural selection and geographical isolation along with reproductive isolation is necessary.

Question:11 . Will geographical isolation be major factor in the speciation of a self- pollinationg plant species?

Answer: No, geographical isolation will not be a major factor for the speciation of a self ? pollinating plant species because geographical isolation helps in the speciation of cross- pollinating organisms. Since self- pollination occurs within the same plant, geographical isolation cannot work as a potent factor of speciation.

Question:12 . Will geographical isolation be a major factor in the speciation of an organism that reproduces asexually? Why or why not?

Answer: Geographical isolation occurs between sexually reproducing organisms. It may not be a major factor in the speciation of asexually reproducing organisms because asexual reproduction involves only a single organism.

Question: 13 . Give an example of characteristic being used to determine how close two species are in evolutionary terms.

Answer: The closeness of two species in evolutionary terms can be determined by:

(a) Number of chromosomes

(b) Degree of pairing between the DNA nucleotide sequences with that of other species

(c) Similarities in composition of species proteins in haematin crystals.

Question: 14. Can the wing of a butterfly and wing of a bat be considered homologous organs? Why or why not?

Answer: Homologous organs are those organs which are similar in structure and origin but differ in function in case of wings of a butterfly and a bat, the function is same but the structure is different. Wings of a butter fly are merely the outgrowth of body wall, while the wings of a bat constitute bones and extension of skin. Hence, they cannot be considered as homologous organs.

Question: 15.What are fossils? What do they tell us about the process of evolution?

Answer: Fossils are the remains or impression of the living beings which existed earlier.

(a) Fossils provide us direct evidence of evolution.

(b) The fossils present in different strata of the earth indicate the sequence of evolutionary changes from simpler to more complex forms.

(c) The fossil helps to trace out the phylogeny of living beings.

(d) Certain fossil act as connecting links between two groups of organisms indicating the ancestry of particular group.

Question: 16.Why are human beings, who look so different from each other in terms of size, colour and looks, said to belong to the same species?

Answer: There are different races of human beings which differ in external appearances but they all belongs to the same species, How species because of the fact that

(a) They contain the same number of chromosomes in their cells

(b) Their karyotype is similar.

(c)Their molecular phylogeny involving DNA sequencing is common

(d) They are capable of interbreeding.

Question: 17. In evolutionary terms, can we say which among bacteria, spiders, fish and chimpanzees have a better body design? Why or why not?

Answer: Chimpanzees have a better body design because of the following facts:

(a) There is complete separation of oxygenated and deoxygenated blood due to presence of four- chambered heart and more efficient pulmonary respiratory system.

(b) They have more developed brain.

(c )They have faster metabolic rate.