SEBA Solutions for Class 10 Science Chapter 9 Heredity and Evolution: In-text Questions
This chapter is important for SEBA Class 10 students, as SCERT, Assam, follows NCERT textbooks, and their syllabus contains the whole chapter.
Page Number: 143
- If a trait A exists in 10% of a population of an asexually reproducing species and trait B exists in 60% of the same population, which trait is likely to have arisen earlier?
Answer: Trait B likely arose earlier because it is present in a much larger part of the population (60%). A trait that appears earlier has more time to spread through reproduction across generations. Trait A is found in only 10% of the population. So it is a newer variation that has less time to spread.
- How does the creation of variations in a species promote survival?
Answer: Genetic variations in a species enable the species to adapt better in its environmental changes. When the environment changes, individuals with variations suited to the new conditions survive and reproduce. Over time, this natural selection decides whether a species survives or dies out. So, variation is an important factor in evolution.
Page Number: 143
- How do Mendel’s experiments show that traits may be dominant or recessive?
Answer: Mendel showed that traits can either be dominant or recessive through his monohybrid cross in pea plants. He crossed a pure tall plant (TT) with a pure dwarf pea plant (tt). All the plants in the F₁ generation were tall. It shows that tallness was dominant over dwarfness. Mendel then self-pollinated the F₁ plants to get the F₂ generation. This time, both tall and dwarf plants appeared, in a ratio of 3:1 (3 tall, 1 dwarf).
Mendel concluded that the F1 tall plants were not true-breeding; instead, they carried the traits for both tall and dwarf heights. So a dominant trait expresses itself even if only one copy is present (Tt), whereas a recessive trait requires both copies to be present (tt) to show up. This shows that traits can either be dominant or recessive.
- How do Mendel’s experiments show that traits are inherited independently ?
Answer: Mendel’s experiments show that traits are inherited independently through his dihybrid cross experiment. Mendel chose seed colour and seed shape as the two characters for his experiment. In his dihybrid cross, he crossed pea plants with round yellow seeds (RRYY) and wrinkled green seeds (rryy). All the F₁ plants had round yellow seeds, showing that round shape and yellow colour are dominant traits. When the F₁ plants were self-pollinated, the F₂ generation showed a variety of seeds. In F2 progeny, the yellow round seeds, green round seeds, yellow wrinkled seeds, and green wrinkled seeds were seen in the ratio of 9:3:3:1. He further observed that the round yellow and wrinkled green were the original parent combinations, while round green and wrinkled yellow are new combinations. This showed that the gene for seed shape and the gene for seed colour don’t travel together. Each pair of genes separates on its own during gamete formation, so the two traits get inherited independently of each other.
- 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 which blood group is dominant. Each parent carries two alleles for blood group. A man with blood group A can have either AA or AO genotype. A woman with blood group O must have the OO genotype. Since their daughter has blood group O. This means she must received one O allele from each parent. This is possible if the father has the AO genotype. So, if the father is AO, the daughter will inherit O from both sides and express blood group O—regardless of whether A is dominant or O is dominant.
- How is the sex of the child determined in human beings?
Ans: In human beings, the sex of the child is determined by the father. A woman has two X chromosomes (XX). So, every egg carries only one X chromosome. A man has one X and one Y chromosome (XY). So, half of the sperm cells carry one X chromosome and the other half carry a Y chromosome. When fertilisation happens, two outcomes are possible:
- When a sperm carrying an X chromosome fertilises an egg, the child will have XX chromosomes and will be a girl.
- When a sperm carrying a Y chromosome fertilises an egg, the child will have XY chromosomes and will be a boy.
Since both types of sperm are produced in equal numbers, there is a 50% chance of having a boy and a 50% chance of having a girl.
Page Number: 150
- What are the different ways in which individuals with a particular trait may increase in a population ?
Answer: Individuals with a particular trait may increase in a population in the following ways:
- Natural selection: It is the process by which organisms with favourable traits survive and reproduce more successfully than others. Over many generations, these beneficial traits become common in the population. For example, some mosquitoes carry a trait that makes them resistant to a certain pesticide. These mosquitoes survive and reproduce, while the others die out.
- Genetic drift: It is the random change in the frequency of traits or genes in a population due to chance events. This often happens in small populations. If an accident or disaster wipes out most individuals, the few survivors pass their traits to the next generation. This can cause a particular trait to become more common.
- Why are traits acquired during the lifetime of an individual not inherited?
Answer: A child inherits traits through the germ cells of its parents. So, only the changes present in these germ cells are passed on to the next generation. Traits acquired during a person’s lifetime, such as the muscles developed by a bodybuilder through exercise, affect only the body (somatic) cells and not the germ cells. Since these changes do not occur in the DNA of the germ cells, they cannot be inherited by the offspring.
