Class 10 LAKHMIR SINGH AND MANJIT KAUR Solutions Biology Chapter 4 - Heredity and Evolution
Heredity and Evolution Exercise 191
(b) Turtle (Chrysema Picta)
(b) Recessive; Dominant.
(c) Free; attached.
(d) 23; 46.
(f) Sex; Female; male.
Heredity and Evolution Exercise 192
(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.
(b) Snails: This indicates that sex is not determined genetically in some animals.
(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.
(b) Dwarf plant.
(b) It is an example of monohybrid cross.
(c) (i) tt (ii) TT.
(b) Three tall plants and one dwarf plant.
(b) Wrinkled seed: round is dominant: wrinkled is recessive.
(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.
Heredity and Evolution Exercise 193
(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.
(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.
(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.
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.
Heredity and Evolution Exercise 208
Heredity and Evolution Exercise 209
(ii) - b.
(iii) - e.
(iv) - a.
(v) - d.
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.
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.
(b) Wings of birds and wings of insects.
(c) Homo sapiens.
(d) Wild Cabbage.
Heredity and Evolution Exercise 210
(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.
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.
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.
(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.
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.