NCERT Class 10 Science Solutions: Heredity and Evolution

Question:

Evolutionary relationships are determined by common ancestry. Organisms that share a more recent common ancestor will have more in common genetically and phenotypically. Comparing the evolutionary history of different species helps us understand these relationships.


The question asks about evolutionary commonality. This means we need to consider which organism we share the most recent common ancestor with.
A. A Chinese school-boy is a human, and while humans share common ancestry with all other humans, this option is a distractor as it compares humans to other humans, not to different species in an evolutionary context.
B. A chimpanzee is a primate, and primates are our closest living relatives. Humans and chimpanzees diverged from a common ancestor relatively recently in evolutionary history.
C. A spider is an arthropod. While we share very ancient common ancestors with arthropods, the divergence happened much further back in time compared to primates.
D. A bacterium is a prokaryote. The common ancestor of bacteria and eukaryotes (which include humans) existed extremely far back in evolutionary time, making our genetic and physical differences vast.

Therefore, in evolutionary terms, we have more in common with a chimpanzee because it is our closest living relative.

Question:

Genetic diversity, inbreeding, genetic drift, bottleneck effect, population size, evolutionary fitness.


The small number of surviving tigers is a cause of worry from the point of view of genetics due to several interconnected reasons:

Reduced Genetic Diversity: A small population size means a limited gene pool. This reduces the variety of genes within the tiger population. With less genetic diversity, the population is less able to adapt to environmental changes, diseases, or new challenges.

Increased Risk of Inbreeding: In a small, isolated population, individuals are more likely to mate with close relatives. Inbreeding can lead to the expression of harmful recessive genes, resulting in reduced fertility, increased susceptibility to diseases, birth defects, and a general decline in the health and viability of the population. This is known as inbreeding depression.

Genetic Drift: In small populations, random fluctuations in the frequency of genes can have a significant impact. This phenomenon, called genetic drift, can lead to the loss of beneficial alleles or the fixation of harmful alleles purely by chance, further diminishing the population’s genetic health and adaptability.

Bottleneck Effect: The severe reduction in tiger numbers due to hunting, habitat loss, etc., has created a genetic bottleneck. This means the surviving population may not represent the full genetic diversity of the original, larger population. Some genes may have been lost entirely during this bottleneck.

Impact on Evolutionary Fitness: Overall, these genetic issues lead to a decrease in the evolutionary fitness of the tiger population. Their ability to survive and reproduce successfully in the long term is compromised, making them more vulnerable to extinction.

In essence, a small number of tigers means a weakened genetic makeup, making them less resilient and more susceptible to future threats.

Question:

Speciation is the evolutionary process by which new biological species arise. Geographical isolation is a key mechanism for allopatric speciation, where populations of a species become separated by a physical barrier, preventing gene flow. Asexual reproduction involves a single parent producing genetically identical offspring.


Geographical isolation will NOT be a major factor in the speciation of an organism that reproduces asexually. This is because speciation, especially allopatric speciation, relies on the interruption of gene flow between populations. In sexually reproducing organisms, gene flow occurs through the interbreeding of individuals. When a geographical barrier separates a population of sexually reproducing organisms, the lack of gene flow allows the isolated populations to diverge genetically over time due to different selective pressures, mutations, and genetic drift, eventually leading to the formation of new species.

However, organisms that reproduce asexually produce offspring that are essentially clones of the parent. There is no mixing of genes or recombination of genetic material. Therefore, even if geographical isolation occurs, each isolated population will evolve independently but without the process of interbreeding and subsequent genetic divergence that is central to speciation in sexually reproducing organisms. While mutations and environmental differences might lead to some phenotypic differences within the asexual lineages, the biological definition of speciation typically involves reproductive isolation, which is not a factor for asexual reproducers in the same way. If a population of asexual organisms becomes geographically isolated, it will likely continue to exist as a distinct population, but the fundamental process of speciation as understood through reproductive isolation driven by gene flow disruption is absent.

Question:

Dominant and recessive alleles. A dominant trait is expressed even if only one copy of the dominant allele is present. A recessive trait is only expressed if two copies of the recessive allele are present. Inheritance patterns, particularly the observation of specific traits in offspring and parents.


We cannot definitively say whether light eye colour is dominant or recessive based on this single observation alone.

Here’s why:

Scenario 1: Light eye colour is dominant.
If light eye colour is dominant, then a person with light eyes can have one of the following genotypes:
* LL (homozygous dominant – two dominant alleles for light eyes)
* Ll (heterozygous – one dominant allele for light eyes and one recessive allele for dark eyes)
If the parents both have light eyes, they could both be Ll. In this case, they can have offspring with light eyes (LL, Ll) and offspring with dark eyes (ll). So, if light eyes are dominant, it’s possible for light-eyed parents to have light-eyed children.