- Why are the small numbers of surviving tigers a cause of worry from the point of view of genetics?
Answer: A small population means a small gene pool. As the size of the tiger population decreases, the genetic pool of the species decreases too. Variations are essential for a species to adapt to environmental changes and survive. But with so few tigers reproducing, useful variations become rare. If a disease spreads through the tiger population, most tigers may lack the variation needed to survive it. This could wipe out the entire population, since there aren’t enough individuals with different genetic traits to resist the disease.
Chapter– 9 Page: 151
- What factors could lead to the rise of a new species?
Answer: The formation of a new species is called speciation. Important factors include:
- Genetic variations and Mutation
- Natural selection
- Genetic drift
- Geographical isolation
- Reproductive isolation.
- Differences in environmental conditions
- Will geographical isolation be a major factor in the speciation of a self-pollinating plant species? Why or why not
Answer: No, geographical isolation will not be a major factor in the speciation of a self-pollinating plant species. A self-pollinating plant can reproduce using its own pollen. No external agent is required for pollination; it is also not dependent on another plant for pollination. As a result, geographical isolation has little effect on gene flow, and it does not play a significant role in the formation of new species. However, variations may still arise due to accidental mutations, genetic drift, or natural selection over long periods.
- 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 organism that reproduces asexually. This is because asexual reproduction involves only a single parent and does not involve meiosis or gene flow between individuals. Variation occurs in these organisms due to errors during DNA copying (mutations).
In-text Questions – Page 156
- Give an example of characteristics being used to determine how close two species are in evolutionary terms.
Ans: Evolutionary relationships can be traced by looking at similar characteristics shared between organisms. If two different species share common features, it indicates they inherited them from a common ancestor. The presence of feathers is a good example. Fossils show that dinosaurs had feathers. Dinosaurs did not use these feathers to fly; instead, they used them for insulation against the cold. Later on, birds adapted these feathers for flight. This shared characteristic proves that birds are closely related to reptiles, since dinosaurs were reptiles. It also shows that the evolution of wings and flight actually started within the reptile group.
- Can the wings of a butterfly and the wings of a bat be considered homologous organs? Why or why not?
Answer The wing of a butterfly and the wing of a bat cannot be considered homologous organs as they have different basic structural designs and origins. In fact, they are analogous organs as they perform the same function of flight even though their origin and structure are different.
- What are fossils ? What do they tell us about the process of evolution ?
Answer: Fossils are the preserved traces or remains of dead organisms that lived millions of years ago. They form when a dead organism, or its hard parts, get buried in mud before they can fully decompose.
They provide following information on the process of evolution.
- Fossils provide information about extinct organisms and the time period when they lived.
- They help us to understand the evolutionary relationship between different species.
- Fossils reveal how complex organisms developed gradually from simpler ones.
Page Number: 158
- 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 ?
Ans: All human beings belong to the same species, Homo sapiens. This is because all human beings are capable of interbreeding with each other and producing offspring irrespective of colour, size, language, race, etc. The visible differences of human beings, like Skin colour, looks, and size are caused by environmental factors and minor genetic variations. Despite these external differences, our basic body design and internal cellular structure remain the same.
- In evolutionary terms, can we say which among bacteria, spiders, fish and chimpanzees have a ‘better body design’ why or why not ?
Answer: No, we cannot say that bacteria, spiders, fish, or chimpanzees have a better body design in evolutionary terms. Evolution does not produce a “better” body design; it produces body designs that are best suited to an organism’s specific environment and way of life. For example, fish have a streamlined body design essential for living in water, while a spider or chimpanzee would fail to survive there. On the other hand, bacteria have a simple body design, yet they can survive in extreme conditions.
Chapter End Questions
- A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were short. This suggests that the genetic make-up of the tall parent can be depicted as:
(a) TTWW
(b) TTww
(c) TtWW
(d) TtWw
Answer: (c) TtWW - An example of homologous organs is :
(a) our arm and a dog’s fore-leg
(b) our teeth and an elephant’s tusks
(c) potato and runners of grass
(d) all of the above
Answer: (d) All of the above - In evolutionary terms, we have more in common with :
(a) a Chinese school-boy
(b) a chimpanzee
(c) a spider
(d) a bacterium
Answer: (a) A Chinese school-boy - A study found that children with light-coloured eyes are likely to have parents with light-coloured eyes. On this basis, can we say anything about whether the light eye colour trait is dominant or recessive? Why or why not?
