Class 10 NCERT Solutions Biology Chapter 9 - Heredity and Evolution

As the population of the given species reproduce asexually there would be only very minor differences generated due to small inaccuracies in DNA copying, so trait B is likely to be arisen earlier as it is present in 60% of the population. Trait A which exists in 10% of the population may be originated due to variations.

Concept insight: More percentage of one trait in a population indicates the earlier evolution of that particular trait.

Favourable variations help a species to adapt to changes in their environment and they promote survival of a species.

Concept insight: List the advantages of variations.

Mendel took one tall pea plant and one short pea plant and produced progeny from them.    

Parents:                          TT                                        tt                                        

                                    (tall plant)                           (small plant)    

Gametes:                         T T                                       t t  

Progeny: F1                       Tt                                       Tt                                                      

                                                     (tall plant)  

F2                                                  TT          Tt         tt                                                    

                                                    (tall)      (tall)     (short)    

From this cross he found that in nature tallness is dominant over shortness.   Because in F2 generation heterozygous plant contains one gene of tallness and one of shortness of which tallness expressed itself. Thus he concluded that in a contrasting pair of characters one character is dominant and another is recessive.

Concept insight: Recall Mendel's law of inheritance.

Mendel conducted a dihybrid cross. He took two characters:

(i) Round and yellow seeds (RRYY)

(ii) Wrinkled and green seeds (rryy)

In F1 generation, he got plants with Round and yellow seeds (RrYy) Then he self crossed F1 plants. He got the following result:

(i) Round yellow - 9

(ii) Round green - 3

(iii) Wrinkled yellow - 3

(iv) Wrinkled green- 1

From the following result, it can be concluded that genes controlling different characters are not linked. Thus, in F2 generation, he got two new characters (round, green and wrinkled, yellow) other than parental characters (round, yellow and wrinkled, green). 

Concept insight: Recall Mendel's second law of inheritance.

No, the information provided is not enough to tell whether blood group A or O is dominant. Every character is controlled by a pair of alleles. And here, it is not mentioned whether the man and woman have homozygous or heterozygous traits.

Concept insight: Recall Mendel's law of inheritance.

Human male has one X-chromosome and one Y-chromosome. In other words, half the male gametes have X-chromosomes and the other half have Y-chromosomes. On the other hand, a female has two X-chromosomes. Thus all female gametes have only X-chromosomes. If a sperm carrying Y-chromosome fertilizes an ovum then the child born will be a boy. On the other hand, if a sperm carrying X-chromosomes fertilizes an ovum then the child born will be a girl.

Concept insight: Recall types of chromosomes in human beings.

The population of individuals with particular traits may increase in the following ways:

(i) By the natural selection of variations brought during the course of time.

(ii) By accidental changes in the frequency of some genes in a population i.e. genetic drift.

(iii) Variations

Concept insight: List the causes of variation.

Traits acquired by an organism during its lifetime are known as acquired traits. These traits are not inherited because they do not cause any change in the DNA of the organism.      

Concept insight: Student should remember the cause of change in any trait-environmental or genetic.

(i) Small numbers of tigers means that fewer variations in terms of genes are available. This means that when these tigers reproduce, there are less chances of producing progeny with some useful variations.

(ii) If any natural calamity occurs and kills these small numbers of surviving tigers, they will become extinct resulting in the loss of these genes forever.

(iii) Less number of species means less number of diversity and lesser number of traits which reduces the chances of adaptability with respect to change in the environment.

Concept insight: Important from Exam Point of View.

Factors responsible for the rise of a new species are:  

(i) Genetic variation

(ii) Natural selection 

(iii) Genetic drift.  

Concept insight: List the causes of variations.

No,  However, since the plants are self-pollinating, which means that the pollens are transferred from the anther of one flower to the stigma of the same flower or of another flower of the same plant, so geographical isolation cannot prevent speciation in this case.

Concept insight: Recall speciation and factors affecting speciation.

No. Geographical isolation prevents gene flow between populations of a species whereas asexual reproduction generally involves only one individual. In an asexually reproducing organism, variations can occur only when the copying of DNA is not accurate. Therefore, geographical isolation cannot prevent the formation of new species in an asexually reproducing organism.

Concept insight: Recall speciation and factors affecting speciation.

Analogous organs are one such evidence that is used to determine how close two species are related. The presence of feathers in dinosaurs and birds indicates that they are evolutionarily related. Dinosaurs had feathers not for flying but instead these feathers provided insulation to these warm-blooded animals. However, the feathers in birds are used for flight. This proves that reptiles and birds are closely related and that the evolution of wings started in reptiles.