Scenario 2: Light eye colour is recessive.
If light eye colour is recessive, then a person with light eyes must have the genotype ll (homozygous recessive – two recessive alleles for light eyes).
If both parents have light eyes (meaning they are both ll), then all of their offspring will inherit one ‘l’ allele from each parent, resulting in an ‘ll’ genotype. Therefore, all of their children will also have light eyes.

Since both scenarios are consistent with the observation that children with light-coloured eyes are likely to have parents with light-coloured eyes, we cannot conclude whether the trait is dominant or recessive without more information. We would need to observe cases where light-eyed parents have dark-eyed children, or dark-eyed parents have light-eyed children, to make a determination.

Question:

Evolutionary relatedness between species is determined by comparing homologous structures (similar structures due to shared ancestry) and analogous structures (similar structures due to similar function but different evolutionary origins). Homologous structures are stronger indicators of closer evolutionary relationships.


To determine how close two species are in evolutionary terms, we can compare their anatomical features, specifically focusing on homologous structures. Homologous structures are body parts that have a similar underlying structure because they were inherited from a common ancestor, even if they have different functions in the descendant species.

For example, consider the forelimbs of a human, a cat, a whale, and a bat.
1. Human arm: Used for grasping and manipulation.
2. Cat foreleg: Used for walking and running.
3. Whale flipper: Used for swimming.
4. Bat wing: Used for flying.

Despite their different functions, the bones in the forelimbs of all these animals share a similar basic arrangement: one long bone (humerus), two forearm bones (radius and ulna), a series of wrist bones (carpals), hand bones (metacarpals), and finger bones (phalanges). This striking similarity in skeletal structure, despite the varied functions, indicates that these species evolved from a common ancestor that possessed this basic forelimb structure. The more similarities in homologous structures and their arrangement, the closer the evolutionary relationship between the species. In contrast, analogous structures, like the wings of a bird and a butterfly, are superficially similar due to similar function (flight) but have very different underlying structures and evolutionary origins, and thus are not good indicators of close evolutionary relatedness.

Question:

This question relates to the concept of Natural Selection, a key mechanism of evolution. Natural selection is the process by which organisms better adapted to their environment tend to survive and produce more offspring. It hinges on the idea of differential survival and reproduction based on advantageous traits.


I do not agree with the statement. While variations that confer an advantage to an individual organism are more likely to survive and reproduce, leading to their increased prevalence in a population over time, it is not the *only* type of variation that survives.

Here’s why:

* Neutral Variations: Many variations are neutral. They neither benefit nor harm the organism. These neutral variations can persist in a population and even increase in frequency due to genetic drift, which is random changes in allele frequencies.
* Disadvantageous Variations: Sometimes, disadvantageous variations can also survive. This can happen if the disadvantage is not severe enough to prevent reproduction, or if the population is small and genetic drift plays a stronger role. In some cases, a disadvantageous trait might even be maintained if it’s linked to a beneficial trait through genetic linkage.
* Environmental Fluctuations: What is advantageous in one environment might be neutral or even disadvantageous in another. As environments change, variations that were previously not advantageous might become so, and vice-versa.

Therefore, it’s more accurate to say that variations that confer an advantage have a *higher likelihood* of surviving and becoming more common in a population, but not *only* those variations survive.

Question:

Speciation is the evolutionary process by which new biological species arise. Geographical isolation is a key mechanism for speciation, leading to reproductive isolation between populations. Self-pollination means that a plant can reproduce with its own pollen, which limits gene flow between individuals within a population.


No, geographical isolation will not be a major factor in the speciation of a self-pollinating plant species.

Here’s why:

Self-pollination inherently limits gene flow between different individuals within a population. Since the plant can readily reproduce with itself, the genetic makeup of individual plants tends to be very similar.

Geographical isolation works by preventing gene flow between populations that are physically separated. This separation allows populations to diverge genetically over time due to different selective pressures, mutations, and genetic drift, eventually leading to reproductive isolation and speciation.

In a self-pollinating species, even if populations are geographically isolated, the low degree of gene flow *within* the population due to self-pollination means that genetic divergence between isolated groups will be very slow or non-existent. The primary barrier to speciation in self-pollinating plants is already the self-pollination mechanism itself, which maintains genetic uniformity. Therefore, external geographical isolation becomes a less significant driver for the formation of new species compared to cross-pollinating species where geographical isolation is crucial for preventing gene flow.