Answer: No, this information is not complete. On this basis, we cannot decide whether the light colour trait is dominant or recessive. To know whether the trait is dominant or recessive, we need information about the parents’ genotypes or the inheritance pattern when light-eyed and dark-eyed parents have children to see which trait overpowers the other.
5.How are the areas of study – evolution and classification— inteilinked?
Answer: Evolution and classification are closely related. Classification groups organisms on the basis of their similarities and differences. When two organisms have more characteristics in common, they are considered more closely related. This also suggests that they may have a more recent common ancestor. Therefore, classification helps us understand the evolutionary relationships among organisms. The similarities and differences seen during classification give us an idea about how organisms have evolved from their ancestors.
- Explain the terms analogous and homologous organs with examples.
Answer: Analogous organs: Organs that have different structural designs and origins but perform similar functions are called analogous organs.
Example: The wings of a bird and the wings of a bat (or insect) are analogous organs.
Homologous organs: Organs that have the same basic structure or design and origin but perform different functions are called homologous organs.
Examples: The forelimbs of a frog, a lizard, a bird, and a human are homologous organs.
- Outline a project which alms to find the dominant coat colour in dogs.
Answer: A homozygous black (BB) male dog and a homozygous white (bb) female dog are taken and mated to produce offspring in the F1 generation. If the progeny are all black dogs(Bb), then the dominant coat colour is black. For the F₂ generation, the F₁ black dogs are crossed with each other. This cross results in a phenotypic ratio of 3 Black : 1 White, which confirms that black is the dominant trait.
- Explain the importance of fossils in deciding evolutionary relationships.
Answer: The study of fossils helps us know about the species that are no longer alive. It also provides evidence about the process of evolution. By studying fossils, scientists can arrange organisms in a chronological sequence to show how they gradually evolved. Some fossils also show features of two different groups acting as connecting links between them.
For example, Archaeopteryx had feathers and wings like a bird, but it also had teeth and a long tail like reptiles. It is therefore considered a connecting link between reptiles and birds and provides evidence for their evolutionary relationship.
- What evidence do we have for the origin of life from inanimate matter?
Answer: In 1929, the British scientist J.B.S. Haldane suggested that life must have developed from the simple inorganic molecules present on the early Earth. He proposed that the conditions on the early Earth could have led to the formation of complex organic molecules needed for life.
In 1953, Stanley L. Miller and Harold C. Urey provided experimental evidence for this idea. They created an atmosphere similar to that thought to exist on the early Earth, containing methane, ammonia and hydrogen sulphide, along with water, but no oxygen. The gases were maintained at a temperature just below 100°C, and electric sparks were passed through them to simulate lightning. After about a week, about 15% of the carbon from methane had been converted into simple carbon compounds, including amino acids, which form protein molecules. This experiment showed that life originated from inanimate matter (or lifeless matter) like inorganic molecules.
- Explain how sexual reproduction gives rise to more viable variations that asexual reproduction. How does this affect the evolution of those organisms that reproduce sexually ?
Answer: Sexual reproduction produces more variations than asexual reproduction because it involves the combination of genetic material from two distinct parents. During sexual reproduction, DNA from both parents is combined, and minor errors in DNA copying can also produce additional variations. As a result, the offspring are genetically different from their parents and from one another.
These variations are crucial for evolution because they drastically increase the chances of survival of organisms in changing environmental conditions. Favourable variations help individuals adapt better, allowing them to be passed to the next generation and accumulate over time. Thus, sexual reproduction provides more opportunities for evolution than asexual reproduction.
- How is the equal genetic contribution of male and female parents ensured in the progeny?
Answer: The equal genetic contribution of male and female parents is ensured in the progeny through the inheritance of equal numbers of chromosomes from both parents. In humans, there are 23 pairs of chromosomes. Of these, 22 pairs are called autosomes, while the remaining pair is known as sex chromosomes, represented as X and Y.
During gamete formation, a male gamete contains 22 autosomes and either an X or a Y chromosome, while a female gamete contains 22 autosomes and one X chromosome. During fertilisation, a male gamete fuses with a female gamete to form a zygote. The zygote receives exactly half of its genetic material from the male parent and half from the female parent. In this way, both parents make an equal genetic contribution to the progeny.
- Only variations that confer an advantage to an individual organism will survive in a population. Do you agree with this statement? Why or why not?
Answer: No, I do not agree with the statement that only advantageous variations will survive in a population. Natural selection favours variations that give an organism a survival advantage. Such organisms are more likely to survive, reproduce and pass these favourable variations to their offspring. However, some variations may not provide any survival advantage but can still remain in a population. In small populations, such changes in the frequency of genes may occur by chance. This is called genetic drift. Therefore, both advantageous and neutral variations can survive in a population.