NCERT Solutions Class 10 Science Chapter 8 Heredity and Evolution

Trait B is more probable to arise early as this trait has already been existing and replicating in a larger percentage of the population as compared to trait A.

 Genetic variations enable the species to better adapt to changes in its environment. Moreover, it is an important force in evolution as it allows the frequency of alleles to increase or decrease through natural selection. These variations will determine the difference between extinction or continuation of the species.

Mendel showed that the traits could either be dominant or recessive through his experiments that focused on the mono-hybrid cross. The experiment involved him crossing tall (TT) pea plants with dwarf (tt) pea plants. The resultant plants which formed after fertilisation represented the F1 (or filial) generation. All the F1 plants were tall. Mendel then proceeded to self-pollinate the filial generation plants, and the result was that 1/4th of the plants obtained in the F2 generation were dwarfs. From this experiment, Mendel concluded that the F1 tall plants were not true-breeding; instead, they carried the traits for both tall and dwarf heights. A portion of the plants was tall due to the fact that the traits for tallness were dominant over the traits for dwarfness. This cements the notion that traits can either be dominant or recessive.

The wing of a butterfly and the wing of a bat cannot be considered homologous organs as they do not share a common ancestor. Even though both structures aid in flying, they have evolved separately. To prove this, the wings of a butterfly are composed of two chitinous membranes, whereas the wings of a bat are composed of a bony skeleton, complete with blood vessels. Hence, these aren’t homologous organs but rather analogous organs.

Mendel’s experiments show that traits are inherited independently through his dihybrid cross experiment. The experiment involved him using two traits – namely, seed shape and seed colour. The colour yellow (YY) is dominant over green (yy), while the round shape (RR) is dominant over the wrinkled shape (rr). The F2 progeny of the dihybrid cross resulted in a phenotypic ratio of 9:3:3:1; therefore, 9 plants with round yellow (RRYY) seeds, 3 plants with round green (RRyy) seeds and 3 plants with wrinkled yellow (rrYY) seeds and one with wrinkled green seeds (rryy). He further observed that the wrinkled greens and the round yellow are parental combinations while the round green and wrinkled yellow are new. A dihybrid cross between two seeds with dominant traits (RRYY) and non-dominant traits (rryy) resulted in the production of 4 types of gametes (RY, Ry, rY and ry). This means each of the gametes segregates independently of the other, and each with a frequency of 25% of the total gametes produced.

Given information is not enough to tell us which characteristics are dominant – blood group A or O. Blood type A is always dominant in ABO blood, and blood type O is always recessive. Here, the father’s blood group may be genotypically AA (homozygous) or AO (heterozygous), whereas that of the mother can be OA or OO.

 The sex of the child in humans is determined by the males. Males have XY chromosomes, while females have XX chromosomes. Hence, if:

• The male’s X chromosomes combine with the female’s X chromosomes, the mother gives birth to a girl

• The male’s Y chromosome combines with the female’s X chromosome, the mother gives birth to a boy

An individual attribute could increase a population in the following 2 ways:

(a) Natural selection: If an attribute is useful to a population, it’ll increase naturally.

For example, mosquitoes which are resilient against a particular pesticide will pass on their genes so that future generations become resistant as well. The mosquitoes which are affected by the pesticide die out.

(b) Genetic drift: If a species faces a catastrophic event where most of the population is wiped out, the surviving population can pass on their traits to the following generations. This may result in a rise in the attribute within the population.

Traits acquired during a lifetime cannot be inherited by successive generations as the changes do not reflect in the DNA of the germ cells. For instance, a football player cannot pass on his skills to his offspring as they are limited to non-reproductive cells only.

 As the size of the tiger population decreases, the genetic pool of the species decreases too. This results in a limitation on the variations which will be introduced within the genetic makeup of the tigers. This lack of variation will result in serious implications. For example, if an illness spreads within the tiger population, it can potentially wipe out the whole population, possibly causing their extinction.

Factors that would result in a new species are as follows:

(a) Mutation

(b) Genetic drift

(c) Natural selection

(d) Geographical isolation

(e) Generative isolation for prolonged periods

(f) Environmental factors on the isolated populations

(g) Quantum of genetic variant transmissible from one generation to the following generation

 In the pollination of plant species, geographical isolation is usually not a major factor, as no new trait will become part of the genetic makeup of a self-pollinating plant species. However, there are some possibilities of some environmental changes which could result in some variations.

In the case of asexually reproducing organisms, geographical isolation can’t be considered a factor. This is due to the fact that meiosis does not occur during asexual modes of reproduction.

Let us take the instance of humans and chimpanzees. Chimpanzees are able to express a wide range of emotions, such as busting out in laughter or smiling – this trait was once thought to be a feature exclusive to humans. The smile can be linked to the activation of the brain’s limbic system, where the orbicularis oculi muscle involuntarily contracts and raises the cheeks, forming wrinkles around the eyes. This implies that the smile is a true and genuine smile. Interestingly, this type of reflex has a name – the Duchenne smile. Moreover, research has shown that chimpanzees share 98.6% of our DNA – This means that humans and chimpanzees shared a common ancestor aeons ago. It is also important to note that chimpanzees are the closest living relatives to humans.

