# NCERT Solutions for Class 10th Science Chapter 9 Heredity and Evolution

NCERT Solutions for Class 10th Science Chapter 9 Heredity and Evolution

Page 143
Q1. 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 population, which trait is likely to have arisen earlier ?
Ans: As species are asexually reproducing, there would be only very minor differences generated due to small inaccuracies in DNA copying, so trait B, which exists in 60% of the same population may get inherited earlier while trait A, which exists in 10% of the population may be originated late due to variations. Thus, trait B have arisen earlier since it is present in 60% of the same population.

Q2. How does the creation of variations in a species promote survival ?
Ans: Natural selection selects the individuals having useful variations which ensure their survival in the prevailing conditions of environment. Variant individuals that can withstand or cope with prevailing environment will survive better and will increase in number through differential reproduction.

Page 147
Q1. How do Mendel’s experiments show that traits may be dominant or recessive ?
Ans: Mendel took pea plants with contrasting characteristics — tall plant and dwarf (short) plant. On cross pollination, he got all tall plants in F1 generation. Then by self pollination of F1 tall plants, he produced second generation (F2) consisting of tall and short plants in the ratio of 3 : 1. Then he concluded that, ‘T’ (tall) trait is dominant while ‘t’ trait for shortness is recessive.

Q2. How do Mendel’s experiments show that traits are inherited independently ?
In a dihybrid cross made by Mendel, it was observed that when two pairs of traits or characters were considered; each trait expressed independent of the other. Thus, Mendel was able to propose the Law of Independent Assortment which says about independent inheritance of traits.

Q.3. A man with blood group A marries a woman with blood 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 ?

No. This information is not sufficient to determine which of the traits − blood group A or O − is dominant. This is because we do not know about the blood group of all the progeny.Blood group A can be genotypically AA or AO. Hence, the information is incomplete to draw any such conclusion.

Q4 How is the sex of the child determined in human beings?

The females carry two X-chromosomes. Females produce one type of gametes (eggs) with same type of chromosomes (22 + X). Males have one X and one Y- chromosome. Among the male gametes, half of the sperms carry X-chromosome (22 X) and half
carry Y-chromosome (22 + Y). Thus, female is homogametic and male is heterogametic. When a sperm carrying X- chromosome fertilises an egg, the zygote develops into female (XX condition). When sperm carrying Y-chromosome fertilises an egg, the zygote develops into a male (XY condition). Thus, sex is determined at the time of fertilisation.

Page 150
Q1. What are the different ways in which individuals with a particular trait may increase in a population ?
Different ways are : variation, natural selection and genetic drift (isolation).

Q2. Why are traits acquired during the lifetime of an individual not inherited ?
Because acquired characters bring changes only in non-reproductive tissues and cannot change the genes of the germ cells. Thus, acquired traits cannot be passed to next generation.

Q3. Why are the small numbers of surviving tigers a cause of worry from the point of view of genetics ?
(i) If any natural calamity occurs and kills these small number of surviving tigers, they can become extinct resulting in the loss of some genes forever.
(ii) Small number will lead to little recombination and, therefore, lesser variations. These both are very important for giving better survival chances to the species.
(iii) Less number of species means lesser extent of diversity and lesser number Of traits which reduces the chances of adaptability with respect to the change in the environment.

Page 151:

Q1. What factors could lead to the rise of a new species ?
Genetic variations, natural selection and reproductive isolation could lead to the rise of a new species.
Q2 Will geographical isolation be a major factor in the speciation of a self-pollinating plant species ? Why or why not ?
No, because pollination occurs on the same plant in self-pollinating plant species.
Q3 Will geographical isolation be a major factor in the speciation of an organism that reproduces asexually ? Why or why not ?
No, because asexual reproduction involves single parent or organism.

Page 156:

Q1: Give an example of characteristics being used to determine how close two species am in evolutionary terms ?
Homologous organs, analogous organs and vestigial organs help to identify evolutionary relationships amongst the species.
Q2: Can the wing of butterfly and the wing of a bat be considered homologous organs ? Why or why not ?
No, wing of a bat and wing of a bird cannot be considered as homologous organs because they have different basic structure.
Q3: What are fossils ? What do they tell us about the process of evolution ?
Fossils are the impression or remains of ancient life found preserved in the sedimentary rocks. Fossils are direct evidences of evolution. Fossils also help to identify evolutionary relationship between apparently different species. They also tell about the extent of evolution that has taken place.

Page 158:
Q1: 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 ?
They look different because of interaction of genes with environment which results in change in their appearance. But they belong to the same species as they have same number of chromosomes and can breed among themselves.

Q2: In evolutionary terms, can we say which among bacteria, spiders, fish and chimpanzees have a ‘better body design’ why or why not ?
No, because different designs are the product of evolution and different species have different body design to suit or adapt to their environment.

Q10: Explain how sexual reproduction gives rise to more viable variations that asexual reproduction. How does this affect the evolution of those organisms that reproduce sexually ?
Variations occurring during sexual reproduction may be due to:
(i) Separation of homologous chromosomes (by chance only) during gamete formation.
(ii) Crossing over (recombination) of homologous chromosomes.
(iii) Fertilisation of gametes to form zygote.
(iv) Errors during DNA copying or mutations. In asexually reproducing organisms only errors during DNA copying or mutations cause variations.
Since the extent of variations is much larger in sexually reproducing organisms, therefore, the chances of evolution is also much in sexually reproducing These variations enable the organisms to adapt themselves to the changing conditions and also help to face the struggle for Over the time, they and rise to new species.

Q11: How is the equal genetic contribution of male and female parents ensured in the progeny?
Genetically organisms are of types
(i) Haploid : They have single set of chromosomes, where each chromosome is represented singly. As the chromosomes are the bearer of genes so haploids have single set of genes. A single gene determines the expression of character.
(ii) Diploid : ‘They have two sets Of homologous chromosomes, where the chromosome occur in pair, one maternal contributed by the mother through her ovum and the second Of the pair is contributed by the male parent through his sperm. The resultant cell zygote produces by the fusion of male and female gametes have two sets of chromosomes— each set contributed’ by each parent. In diploids a character is controlled by two genes/factors. Both the father and mother contribute practically equal amount of genetic material to the child. It means that each trait can be influenced by both paternal and maternal DNA.

Q12: Only variations that confer an advantage to an individual organism will survive in a population. Do you agree with this statement ? Why or why not ?
No, many of the times the variations are not advantageous to an individual organism but still survive in a population, e.g., take the case of free ear lobe and attached ear lobe. Most of the other variations not only give survival advantage to an individual but also contribute to genetic drift. Thus, we can say that most of the variations lead to better adaptation of an organism to the changing environment. In this way, it gives survival advantage to that organism and will also survive in the coming population.