Page 143
1. 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?
Trait B most likely appeared earlier and has spread to 60% of the population, while trait A seems to be newer and has spread to only 10% of the population.
2. How does the creation of variations in a species promote survival?
Variations provide stability to a species' population by helping prevent total extinction during unfavorable conditions. Since the natural environment is always changing, those variations that suit the new conditions enable the species to survive.
Page 147
1. How do Mendel’s experiments show that traits may be dominant or recessive?
Mendel crossed pure tall and pure dwarf pea plants. In the first generation (F1), all the plants were tall. When these F1 plants were self-pollinated, the second generation (F2) produced both tall and dwarf plants in a 3:1 ratio. From these observations, he concluded that the traits that appear in the F1 generation are dominant (tall), while those that do not appear are recessive (dwarf).
2. How do Mendel’s experiments show that traits are inherited independently?
Mendel performed dihybrid crosses using two pairs of contrasting traits. The traits that appeared in the F1 generation were considered dominant. When the F1 plants were self-pollinated to produce the F2 generation, plants with various combinations of traits were observed. Some plants showed both dominant traits, some both recessive, and others a mix. This demonstrated that traits are inherited independently of one another.
3. 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?
No, the given information is not sufficient. Every individual carries two alleles. A recessive trait is expressed only when both alleles are recessive. If blood group A is dominant and O is recessive, the daughter can have blood group O only if she inherits one O allele from each parent. This means the father must have one O and one A allele, and the mother must have two O alleles.
4. How is the sex of the child determined in human beings?
In human beings, half of the male gametes (sperms) carry an X chromosome, while the other half carry a Y chromosome. All female gametes (eggs) carry only X chromosomes. When fertilization occurs:
- If a sperm with an X chromosome fertilizes the egg (X), the zygote will be XX and develop into a female.
- If a sperm with a Y chromosome fertilizes the egg (X), the zygote will be XY and develop into a male.
Since either type of sperm can fertilize the egg, there is an equal (50-50) chance of having a male or female child. Therefore, neither parent can be held responsible for determining the child’s sex.
Page 150
1. What are the different ways in which individuals with a particular trait may increase in a population?
Individuals with a particular trait may increase in a population in the following ways:
- If the trait provides a survival advantage through natural selection.
- If there is a sudden increase in the frequency of the trait due to genetic drift.
2. Why are traits acquired during the life-time of an individual not inherited?
Traits acquired during an individual's lifetime involve changes in non-reproductive (somatic) cells, which are not involved in the process of reproduction. Therefore, such traits cannot be passed on to the next generation.
3. Why are the small numbers of surviving tigers a cause of worry from the point of view of genetics?
A small population of tigers poses a genetic concern because it results in very limited genetic variation. This lack of diversity reduces their ability to adapt to rapid environmental changes, which could threaten their survival and potentially lead to extinction.
Page 151
1. What factors would lead to the rise of a new species?
Several factors can lead to the emergence of a new species:
- Geographical isolation of a population due to barriers like mountains, rivers, or seas. This results in reproductive isolation, preventing gene flow between the separated populations.
- Genetic drift, which involves random changes in gene frequencies by chance.
- Variations that occur in individuals through the process of natural selection.
2. Will geographical isolation be a major factor in the speciation of a self-pollinating plant species? Why or why not?
No, geographical isolation is not a major factor in the speciation of a self-pollinating plant species because such plants do not rely on external agents or other individuals for reproduction.
3. Will geographical isolation be a major factor in the speciation of an organism that reproduces asexually? Why or why not?
No, geographical isolation does not significantly affect the speciation of an asexually reproducing organism since reproduction occurs without the involvement of another organism.
Page 156
1. Give an example of characteristics being used to determine how close two species are in evolutionary terms.
When different organisms share similar characteristics, it indicates that they have inherited them from a common ancestor. This similarity helps determine how closely related the species are. For example, bats and birds both have wings, which suggests they are more closely related to each other than to organisms like lizards or squirrels, which lack wings.
2. Can the wing of a butterfly and the wing of a bat be considered homologous organs? Why or why not?
No, the wings of a butterfly and the wings of a bat are not homologous organs. Although both are used for flying, they differ in their basic structural design. Therefore, they are classified as analogous organs.
3. What are fossils? What do they tell us about the process of evolution?
Fossils are the preserved remains or traces of dead organisms, usually found in sedimentary rocks. They provide important information about the process of evolution, such as:
- The changes that have occurred on Earth and the organisms that lived during those times.
- The gradual development of complex organisms from simpler ones.
- Evidence that birds evolved from reptiles.
- The origin of angiosperms from pteridophytes and gymnosperms.
- Insights into the evolutionary journey of human beings.
Page 158
1. 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?
Despite physical differences such as size, skin colour, or appearance, all human beings belong to the same species because there is no reproductive isolation between them. They can interbreed and produce fertile offspring, which is the defining feature of a species.
2. In evolutionary terms, can we say which among bacteria, spiders, fish and chimpanzees have a ‘better’ body design? Why or why not?
No, we cannot say that one has a "better" body design than the others. Bacteria, being among the earliest organisms to appear, have survived for millions of years by adapting to changing environments. The same applies to spiders, fish, and chimpanzees. Each organism has a body design that is best suited to its environment, allowing it to survive and thrive. Therefore, all existing organisms have body designs that are effective in their respective ecological niches.