LAKHMIR SINGH AND MANJIT KAUR Solutions for Class 10 Biology Chapter 4 - Heredity and Evolution

Sexual Reproduction

Free Earlobes.

Gametes.

B.

Genes.

Tt.

Sex Chromosomes.

Sperm.

False.

Baby boy

XY.

Gregor Mendel

Pea plants.

Black hair.

A, B, AB and O.

(a) Lizard (Agama agama)
(b) Turtle (Chrysema Picta)

(a) Pairs.

(b) Recessive; Dominant.

(c) Free; attached.

(d) 23; 46.

(e) Four.

(f) Sex; Female; male.

(a) Tall - Tt will have tall plants because of the presence of T which is dominant gene and t is recessive gene.

(b) Dwarf plants: It is dwarf due to the presence of both the recessive genes.

(c) Tall plants: These plants are tall due to the presence of both the dominant genes.

Equal chance of having blood group O or blood group B.

(a) Gregor Mendel

(b) Snails: This indicates that sex is not determined genetically in some animals.

The characteristics or traits in animals and plants are controlled by genes. For example the transmission of color of hair from the parents to the child. If a mother has black hair and the father has blonde hair and the child has black hair then the transmission of genes for hair colour from the mother and father to the child is as follows: Mother's cell contains two genes HH for black hair. Both the genes HH are dominant genes, so the mother has black hair. Father's cell contains two genes (hh) for blonde hair. The two genes hh are recessive genes, so the father has blonde hair. Now, during the process of reproduction, the mother transmits one of the dominant genes H for black hair to the child and the father transmits one of his recessive genes h for blonde hair to the child. Due to this, the child has the genes Hh for her hair. Now the gene H for black hair is the dominant gene but the gene h for blonde hair is the recessive gene. The dominant gene H for black hair shows its effect due to which the child has black hair.

The sex of a child depends on what happens at fertilisation:
(a) If a sperm carrying X chromosome fertilises an ovum (or egg) which carries X chromosome, then the child born will be a girl. This is because the child will have XX combination of sex chromosomes.
(b) If a sperm carrying Y chromosome fertilises an ovum (or egg) which carries X chromosome, then the child born will be a boy. This is because the child will have XY combination of sex chromosomes.

(a) Tall plant.
(b) Dwarf plant.
(c) Female.
(d) Male.

(a) In the F₁ generation, all plants produced will be tall and in F₂ generation three tall plants and one dwarf plant will be produced.

(b) It is an example of monohybrid cross.

It is a monohybrid cross. Example - when two hybrids tall Pea plants cross bred with each other, they will produce three tall plants and one dwarf plant in F₂ generation.

(a) tt.

(b) TT.

(c) (i) tt (ii) TT.

(a) (i)23.

(ii) 46.

(b) Three tall plants and one dwarf plant.

(a) 46.

(b) 23.

(c) 23.

(d) 46.

(e) 46.

(a) Genes.
(b) Chromosomes.

No, because mother has a pair of X chromosomes. All new born babies will inherit an X chromosome from mother whether they are baby boys or baby girls.

(a) Green seed: Yellow is dominant: Green is recessive.

(b) Wrinkled seed: round is dominant: wrinkled is recessive.

(a) The transmission of characters from parents to the offspring's is called heredity. The units of heredity are genes.                
(b) According to Mendel's first law of inheritance:  The characteristics (or traits) of an organism are determined by internal 'factors' which occur in pairs. Only one of a pair of such factors can be present in a single gamete.

(a) Mendel choose pea plants for studying inheritance because pea plants had a number of clear cut differences which were easy to tell apart. Another reason for choosing pea plants are they were self pollinating and many generations can be produced in a short time span.

(b) According to Mendel's second law of inheritance: In the inheritance of more than one pair of traits in a cross simultaneously, the factors responsible for each pair of traits are distributed independently to the gametes.

(a) The differences in the characters among the individual of a species is called variation.

(b) Human height and free ear lobe are the traits which show variation in humans.

(c) Due to the creation of variations, a species can adjust to the changing environment around it. And this promotes the survival of the species in the changing environment.

Example: The accumulation of 'heat resistant' variation (or trait) in some bacteria will ensure its survival even when the temperature in its environment rises too much due to a heat wave or some other reasons. On the other hand, the bacteria which did not have this variation to withstand heat would not survive under these circumstances and die.

(a) Genes are the units of heredity which transfer characteristic from parents to their offspring's during reproduction. Genes are located on the chromosomes.

(b) The gene which decides the appearance of an organism even in the presence of an alternative gene is known as dominant gene. The gene which can decide the appearance of an organism only in the presence of another identical gene is called a recessive gene. The dominant gene is represented by a capital letter and the corresponding recessive gene is represented by the corresponding small letter. For example, in pea plants, the dominant gene for tallness is T and the recessive gene for dwarfness is t.

(c) There is a pair of genes for each characteristic of an organism, one is dominant gene and the other is recessive gene. Each parent passes only one of the two genes of the pair for each characteristic to its progeny through gametes. Thus, the male gamete and the female gamete carry one gene for each characteristic from the gene pairs of the parents. When a male gamete fuses with a female gamete during fertilisation they make a new cell called zygote which grows and develops to form a new organism having characteristics from both the parents which it has inherited through genes.

