Heredity and Evolution Question Answers: NCERT Class 10 Science

B trait has 60% population and A trait has 10% population. It is clearly seen that A trait has less population than B trait. B traits have arisen earlier because of the higher population than A trait, i.e, 60%. And the A trait has 10% population which is less, it means that it has arisen some time ago in the environment.

Variation is the only factor of evolution. Evolution is the gradual changes in some features or functions of the pre-existing species. Every individual organism varies in their size, shape, behaviour from other organisms. Variations in the environment ensure survival because when any organism adapts themselves according to the variations there will be long term survival for that species, those species will survive who will struggle for their existence. The organisms who live in unfavourable conditions have to do more efforts for survival and those who survive in the most drastic conditions will be the fittest survival organism also known as Natural selection theory.

The allele which always expresses itself in the presence of contrasting allele is a dominant allele and which does not express itself in the presence of its contrasting dominant allele is known as recessive allele.

Mendel showed the traits dominant or recessive by the monohybrid cross which is shown by the law of dominance in pea plants, in which the allele is expressed itself is dominant or the unexpressed one is recessive. He crossed the homozygous tall pea plant with genotype TT to the homozygous dwarf pea plant with genotype tt. Then, the F1 (filial generation) is formed with the result of phenotype Tt where T shows the tallness, i.e, dominant or t shows the dwarfness which remain unexpressed, i.e, recessive. And when he crossed again, he found the 3:1 phenotypic ratio and 1:2:1 genotypic ratio in F2 generation.

Mendel gave the law of independent assortment from the dihybrid cross.

Mendel performed an experiment in which he made crosses of pea plant with the round yellow seeds and one with the wrinkled   green seeds. And in the F1 progeny, round yellow seeds are dominant over the wrinkled green seeds. And in the F2 generation outcome, he saw all the characters are inherited independently by the use of punnett square that helps to demonstrate the possible combinations of the gametes in F2 progeny. Those independently inherited combinations were round yellow, round green, wrinkled yellow and wrinkled green as shown below :

This punnet square is showing the 16 possible combination of gametes with the phenotypic ratio 9:3:3:1 and 1:2:1:2:4:2:1:2:1 genotypic ratio in F2 generation. Every pair is showing the different characters, showing independent assortment.

This information is not enough to tell which of the traits A or O dominant. In human hereditary characters, A, B, ABO blood group is dominant or O is recessive. But in the above case we cannot detect which blood group is dominant. If father’s blood group is A, genotypes will be homozygous AA or heterozygous AO. And women having the blood group O, then the genotype will be homozygous OO and heterozygous AO. 

If A is dominant, the cross of father genotype I^AI^A with mother have I^oI^o and the progeny will have 50- 50 percent of the genetic material, where the father blood group is A and mother’s is O.

                       I^A                I^A

                      I^o                        I^o

 If the blood group O is dominant, when we cross the father genotype I^AI^O with the mother genotype I^oI^o, the child will contain blood group O.

                       I^A                I^o

                      I^o                        I^o

So, in both cases the child will have O. So, this is not clear which trait is dominant.

In organisms, a specific pair of chromosomes determine the sex of the individual called sex chromosome. In human beings, sex is determined by the XX-XY chromosomes. Females contain XX chromosomes and males contain XY chromosomes.

So, if the male gamete X chromosome fuses with the female X chromosome then the child will be female or when male gamete Y fuses with the female X chromosome then it will be the male child.

There are many different ways in which individuals with a particular trait may increase in a population and these are following :

• Genetic drift, natural selection, well nourishment of the child, geographical isolation, mutation etc.
• Genetic drift is when there is any calamity that occurs in the population, if some of them individuals not living then the remaining ones shift to the new place and they can pass their genes to the next generation or can increase the population.
• Natural selection occurs for the survival of the fittest organisms who adapt themselves according to the variations in environmental conditions, and variation is the important factor of the evolution. So, the maturation and adaptation factors of an individual to the environment may lead to increase in the population.
• If the well nourishment of the child is done by parents, the population will start increasing.
• Mutation plays the major role in genetic variations. It alters the genes expression or gene sequence in the chromosome may lead to increase in a particular trait.

