NCERT Class 10: Science-Intext Solutions

Chapter 09-Heredity and Evolution-Intext Solutions

NCERT Book Page Number- 142
Q.1. 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 species, which trait is likely to have arisen earlier?
Ans. In asexual reproduction, offspring is produced from single parents. There may be small inaccuracies in DNA copying which can develop new traits. They will be in smaller proportion than the traits already present. Therefore, trait B which exists in 60% of population must have arisen earlier than trait A which occurs in 10% of the population.
Q.2. How does creation of variations in a species promote survival?
Ans. The variations generated in offspring do not have equal chances to survive and get inherited in the next generation. The inheritance of such characteristics or variations depends on a number of environmental factors as well as on the nature of variation. Same variations are preadaptations which can be beneficial under certain environmental condition. For example, in a heat wave most of the bacteria will die but a few having pre-adaptation or variation to tolerate heat wave, will survive and multiply.
NCERT Book Page Number- 147

Q.1. How do Mendel’s experiments show that traits may be dominant or recessive?
OR
Illustrate Mendel’s experiment to show that traits may be dominant or recessive by taking tall/short plants as a character in garden pea.
Ans. Mendel took pea plants with contrasting characters some with short and some with tall stem. He produced the progeny of first generation (F1) from them. All the plants were tall in F1 generation, there was no intermediate characteristic. Mendel used the progeny of F1 as parent plants and produced the progeny of F2 generation to test whether the tallness of F1 progeny was same as their parents. He noticed that the progeny of F2 generation were not all tall. One fourth of progeny were short. This characteristic of shortness proves that both the characteristics (tallness and shortness) were inherited from the parents to F1 progeny. In F1 progeny only tallness character was expressed. However, the second-generation progeny (F2 progeny) expressed both the characters in a particular ratio i.e. 3 : 1 phenotypically and 1 : 2 : 1 genotypically. In a cross, trait (T) which expresses itself in the hybrid (Tt) is called dominant. In such hybrid only T is sufficient to express its trait i.e. tallness. While the trait which does not express itself in the hybrid is called recessive trait. In such a trait both the copies should be tt.
Q.2. How do Mendel’s experiments show that traits are inherited independently?
Ans. Mendel crossed a variety of pea plant with round, yellow seeds with another variety having wrinkled green seeds, the F1 progeny showed only round yellow seeds. After self-fertilisation of F1 plants, the F2 progeny obtained, showed four different types of plants having seeds in a ratio 9 (round, yellow) : 3 (round, green) : 3 (wrinkled, yellow) : 1 (wrinkled, green). This is called a dihybrid ratio (9 : 3 : 3 : 1). This shows that each trait is inherited independently of other.


Q.3. 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. No, this information is not enough to tell that which of the traits is dominant. As you know a recessive trait appears only when the two alleles are similar. Therefore, there can be two possibilities.
• Blood group O is dominant and blood group A is recessive. In this case father should have both alleles of A (IAIA) and mother can be homozy-gous or heterozygous (I°I° or I°IA) for allele of O. The daughter will have one dominent allele of O (IAI°).
• Blood group A is dominant and blood group O is recessive. In this case mother and daughter should have both alleles of O (I°I°) and father may be homozygous or heterozygous (IAIA, IAI°) for allele A. As both the possibilities can occur, the given information is unable to tell whether the allele for blood group A or O is dominant.


Q.4. How is the sex of the child determined in human beings?
Ans. It is assumed that half the children of a couple will be girls and half will be boys. All children of the couple will inherit similar chromosomes from the mother but the sex of the children will depend on the chromosomes they inherit from the father. If a child inherits (22 + X) chromosomes from the father that will be a girl but when a child will inherit (22 + Y) chromosomes from the father that will be a boy. Therefore, the sex of a child is determined by the inheritance of X or Y chromosome from the father.


NCERT Book Page Number- 150

Q.1. What are the different ways in which individuals with a particular trait may increase in a population?
Ans. The different ways in which individuals with a particular trait may increase in a population are following :
• Food availability – Individuals with a particular trait may have extra abundance of food in their environment. They will naturally increase in number.
• Genetic drift – Individuals with a particular trait may increase in a population due to ge¬netic drift. In it, there is a seasonal or accidental decline in population. The survivors have certain combination of traits which increase in number with increase in population.
• Natural selection – The particular trait which has survival value is picked up by natural selection. It increases in population through differential reproduction.