(a) Mendel first crossed pure bred tall pea plants with pure bred dwarf pea plants and found that only tall pea plants were produced in the first generation. No dwarf pea plants were obtained in F1 generation. Mendel concluded that the first generation showed the traits of only one of the parent plants; tallness. The trait of other parent plant, dwarfness, did not show up in the progeny of first generation.

Mendel then crossed the tall pea plants of the first generation (F₁ Generation) and found that tall plants and dwarf plants were obtained in the second generation (or F₂ generation) in the ratio of 3:1. Mendel noted that the dwarf trait of the parent pea plant which had seemingly disappeared in the first generation progeny reappeared in the second generation. In this way, Mendel's experiments with tall and dwarf pea plants showed that the traits may be dominant and recessive.





(b) When Mendel crossed pure-bred tall pea plants with pure-bred dwarf pea plants, he found that only tall pea plants were produced in the F₁ generation. When Mendel further crossed the tall pea plants of the F₁ generation, he found that the tall plants and dwarf plants were obtained in the ratio 3:1 in the F₂ generation. Mendel noted that all the pea plants produced in the F₂ generation were either tall or dwarf. There were no plants with intermediate height (or medium height) in-between the tall and dwarf plants. In this way, Mendel's experiment showed that the traits (like tallness and dwarfness) are inherited independently. This is because if the traits of tallness and dwarfness had blended (or mixed up), then medium sized pea plants would have been produced.

Evolution.

Charles Robert Darwin.

False.

The presence of feathers on birds and dinosaurs shows that they are closely related to each other.

Planaria.

Wild Cabbage.

Sea water.

(i) - c.

(ii) - b.

(iii) - e.

(iv) - a.

(v) - d.

A trait of an organism which is 'not inherited' but develops in response to the environment is called an acquired trait.

Example - If a beetle does not get sufficient food for a considerable time, its weight will be reduced due to starvation. The low weight of the beetle is an example of acquired trait.

A trait of an organism which is caused by a change in its genes (DNA) is called an inherited trait. Example - The change of colour from red beetle to green beetle is an example of inherited trait.

For a trait of an organism to be inherited, it should bring about a change in the genes present in the reproductive cells or gametes of that organism. The traits acquired during the life time of a person do not bring about a change in the genes present in its reproductive cells or gametes and hence they are not inherited by the offsprings.

The wings of a butterfly and the wings of a bat cannot be considered homologous because though the function of wings in both the cases is same but they have different basic design. The butterfly has a fold of membranes as wings which are associated with few muscles but has no bones whereas the wings of bat are supported by bones.

Forelimb of humans and forelimb of lizard are the homologous organs and the wing of insect and the wing of bird are analogous organs.

The remains of dead animals or plants that lived in the remote past are known as fossils. The fossils provide evidence of evolution. For example, a fossil bird called Archaeopteryx looks like a bird but it has many features which are found in reptiles. This is because Archaeopteryx has feathered wings like those of birds but teeth and tail like those of reptiles. Therefore, Archaeopteryx is a connecting link between the reptiles and birds and suggests that birds have evolved from reptiles. Thus, fossils provide evidence that the present plants and animals have originated from the previously existing ones through the process of continuous evolution.

The changes in DNA during reproduction are mainly responsible for evolution. The changes which take place in the DNA of species go on accumulating from one generation to the next. So, if the changes in the DNA of any two species are less, then the two species are quite close to one another in evolutionary terms. But if the changes in the DNA of two species are much more, then the two species will be far apart from one another in evolutionary terms. Thus, it is the characteristic of the extent of change in the DNA which is being used to determine how close two species are in evolutionary terms.

The presence of homologous organs in different animals provides evidence for evolution by telling us that they are derived from the same ancestor who had the 'basic design' of the organ on which all the homologous organs are based. 

Sometimes a species may completely die out. It may become extinct. Once a species is extinct, its genes are lost forever, it cannot reemerge at all. The small numbers of surviving tigers are a cause of worry because if they all die out and become extinct, their genes will be lost forever. Our coming generations will not be able to see Tigers at all.

Geographical isolation cannot be a major factor in the speciation of an asexually reproducing organism because it does not require any other organism to carry out reproduction.

Human evolution has been studied by using the various tools of tracing evolutionary relationships like excavating (digging earth), carbon-dating, studying fossils and determining DNA sequences.

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

Some amount of variations are produced even during asexual reproduction but it is very small. The number of variations produced during sexual reproduction is, however, very large. It is due to these variations that no two human beings look alike. The number of successful variations is maximized by the process of sexual reproduction, so we can conclude that the variation is a necessity for organic evolution. Example: Animal reproduced by sexual reproduction show large number of variations.

(a) A species is a population of organisms consisting of similar individuals which can breed together and produce fertile offspring's.

Plant species - Wheat, Paddy, Sunflower etc.

Animal species - Cat, Dog, Tiger, etc.