Acquired traits are those, people attain during it life- time due to change in environment and on the other hand, inheritance is the transfer of characters as well as variations or information from one generation to the next by the copying of DNA. Traits are acquired during the life- time not inherited because inheritance occurs during the birth of an individual, and traits are acquired when people learn for their future and to make the changes in their work that does not affect the DNA of the germ cells, like in the inherited traits where DNA get affected by the changes. If any mutation occurs in the germ cell it can give rise to the new trait being acquired by the next successive generation.

Small numbers of surviving tigers are the cause of worry from the point of view from genetics because these populations carry some genes which help them to survive in the environment changes, which is important for the evolution of the tiger's population. Small number of tiger populations indicates less variation which is important for survival. If in case, any drastic condition like natural calamity, diseases, or hunting occurs then the small numbers of surviving tigers can be on the stage of extinction or can be fully extinct from the environment and their genes will be lost forever.

Genetic drift, natural selection, genetic variation in population, mutation, speciation, reproductive isolation. These all factors could lead to the rise of a new population, i.e, show patterns of evolution in different ways.

Genetic Variation in the Population: A species may have many populations in different places. Population is involved in this not an individual because the individual can die suddenly, but the population will not die in the sudden way it will continue and lead to the generation of new species.

Genetic Drift: It is the chance of elimination of genes when the part of a population dies during the natural calamity or migrates due to any reason. Some of the population dies, but the remaining part of the population migrants when mates with same or different species may lead to rise of a genetic variability or rise of a new species.

Natural Selection: This theory is given by Darwin, which represented the struggle for existence or elimination of the unfit members. Some members of the population have special genes which are able to grow up or reproduce at the higher level and transfer their special genes to the next generation and so on. Genes of the some members who reproduce the offspring at the higher level will become the strongest element  in the gene pool which leads to change in gene frequency of the population and the rise of the new species.

Mutation: Mutation is the major source of the variation. When mutation occurs in the genes the new phenotype produced by the genetic variations, and these variations can have some advantages or some disadvantages, that alters the gene pool. Or when variations occur in the positive way, it will give rise to the new species.

Speciation: When the environmental changes occur or the part of population migrates to the new place due to some variations, may lead to the rise of a new species. If the population becomes adapted and if its members have some genetic variations, i.e, adapted to the new environmental changes give rise to the new population. And, the origin of new species from the existing one with the new acquired characters is known as speciation.

Reproductive Isolation: It is the mechanism which checks the population of the two different species from interbreeding (that breed with the other species). In brief, it preserves the integrity of the species by checking hybridization, they ensure that the different species will not breed. It may, however, be the origin of new species that occurs by the accumulation of the genetic variation in the population leads to evolution. There are many isolating mechanisms which leads to the barrier of the interbreeding species including:- geographical isolation, genetic isolation, ecological isolation, incompatibility, hybrid sterility, hybrid breakdown etc.

No, geographical isolation will not be the major factor in the speciation of the self pollinating plant species because geographical isolation is the part of the reproductive isolation which occurs before the mating of the individuals. Geographical isolation is the separation of a group of related organisms by physical barriers like sea, mountain, desserts etc. In the self pollinating plants new variants or new traits are not formed, only homozygous will form, but in cross pollination variations can evolve. So, geographical isolation cannot be the major factor in the speciation of self pollinating plants.

Asexual reproduction occurs during the formation of two daughter cells from one parent only. There will be no variation when there is only one parent involved. And, where there is no variation speciation cannot occur. As in the self pollinating flowers geographical isolation is not the major factor in the speciation, this can also be seen in the case of asexual reproducing organisms. Where there is no variation, competition or genetic recombination, speciation will not occur and as well as geographical isolation will not occur because it refers to the separation of group of related organisms by some barriers.