Fossils are the preserved remains of animals or plants, or other organisms that died out millions of years ago. These fossils tell us about a lot of extinct animals and also give insights into how evolution might have occurred. Fossils can be used to understand how an organism would have lived and what it may have looked like. More importantly, we can correlate with fossils as well as extant organisms to understand their relationships. For instance, scientists were able to recover protein sequences from a dinosaur called the T-rex, which confirmed its avian lineage. This means birds are the extant relatives of (avian) dinosaurs. Moreover, the pattern of fossil distribution gives us an idea of the time in history when various species were formed or become extinct.

While human beings do vary in colour and general appearance, their genetic makeup is identical to any other human. One of the speculations put forth for our drastic changes is due to evolutionary pressure, where the need to be easily recognised pushed us towards having widely different faces.

Body designs are the result of environmental needs and pressure. Hence, we can’t conclude that one organism has a better body compared to another. For instance, fish have evolved a streamlined design as it is best suited for an aquatic environment. On the other hand, a spider or a chimpanzee might be ill-equipped to survive in such aquatic environments.

Correct answer – (c)

TtWW might be the genetic makeup of the tall parent. Since half the progenies are short, this implies that the parent plant also will have a collection of short genes; all progenies bore violet flowers, further suggesting that violet colour is dominant over white.

Correct answer – (d)

Homologous organs have the same origin as each of the above organs but different functions. Homologous organs can be defined as the organs of various animals having similar basic structures but different functions. For example, a whale’s flippers, a frog’s forelimbs, and a man have the same basic structures but perform different functions, which is why they are called homologous organs.

Correct answer – (a)

Humans and chimpanzees are related since they belong to the identical order (Primates) and the same family (Hominidae). However, a schoolboy, regardless of ethnicity, is still a Homo sapien.

Knowledge of at least 3 generations is required to find if an attribute is dominant or recessive. Hence, it is not possible to identify if the given trait is dominant or recessive.

lassification and evolution are two related fields of biology. Evolution pertains to how organisms evolve, and classification deals with finding out how two species are related to each other. For example, evolution and fossil evidence point to the fact that Australopithecus afarensis is considered one of our earliest ancestors. And classification tells us that Australopithecus afarensis belongs to the genus Homo, which is also the same genus as modern humans.

Homologous organs are those organs that have the basic structural design as well as the origin; however, they serve different functions. For example, the forelimbs of humans and the wings of bats are anatomically similar.

Analogous organs are those organs that have a different structural design as well as origin; however, they perform similar functions. For example, the wings of birds and insects.

Dogs have a certain set of genes that govern coat colour. There are a minimum of eleven known sequence series (A, B, C, D, E, F, G, M, P, S, T) that influence the colour of a dog. A dog inherits one copy from each of its parents. As an example, within the B series, a dog is genetically black or brown. Assume that one parent is homozygous black (BB), whereas the other parent is homozygous brown (bb).

In this case, all the offspring are going to be heterozygous (Bb).

Since black (B) is dominant, all the offspring are going to be black. However, they are going to have each B and b alleles. If such heterozygous pups are crossed, they are going to produce 25 homozygous blacks (BB), 15 heterozygous black (Bb), and 25 homozygous brown (bb) offspring.

Fossils give evidence about:

(a) The organism and their paleobiology

(b) Even the behaviour of an organism can be deduced to some extent (for example, palaeontologists) have unearthed a site with more than 10,000 skeletons of a dinosaur called Hadrosaurus. This implies that the dinosaur lived in herds.

(c) Fossils also provide insight into the evolutionary history of animals and plants (for instance, palaeontologists have discovered that whales had evolved from goat-sized land-dwelling animals called Pakicetus).

The evidence on the origin of life from inanimate matter was provided by Stanley L. Miller and Harold C. Urey’s experiment, which was conducted in 1953. They created an artificial environment which was reminiscent of the early earth’s atmosphere – it contained ammonia, hydrogen and other gases which were thought to have existed during primordial earth.

This concoction of gases was kept at a temperature slightly below 100 ° C. Additionally, sparks were generated to simulate lightning, which was also thought to be common during that period. At the end of the experiment, he was able to create 11 out of the 20 amino acids which are required for life.

Sexual reproduction causes a lot of viable variations because of the following reasons:

(a) Error in copying of DNA (though it was rare)

(b) Random segregation of paternal and maternal chromosomes at the time of sex cell formation.

(c) Exchange of genetic material between homologous chromosomes during the formation of gametes.

(d) Accumulation of variations occurred because of reproduction over generation after generation, and choice naturally created wide diversity.

(e) In the case of asexual reproduction, variation is severely limited as there is only one parent involved. Hence, the offspring is genetically similar to the parent

Equal genetic contribution of male and female parents is ensured in progeny through the inheritance of equal numbers of chromosomes from both parents. There are 23 pairs of chromosomes, but not all are paired. The 22 pairs are called autosomes, while the remaining 1 pair is called the sex chromosomes (represented as X and Y.)

Females have two sets of X-chromosomes, while males have one X-chromosome and one Y-chromosome.

During the process of reproduction, fertilisation takes place, where the male gamete fuses with the female gamete, and it results in the formation of a diploid zygote. Furthermore, the zygote receives an equal contribution of genetic material from both parents. The male contributes 22 autosomes plus one X or Y chromosome. The female contributes 22 autosomes plus one X-chromosome.

The statement holds true – only variations provide an advantage to individual organisms that will survive in a population. For example, variations that lead to the increase in heat resistance in bacteria are very useful for survival if they find themselves in an environment where there is a sudden increase in ambient temperature. This will determine the difference between life and death for the bacteria.