(b) The important factors which could lead to the rise (or formation) of a new species are the following:

(i) Geographical isolation of a population caused by various types of barriers (such as mountain ranges, rivers and sea). The geographical isolation leads to reproductive isolation due to which there is no flow of genes between separated groups of population.

(ii) Genetic drift caused by drastic changes in the frequencies of particular genes by chance alone.

(iii) Variations caused in individuals due to natural selection.

The evidence was given by Stanley L. Miller and Harold C. Urey in 1953. They assembled an apparatus to create an early earth atmosphere which was supposed to consist of gases like ammonia, methane and hydrogen sulphide, but no oxygen) over water. This was maintained by them at a temperature just below 100^o C and electric sparks were passed through the mixture of gases to stimulate lightning. At the end of a week, they found that 15% of the carbon (from methane) had been converted to simple compounds of carbon including amino acids which make up protein molecules.

Geographical isolation is the major factor in the speciation of sexually reproducing animals because it interrupts the flow of genes between their isolated populations through the gametes.

Methane, Ammonia and Hydrogen Sulphide.

The Origin of Species.

Homologous Organs.

Dinosaurs and Archaeopteryx.

(a) Wings of bird and wings of bat.

(b) Wings of birds and wings of insects.

The human beings who look so different from each other in terms of size, color and looks are set to belong to the same species because they can inter breed to produce fertile offsprings.

Cabbage, Broccoli, Cauliflower, Kohlrabi and Kale.

Wild cabbage.

(a) Homologous: Analogous.

(b) Stages.

(c) Homo sapiens.

(d) Wild Cabbage.

(e) Darwin.

(a) The theory of origin of life on earth was given by J.B.S Haldane. He suggested in 1929 that life must have developed from the simple inorganic molecules (such as methane, ammonia, hydrogen sulphide, etc.) which were present on the earth soon after it was formed. He said that the conditions on earth at that time (including frequent lightning) could have converted simple inorganic molecules into complex organic molecules which were necessary for life. These complex organic molecules must have joined together to form first primitive living organisms. Haldane also suggested from theoretical considerations that life (or living organisms) originated in the sea water.

(b) Those species which are now extinct are studied by studying their fossils which are found during the digging of earth.

Evolution is the sequence of gradual changes that takes place in the primitive organisms over millions of year in which new species are produced.
Darwin's theory of evolution is known as 'The Theory of Natural Selection'. It can be described as follows:
(i) Within any population there is natural variation. Some individuals have more favourable variations than others.
(ii) Even though all species produce a large number of offspring's, populations remain fairly constant naturally.
(iii) This is due to the struggle between members of the same species and different species for food, space and mate.
(iv) The struggle for survival within populations eliminates the unfit individuals. The fit individuals possessing favourable variations survive and reproduce. This is called natural selection.
(v) The individuals having favourable variations pass on these variations to their progeny from generation to generation.
(vi) These variations when accumulated over a long period of time, lead to the origin of a new species.

(a) Analogous Organs: Organs which performs similar function but are different in structure and origin.

Example - wings of a bird and wings of an insect.

Homologous Organ: Organs which have different functions but similar structure and origin.

Example - fore arm of frog, lizard, bird and human.

(b) The presence of analogous organs indicates that even the organisms having organs with different structures can adapt to perform similar functions for their survival under hostile environmental conditions. Thus, the presence of analogous organs in different animals provide evidence for evolution by telling us that though they are not derived from common ancestors, they can still evolve to perform similar functions to survive, flourish and keep on evolving in the prevailing environment.

(a) Natural selection is the process of evolution of a species whereby characteristics which help individual organisms to survive and reproduce are passed on to their offspring, and those characteristics which do not help are not passed on.

(b) Yes, only those variations that confer advantage to an individual organism will survive in a population. This will become clear from the following example. Suppose there is a population of red beetles in the green bushes and a colour variation arises during reproduction so that one beetle is now green in colour (instead of red). This variation offers advantage of survival because the green beetle can mix up with green bushes, it cannot be spotted and eaten up by a crow and hence its population will increase. If, however, the variation had produced a blue coloured beetle, then this colour could not offer any survival advantage because blue beetle in green bushes could be easily spotted by a crow and eaten by it.

(a) The process by which new species develop from the existing species is known as speciation. New species are formed when the population of same species splits into two separate groups which then get isolated from each other geographically by the barriers such as mountain ranges, rivers or the sea.

The geographical isolation of the two groups of population leads to their reproductive isolation due to which no genes are exchanged between them. However, breeding continues within the isolated populations producing more and more generations. Over the generations, the processes of genetic drift (random change in gene frequency), and natural selection operate in different ways in the two isolated groups of population and make them more and more different from each other. After thousands of years, the individuals of these isolated groups of population become so different that they will be incapable of reproducing with each other even if they happen to meet again. We then say that two new species have been formed.

(b) Geographical isolation will not be a major factor in the speciation of a self pollinating plant because it does not depend on other plants for its process of reproduction to be carried out.

Bacteria have simpler body plan when compared with human beings. Both of them have evolved differently. Bacteria can inhabit most of the unfavourable habitats such as hot springs, deep- sea thermal vents and the ice in Antarctica.