Q.2. Why are traits acquired during the life time of an individual not inherited?
OR
Why traits such as intelligence and knowledge cannot be passed on to the next generation?
Ans. Acquired traits develop due to the effects of environmental factors, use and disuse of organs and special (conscious) efforts. These are somatic variations and remain restricted to somatic cells. They are destroyed with the death of the individual and cannot pass to the progeny.
Q.3. Why are the small number of surviving tigers a cause of worry from the point of view of genetics?
Ans. The small number of surviving tigers is always at a risk of degeneration and extinction. Due to the small population, there would be excessive inbreeding that brings about inbreeding depression or degeneration. There are fewer recombinations and variations in a small population which are essential for vigour of the species.

NCERT Book Page Number- 151

Q.1. What factors could lead to the rise of a new species?
Ans. Following factors could lead to the rise of a new species :
(a) Physical barrier – Absence of gene flow amongst sub-populations due to the presence of physical barriers lead to the rise of a new species.
(b) Spatial Isolation – Sub-populations at the two ends of a long range seldom interbreed. They undergo changes in structures, timing and season of breeding resulting in formation of new species.
(c) Gene Mutation – A large mutation can make some members reproductively isolated from the rest. It leads to the rise of a new species.
(d) Genetic drift – It is the random change in the frequency of alleles in a population over successive generations due to error during DNA copying in the gametes. It leads to the rise of a new species.
Q.2. Will geographical isolation be a major factor in the speciation of a self pollinating plant spe¬cies? Why or why not?
Ans. No, geographical isolation will not be a major factor in the speciation of a self pollinating plant species because there is already no gene flow among members of the species.
Q.3. Will geographical isolation be a major factor in the speciation of an organism that reproduces asexually? Why or why not?
Ans. No, geographical isolation will not be a major factor in the speciation of an organism that reproduces asexually because there is no recombination of genes in it. Therefore, variations originating in them do not get diluted but spread to all the subsequent generations.

NCERT Book Page Number- 156

Q.1. Give an example of characteristics being used to determine how close two species are in evolutionary terms.
Ans. The presence or absence of fundamental characteristics and correlated characters determine the closeness of two species. Two species of sponges are closely related as they possess cellular level of organisation. Human beings are close to chimpanzee because they possess similar mammalian and primate characters. Nowadays, closeness of the species is determined by DNA matching.
Q.2. Can the wings of a butterfly and the wings of a bat be considered homologous? Why or why not?
Ans. No, the wings of a butterfly and the wings of a bat are not homologous organs because the wings of a bat are skin fold between the elongated fingers, while the wings of a butterfly are the extension of integument. Thus their structures are different but functions are same. Hence they are analogous organs.
Q.3. What are fossils? What do they tell us about the process of evolution?
Ans. Fossils are the remains or traces and impressions of any organism that lived in the geological past. Fossils provide a direct evidence of evolution and are called written documents of evolution. They directly indicate the presence of different types of organisms in different ages. The path of evolution is known by arranging the fossils in a proper sequence agewise. The early fossils are the simple organisms. Later on different complex forms arose, flourished and died down. They are replaced by newer forms.

NCERT Book Page Number- 158

Q.16. Why are human beings who look so different from each other in terms of size, colour and look are said to belong to the same species?
OR
How different races of human beings belong to the same species?
Ans. Human beings are different in size, colour and look because these characteristics are based on preponderance of specific alleles and their interactions with environment. They are said to belong to the same species because they have a common gene pool and they can marry amongst themselves and can produce fertile offspring.
Q.2. In evolutionary terms, can we say which among bacteria, spider, fish and chimpanzee have a better body design? Why or why not?
Ans. Chimpanzees have a better body design out of the four (bacteria, spider, fish and chimpanzee) because a better body design is the one which has more complexity, more elaboration and more controls which gives the organism a better competitive edge over others.