Concept insight: List the various evidences that show the close relationship between two organisms.

No, wing of a butterfly and wing of a bat cannot be considered as homologous organs because wings of a bat are skin folds in between the fingers and wings of an insect are membrane supported by muscles. They are analogous organs which have different basic structure but have similar appearance and perform similar functions.  

Concept insight: List the various evidences that show the close relationship between two organisms.

Though human beings look so different from each other still they belong to same species because

(i) The number of chromosomes are same.

(ii) They have a common ancestor.

(iii) They interbreed among themselves. So, they are able to produce fertile offsprings.   

Concept insight: List the common characteristics of a species.

In evolutionary terms, we can say that bacteria has a 'better' body design than spiders, fish and chimpanzees because though bacteria is one of the simplest and primitive life forms but still it inhabits and survives in some of the most inhostpitable habitats such as hot springs, deep-sea thermal vents and ice in Antarctica. Other organisms cannot survive in such harsh habitats.

Concept insight: Evolution should not be equated with progress.

(d) All of the above.

Concept insight: Recall Homologous organs.

(b) A Chinese school- boy.  

Concept insight: Recall the common characteristics between two organisms.

Only dominant traits are transferred from the parents to the offspring in the first generation. Therefore, it can be concluded that light eye colour is dominant.  

Concept insight: Recall Mendel's law of inheritance.

The field of evolution and classification are interlinked in the following manner:

(i) The more characteristics two species have in common, the more closely they are related. Classification of species is a reflection of their evolutionary relationship.

(ii) The more number of characteristics shared by two organisms more is the probability of them having common ancestor. Thus, classification of an organism is the reflection of its evolutionary path.

Concept insight: Recall the definition of evolution and classification.

Analogous organs: Those organs which have different basic structure but have similar appearance and perform similar functions are called analogous organs. Example - wings of bird and insect.  

Homologous organs: Those organs which have same basic structure but different functions are called homologous organs. Example: fore limb of humans and fore limb of lizard.  

Concept insight: Recall the definition of homologous and analogous organs.

Dogs have a variety of genes that govern coat colour. There are at least eleven identified. Gene series (A, B, C, D, E, F, G, M, P, S, T) that influence coat colour in dog. A dog inherits one gene from each of its parents. The dominant gene gets expressed in the phenotype. For example, in the B series, a dog can be genetically black or brown.

Let us assume that one parent is homozygous black (BB), while the other parent is homozygous brown (bb) gametes of black parent will be (B,B) and brown parent will be (b,b). In the F1 generation, all offsprings will be heterozygous (Bb). (1 mark)

Since black (B) is dominant, all the offsprings will be black. However, they will have both B and b alleles. If such heterozygous pups are crossed, they will produce 25% homozygous black (BB), 50% heterozygous black (Bb), and 25% homozygous brown (bb) offsprings.

Concept insight: Recall Mendel's law of inheritance.

By studying characters of fossils, we are able to find their descendants and thus we can know what new variations have come. They provide evidence and missing links between two classes. They are helpful in forming a sequence of organisms in the pathway of evolution. Thus, fossils have an importance in deciding evolutionary relationship.

Concept insight: List the importance of fossils.

J.B.S.Haldane suggested that life must have developed from the simple inorganic molecules which were present on earth soon after it was formed. He speculated that the conditions on earth at that time could have given rise to more complex organic molecules that were necessary for life. The first primitive organisms would have evolved from further chemical synthesis.  

Later, Stanley L.Miller and Harold C.Urey conducted experiment to find out about the origin of organic molecules. They assembled an atmosphere similar to that thought to exist on early earth (molecules like ammonia, methane and hydrogen sulphide, but no oxygen over water. This was maintained at a temperature just below 100^oC and sparks were passed through the mixture of gases to stimulate lightning.   At the end of a week, 15% of the carbon had been converted to simple compounds of carbon including amino acids which make up protein molecules. This is how life originated from inanimate matter.  

Concept insight: Recall Stanley L. Miller and Harold C. Urey's experiment.

Sexual reproduction gives rise to more viable variations than asexual reproduction because the genes of the traits are transmitted from one generation to another and dominant characters are expressed.  While in the asexual reproduction, genes cannot be separated from the offspring, only the DNA of one parent is copied.

Variations affects the evolution of those organisms that reproduce sexually.

By this process, more and more variations are produced in the subsequent generationS. In this way, genetic drift will accumulate which causes the formation of new species.  

Concept insight: Recall the importance of sexual reproduction over asexual reproduction.