The more characteristics of two species have in common, they seem to be more closely related. And the more closely they are related, more recently they shared a common ancestor or the evolutionary relationship.

Mostly the homologous characteristics derive from how close two species are in evolutionary terms. For example, mammals have four limbs as birds, reptiles and amphibians possess. Basic structures of the limbs are very similar and they are modified to do different functions. The characteristics can be similar in different organisms because of sharing the common ancestors which helps to identify the evolutionary relationship among two close related species. Another example is the common connection link between the reptiles and birds. The connecting link between the reptiles and the birds are known as archaeopteryx. Aves have the evolutionary characters which are derived from the reptiles, in which the forelimbs of the reptiles modified into the feathers or the wings.

The wing of a butterfly and the wing of a bat cannot be considered as homologous organs. Their wings are functionally the same that are adapted for the flight and they do not have common ancestors. So, they are not homologous organs.

Fossils are the remains or impression of the ancient organisms preserved naturally. During the formation of sedimentary rocks, dead animals of the sea or the lakes and of the land sink down or are buried in the rocks. The animals, thus preserved in the rocks formed the fossils. Fossils are the main evidence which show the process of evolution from how the origination of life occurs in ancient times or how it has been started to the life found on earth in today’s time.

All human beings look very different from each other in size, colour and looks but belongs to the same species because they can breed with each other and  have common ancestors and contains the same DNA sequences of purine and pyrimidine (adenine, guanine, cytosine, thymine, uracil) they are different in size, looks and colour because of the variation in the expression of genes or traits.

A better body design is, which have more complexity in their structural and developmental characteristics and have control on their body.  Every organism has their body design according to their needs for survival in the environment. Among all four organisms bacteria, spider, fish and chimpanzee- chimpanzees have more complex body structure and can do more functions according to the evolutionary terms. But all four organisms have better body design according to their needs to adapt to the environment. So, we cannot decide which organisms have better body design in the evolutionary terms.

(c) TtWW

According to the law of dominance, Mendel showed that the one allele is dominant over the another one (expressed allele is known as dominant allele or unexpressed one is recessive). As shown in this question, the all progeny bears violet flowers but half of them are short, this means that the violet colour flowers are dominant and the dwarf flower which contains white flower is a recessive trait.

(d) all of the above 

All of the above pairs are the examples of homologous organs which have similar basic structure but do not perform the same functions.

(a) a Chinese school-boy 

We have more in common with the Chinese school- boy cause we belong to the same species according to our evolution, i.e, Homo sapiens. Some variations can be seen in their physical appearance or in their behaviour this is only because of the environmental changes.

On the basis of the above information, we cannot say that light eye colour is a dominant trait or a recessive trait. The light eye colour trait can be dominant or as well as may be recessive. If the child has LL genotype, parents could have light coloured eyes with homozygous LL genotype. And, if the child has ll genotype, parents could have light coloured eyes with homozygous ll. So, as a conclusion we cannot find that light coloured eyes traits can be dominant or recessive, for this we should have at least data of three generations.

Study of evolution and classification are interlinked. We can find the evolutionary relationships by identifying the hierarchy of the characteristics (these are the details of appearance and the behaviour) between the two species which shows the evolution. Classification shows the similarities among the two species which allows us to group them and then do the study of them, with the sharing of their common ancestor. The more similar characters are between two species, more closely they seem to be related with sharing of common ancestor. As shown in the given example in the book, A brother and sister are closely related, having the common ancestors before them, i.e, their parents. Their cousin and the girl are related but less if we compared them to the real brother and sister. This is due to cousins having common ancestors, their grandparents. That is how we can relate the classification of the species to the evolutionary relationship.

Homologous organs are the organ whose origin is similar (or are embryologically similar) but perform different functions. Such as the forelimbs of humans and the wings of birds look different externally but their skeletal structure is similar. It means that their origin is similar (as wings in birds are modifications of the forearm) but functions are different - the wings help in flight whereas human forearms help in various activities.

