CLASS X BIOLOGY

#HEREDITY#EVOLUTION

HEREDITY AND EVOLUTION

NCERT TEXTBOOK SOLUTION:

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?

Answer. The trait B which exists in 60% of the population is likely to have arisen earlier. This is because the traits (or variations) produced in an organism during successive generations get accumulated in the populations of the species.

Q2. How does the creation of variations in a species promote its survival?

Answer. 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. For 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 heatwave 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.

Q3. How do Mendel’s experiments show that traits may be dominant or recessive?

Answer. 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 or F1 generation (see Figure 1). No dwarf pea plants (or short pea plants) were obtained in the first generation of progeny.

From this Mendel concluded that the first generation (or F1 cross) showed the traits of only one of the parent plants: tallness. The trait of other parent plants, 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. In other words, in the F2 generation, three-fourth plants were tall and one-fourth were dwarf (see Figure 2). 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. Mendel said that the trait of dwarfness of one of the parent pea plant had not been lost, it was merely concealed or suppressed in the first generation to re-emerge in the second generation. Mendel called the repressed trait of ‘dwarfness’ as ‘recessive trait’ and the expressed trait of ‘tallness’ as the ‘dominant trait’. In this way, Mendel’s experiments with tall and dwarf pea plants showed that the traits may be dominant or recessive.

Q4. How do Mendel’s experiments show that traits are inherited independently?

Answer. 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. Now, when Mendel further crossed the tall pea plants of the F1 generation, he found that 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 was 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 of the traits of tallness and dwarfness had blended (or mixed up), then medium-sized pea plants would have been produced.

Q5. A man with blood group A marries a woman with blood group 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?

Answer. No, this information is not enough to tell us which of the traits, blood group A or blood group O is dominant.

This is because:

(i) if the blood group A is a dominant trait and blood group O is a recessive trait, the daughter can have blood group O, and

(ii) even if the blood group A is a recessive trait but blood group O is the dominant trait, the daughter can still have blood group O.

Let us discuss these two possibilities in detail.

Possibility 1: When blood group A is the dominant trait but blood group O is a recessive trait

When father’s blood group A is a dominant trait, it can have two genotypes: IAIA and IAIO. And when mother’s blood group O is recessive trait it can have only one genotype: IOIO (because it should have two recessive alleles). Now, if one recessive allele IO comes from father and one recessive allele IO comes from mother, then the daughter can also have the genotype IOIO which can give her blood group O.

Possibility 2: When blood group A is a recessive trait but blood group O is the dominant trait

When father’s blood group A is a recessive trait, it can have only one genotype: IAIA (because it should have two recessive alleles). And when mother’s blood group O is dominant trait, then it can have two genotypes: IOIO and IOIA. Now, if one dominant allele IO comes from the mother and one recessive allele IA comes from the father, the daughter will have the genotype IOIA which will again give her blood group O.
#trait#Individual

Q6. How is the sex of the child determined in human beings?

Answer. Genetics is involved in the determination of the sex of a child. This can be explained as follows: The chromosomes which determine the sex of a child are called sex chromosomes. There are two types of sex chromosomes, one is called the X chromosome and the other is called the Y chromosome.

(i) A male (man or father) has one X chromosome and one Y chromosome. This means that half the male gametes or half the sperms will have X chromosomes and the other half will have Y chromosomes.

(ii) A female (woman or mother) has two X chromosomes (but no Y chromosomes). This means that all the female gametes called ova (or eggs) will have the only X chromosome

The sex of a child depends on what happens at fertilisation:

(a) If a sperm carrying an X chromosome fertilises an ovum (or egg) which carries an X chromosome, then the child born will be a girl (or female). This is because the child will have an XX combination of sex chromosomes.

(b) If a sperm carrying Y chromosome fertilises an ovum (or egg) which carries an X chromosome, then the child born will be a boy (or male). This is because the child will have an XY combination of sex chromosomes.

Q7. What are the different ways in which individuals with a particular trait may increase in a population?

Answer. The various ways in which individuals with a particular trait may increase in a population are:

(i) By the process of natural selection in which the characteristics that help the individual organisms to survive and reproduce are passed on to their offsprings, and those characteristics which do not help are not passed on. (ii) By the process of genetic drift caused by drastic changes in the frequencies of particular genes by chance alone.

Q8. Why are the traits acquired during the lifetime of an individual not inherited?

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

Q9. Why are the small number of surviving tigers a cause of worry from the point of view of genetics?

Answer. Sometimes a species (a type of animal or plant) may completely die out. It may become extinct. Once a species is extinct, its genes are lost forever. It cannot re-emerge at all. The small numbers of surviving tigers are a cause of worry from the point of view of genetics 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.

Q10. What factors could lead to the rise of a new species?

Answer. 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.

Q11 Will geographical isolation be a major factor in the speciation of a self-pollinating plant species? Why or why not?

Answer. 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.

Q12. Will geographical isolation be a major factor in the speciation of an organism that reproduces asexually? Why or why not?

Answer. 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.

Q13. Give an example of the characteristics being used to determine how close two species are in evolutionary terms.

Answer. 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.

Q14. Can the wings of a butterfly and the wings of a bat be considered homologous organs? Why or why not?

Answer. The wings of a butterfly and the wings of a bat cannot be considered homologous organs because though the function of wings in both cases is the same (flying) but they have different basic design. The butterfly (which is an insect) has a fold of membranes as wings which are associated with a few muscles but no bones are present. On the other hand, a skeleton made of bones supports the bat’s wings.

Q15. What are fossils? What do they tell us about the process of evolution?

Answer. The remains (or impressions) of dead animals or plants that lived in the remote past are known as fossils. The fossils provide evidence for evolution. For example, a fossil bird called Archaeopteryx looks like a bird but it has many other 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. Archaeopteryx is, therefore, a connecting link between the reptiles and birds, and hence suggests that the birds have evolved from the reptiles. Thus, fossils provide evidence that the present animals (and plants) have originated from the previously existing ones through the process of continuous evolution.

Q16. 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?

Answer. The human beings who look so different from each other in terms of size, colour and looks are said to belong to the same species (Homo sapiens) because they can interbreed to produce fertile offsprings (sons and daughters).

Q17. In evolutionary terms can we say which among bacteria, spiders, fish and chimpanzees have a ‘better’ body design? Why or why not?

Answer. In evolutionary terms, we can say that bacteria have ‘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.

#Evolution#Offsprings

IMPORTANT NOTES

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