Question:

Evolutionary relationships between different organisms.
Homologous organs: Similar in origin and basic structure, but perform different functions, indicating common ancestry.
Analogous organs: Different in origin and basic structure, but perform similar functions, indicating adaptation to similar environments.


Homologous organs are body parts that have a similar underlying structure and developmental origin because they are inherited from a common ancestor. However, due to different environmental pressures and selective forces, these organs have evolved to perform different functions in different species.
Example: The forelimbs of humans, cats, bats, and whales. All these forelimbs have the same basic bone structure (humerus, radius, ulna, carpals, metacarpals, phalanges) because they are derived from a common ancestral limb. In humans, they are used for grasping; in cats, for walking and running; in bats, for flying; and in whales, for swimming.

Analogous organs are body parts that have different underlying structures and developmental origins but perform similar functions. This similarity in function arises because different species have adapted to similar environmental conditions or lifestyles, leading to convergent evolution.
Example: The wings of a butterfly and the wings of a bird. The butterfly’s wing is made of chitinous membranes supported by veins, while a bird’s wing is made of bones, muscles, and feathers. Despite their vastly different structures and evolutionary origins, both are adapted for flight.

Question:

The sex of a child is determined by the chromosomes inherited from the parents. In humans, females have two X chromosomes (XX), and males have one X and one Y chromosome (XY). The Y chromosome carries genes that initiate male development.


In human beings, sex determination is a genetic process that occurs at the moment of fertilization.
1. Chromosomes: Human cells contain 23 pairs of chromosomes. 22 pairs are autosomes, which are common to both males and females. The 23rd pair are sex chromosomes.
2. Sex Chromosomes in Females: Females have two identical sex chromosomes, both X chromosomes (XX).
3. Sex Chromosomes in Males: Males have two different sex chromosomes, one X chromosome and one Y chromosome (XY).
4. Gamete Formation: During the formation of gametes (sperm and egg cells) through meiosis:
* Each egg cell produced by a female will always contain one X chromosome.
* Sperm cells produced by a male can contain either an X chromosome or a Y chromosome.
5. Fertilization: When a sperm fertilizes an egg:
* If a sperm carrying an X chromosome fertilizes the egg (which always has an X chromosome), the resulting zygote will have XX chromosomes, and the child will be female.
* If a sperm carrying a Y chromosome fertilizes the egg, the resulting zygote will have XY chromosomes, and the child will be male.
Therefore, the presence of the Y chromosome in the sperm determines the sex of the child. The father’s sperm contributes the sex-determining chromosome.

Question:

Homologous organs are structures that have a similar underlying anatomical plan and embryonic origin but may have evolved to perform different functions. This similarity in structure points to a common ancestry. Analogous organs, on the other hand, have similar functions but different underlying structures and embryonic origins, indicating convergent evolution.


No, the wing of a butterfly and the wing of a bat cannot be considered homologous organs.

Homologous organs share a common evolutionary origin and underlying structure. For example, the forelimbs of humans, bats, whales, and horses are homologous because they all have the same basic bone structure, inherited from a common ancestor, even though they are used for different purposes (grasping, flying, swimming, running).

The wing of a butterfly is an appendage made of chitinous membranes supported by veins. It is an outgrowth of the exoskeleton.
The wing of a bat is a modified forelimb. It consists of bones (similar to those in a human arm and hand), muscles, nerves, and blood vessels, covered by a membrane of skin.

While both structures are used for flight (a similar function), their underlying anatomy and embryonic development are entirely different. The bat’s wing is a vertebrate limb structure, whereas the butterfly’s wing is an insect exoskeleton structure. Therefore, they are analogous organs, not homologous organs. They have evolved to perform the same function (flight) independently in different evolutionary lineages.

Question:

Species Definition: A group of organisms that can interbreed naturally and produce fertile offspring. Key idea is reproductive isolation and ability to produce fertile young.
Genetic Variation: Despite superficial differences, all humans share a very high degree of genetic similarity. The variations we observe are due to a small percentage of genetic differences and environmental factors.


Human beings are classified as belonging to the same species, Homo sapiens, because they possess the defining characteristics of a species. Primarily, this means that humans, regardless of their apparent differences in size, color, or facial features, are capable of interbreeding with each other and producing fertile offspring. This ability to successfully reproduce and pass on viable genes to the next generation is the most crucial biological criterion for defining a species.

While humans exhibit significant physical diversity, these variations are superficial and represent a relatively small amount of genetic variation when compared to the overall human genome. This genetic variation is a natural phenomenon that arises through processes like mutation and recombination, and it contributes to the rich tapestry of human appearance. However, the underlying genetic blueprint that makes us human is overwhelmingly shared across all populations. Therefore, despite our outward differences, we are all members of the same biological species because we can produce fertile offspring together.