Dogs have different coats. In order to find dominant coat (hair) colour in dogs, select a pure- bred of male and female dog having black colour or pure- bred of male and female having brown colour. Cross the homozygous male BB and homozygous female bb, and then observe the coat colour in dog progeny (offspring). If all progeny have black colour, this means that black colour will be the dominant coat colour in dogs and if progeny will have brown colour then the brown colour will be dominant coat colour.

And, by making the punnett square of F1 generation, we will get Bb, Bb, Bb, Bb ,i.e, all progeny are showing the black colour. So, it is concluded that the dominant colour is black.

Fossils are formed during the formation of sedimentary rocks, dead animals of the sea and the lakes, rivers when they sink down and get buried in the rocks. When there is no oxygen, they prevent the decay of an animal. The animals then preserved in the rocks, which form the fossils. The hard matter remains of the dead animals preserved layer by layer in the sedimentary rocks. The deeper layer had more older fossils and the upper layer had more recent fossils. Palaeontology is the study of past life based on the fossil records. Palaeontology furnishes the evidence for the evolution which tells about the organism that lives in today’s time or that lived in the past and how they originated by showing the fossil records. Ages of the fossils can be determined by some methods like relative dating method, absolute dating method etc. By which age of fossils determine how long they have been in the evolutionary relationships.

Abiogenesis is the generation of life from the inorganic matter. Many scientists gave the theory of origin of life from the non living matter. Then, at last Stanley L. Miller and Harold C. Urey gave the experiment to demonstrate the origin of life from inanimate matter.

Miller and Urey set up an experiment to derive life from inorganic matter to organic. He introduced Methane, Hydrogen and ammonia gases by providing the moist environment above the water containing flask. Then, Miller gave the system an electric current. After a few days he found that flask contained organic compounds and those compounds were amino acids which served as the building block elements of the proteins. By the chromatographic analysis, he confirmed the formation of amino acids. That’s how life originated from the non- living matter.

This is true that sexual reproduction give rise to more variations than asexual reproduction.

• Sexual reproduction is responsible for evolution because these two parents are involved in forming a zygote by sharing their equal amount of genetic material to the offspring that confers a variation. More viable variations occur because error can occur in copying of DNA where mutations occur but this is very rare. 

• Sexual reproduction provides infinite chances of the new combinations of existing traits in two steps:- first, by the segregation (the formation of gametes by random separation of paternal or maternal chromosomes into the daughter cells. And second, at the time of fertilization by random fusion of male and female gametes to form a zygote.

And in case of asexual reproduction,

• Unlike sexual reproduction, Asexual reproduction is not responsible for the evolution because in this type of reproduction process only one parent is involved to form the daughter cells. Very small or we can say no changes or variations occur in the progeny because the offsprings they gave arise are identical to the parent. And the division process is also different in this, it can occur through budding, binary or multiple fission.

So, we can say that sexual reproduction give rise to more viable variations than asexual reproduction.

Equal genetic contribution of male and female parents is ensured by the equal number of inheritance of chromosomes. Human beings possess a normal diploid (2N) chromosome number in human beings is 46. Each body cell contains 23 pairs of chromosomes, with 22 pairs of autosomes and one pair of sex chromosome. The autosomes are similar in male and female but one sex chromosome is different in both. In females, sex chromosomes are same XX and heteromorphic in males, i.e, XY. Sex chromosomes control the sex of an individual. One male haploid gamete and one female haploid gamete form a diploid cell gamete by sharing equal amounts of genetic material from both paternal and maternal parents. If the zygote receives one X chromosome from a female and another X chromosome from male then the child will be female, i.e, XX. And when the zygote receives one X chromosome from a female and another Y chromosome from male then the child will be male.

Variations are the difference among the individuals of species or their offsprings. We are agreeable to this statement that only variations can confer an advantage to an individual organism that will survive in a population. Not all living organisms have equal chances of survival in the environment. The living organisms must change their characters or adapt their characters according to the changing environment, and those who will not adapt, they will not survive. So, the chances of survival only depend on the variations in environment.