Question:

Dominance and recessiveness of traits, monohybrid cross, allele, genotype, phenotype.


Mendel’s experiments demonstrated the concepts of dominance and recessiveness through his work with pea plants. He conducted monohybrid crosses, where he studied the inheritance of a single trait at a time. For example, when he crossed pure-breeding tall pea plants (TT) with pure-breeding dwarf pea plants (tt), the first filial generation (F1) plants were all tall. This indicated that the allele for tallness (T) was dominant over the allele for dwarfness (t), and the allele for dwarfness was recessive.

He observed that even though the F1 generation had the genotype Tt, they expressed the phenotype of tallness. This showed that the dominant allele masked the expression of the recessive allele. When Mendel self-pollinated these F1 generation plants, the F2 generation showed a phenotypic ratio of 3 tall plants to 1 dwarf plant. This segregation of traits in the F2 generation, with the reappearance of the recessive trait (dwarfness), further supported the idea that alleles for traits can be either dominant or recessive. The dominant allele expresses its trait even when only one copy is present, while the recessive allele only expresses its trait when two copies are present.

Question:

Meiosis, Gametes, Fertilization, Chromosomes, Genes


During sexual reproduction, both male and female parents contribute genetic material to the progeny through specialized reproductive cells called gametes (sperm from the male and egg from the female). Meiosis is the process of cell division that produces these gametes. Meiosis ensures that each gamete contains half the number of chromosomes present in the parent’s somatic cells. Typically, in humans and many other organisms, somatic cells are diploid (2n), meaning they have two sets of chromosomes. Through meiosis, gametes become haploid (n), containing only one set of chromosomes. When a male gamete (sperm) fuses with a female gamete (egg) during fertilization, their haploid sets of chromosomes combine to form a diploid zygote, which develops into the progeny. Since each gamete carries one complete set of chromosomes, the zygote formed after fertilization will have the normal diploid number of chromosomes, with one set inherited from the male parent and the other set from the female parent. This process directly ensures that the progeny receives an equal genetic contribution from both parents, with each parent contributing approximately half of the genetic material in the form of their chromosomes.

Question:

Genetic variation, sexual reproduction, asexual reproduction, evolution, natural selection


Sexual reproduction involves the fusion of gametes from two parents, leading to a recombination of genetic material. This recombination, along with processes like crossing over during meiosis, shuffles alleles and creates new combinations of genes in the offspring. This results in a higher degree of genetic variation within a population compared to asexual reproduction, where offspring are essentially clones of the parent, with variations arising only from random mutations.

The increased genetic variation in sexually reproducing organisms is crucial for evolution. It provides a wider range of traits upon which natural selection can act. In a changing environment, individuals with advantageous variations are more likely to survive and reproduce, passing on their beneficial traits. Over time, this differential survival and reproduction leads to the adaptation and evolution of the species, making them better suited to their environment. Asexual reproduction, with its limited variation, can make a population more vulnerable to environmental changes as fewer individuals may possess the traits necessary for survival.

Question:

Inheritance of traits is determined by the genetic material (DNA) present in the germ cells (sperm and egg). Changes in non-germline cells during an individual’s lifetime do not alter the DNA in germ cells, and thus are not passed on to offspring.


Acquired traits are characteristics that an organism develops during its lifetime due to environmental influences or behavioral changes. For example, if a person develops strong muscles by exercising regularly, this acquired trait of muscularity is not inherited by their children. This is because the inheritance of traits is based on the transmission of genetic information, primarily in the form of DNA, from parents to offspring through germ cells (sperm and egg). Changes that occur in the somatic cells (body cells) of an individual during their lifetime, such as muscle development or a scar, do not alter the DNA sequence in their germ cells. Therefore, these acquired characteristics are not encoded in the genes that are passed on to the next generation. Only changes in the germline DNA are heritable.

Question:

In asexually reproducing populations, allele frequencies generally change slowly. Traits that are less frequent are more likely to have arisen more recently, as they haven’t had as much time to spread through the population. Conversely, traits that are more frequent have had more time to propagate.


In an asexually reproducing species, new traits arise due to mutations. Once a mutation occurs, it can be passed down to offspring. Over time, if the trait is advantageous or neutral, its frequency in the population will tend to increase. Conversely, if a trait is disadvantageous, its frequency will tend to decrease.

The question states that trait A exists in 10% of the population, while trait B exists in 60% of the population. A trait that is present in a larger percentage of the population has had more opportunities to spread and become established. This suggests that trait B has been present in the population for a longer duration than trait A. Therefore, trait B is likely to have arisen earlier.