As we have discussed various life processes in the previous chapters of your CBSE class 10 syllabus, we now move on to science ch 9- Heredity and Evolution class 10. Reproduction gives rise to variations in any living organism. However, organisms that sexually reproduce, cause maximum successful variations as compared to asexually reproducing organisms. Take the example of a flower field where you find evidently fewer variations due to asexual reproduction. Humans reproduce sexually and hence certain heredity traits are passed on to us which has resulted in such dynamic changes over the years. In this chapter, we shall study the mechanisms which give rise to variations and the long-term consequences of the accumulation of variations.
This Blog Includes:
- Introduction to Heredity And Evolution Class 10
- Variations and its causes
- Importance of variations
- Terms to know
- Mendel’s contribution to Genetics
- Benefits of choosing the pea plant
- Mendel’s three laws of inheritance
- Monohybrid vs. Dihybrid Cross
- Monohybrid Cross
- Dihybrid Cross
- Sex Determination in Humans
- Acquired Traits vs Inherited Traits
- Darwin’s Theory of Evolution
- Evolution by Stages
- Key Takeaways
- Monohybrid cross
- Dihybrid Cross
- Sex Determination
- Acquired Traits
- Inherited Traits
- Charles Darwin Theory
- Human Evolution
- Tracing Evolutionary Relationship
Introduction to Heredity And Evolution Class 10
Although hidden in plain sight, some of the essential things that we experience around us are heredity and evolution principles—a seed form an orange tree gives rise to an orange tree, not an apple tree. Children always resemble their parents’ looks and personalities. A tail bone in humans is reminiscent of our once tailed ancestors (apes). These facts make it clear that such features play a significant role in nature. Let’s find out how in this chapter of heredity and evolution class 10.
Heredity derives from the Latin word Hereditatem, which means “condition of being an heir.” It refers to the passing of characteristics or traits through parents’ genes to the offsprings (i.e., from one generation to another.) Heredity is seen in sexual reproduction due to which the variations of inherited characteristics are high. The study of heredity and these genetic variations is called genetics.
Variations and its causes
Differences in the traits of parents and offsprings are known as variations. They are caused by mutations (errors in DNA copying), recombination, and random mating. Variations are mainly of two types:
- Somatic Variation: This type of genetic variation occurs in somatic cells (all cells except reproductive cells). As they are not transmitted or inherited by the next generation, they’re also called acquired traits.
- Gametic Variation: This genetic mutation occurs in germline cells (reproductive cells). The next generation may inherit these. Therefore, they’re also known as inherited traits.
Importance of variations
- They form the basis of heredity.
- They are essential as they contribute to evolution.
- It helps the organism to survive and adapt to its changing environment.
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Terms to know
Allele- A specific way in which the different forms of a gene can be physically expressed.
Homozygous- When a specific gene has two copies of the same allele. (two dominant alleles)
Heterozygous- When a specific gene has two different types of alleles. (one dominant and the other recessive)
Mendel’s contribution to Genetics
G.J. Mendel is often known as the father of genetics because of his pioneering work on Pisum sativum (garden pea). Let’s explore his study in depth during the course of this chapter of heredity and evolution class 10. His experiment studied seven pairs of contrasting traits and their inheritance:
|White & wrinkled|
|Terminal pods, flowers on top|
Benefits of choosing the pea plant
- Produces a large number of seeds
- Shorter life cycle
- Various contrasting traits can be observed.
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Mendel’s three laws of inheritance
Law of equal segregation- The two alleles (gene pair) of a heredity trait separate during the formation of gametes (reproductive cells) and are then carried by different gametes.
Law of independent assortment- Alleles of different traits get assorted independently of one another during the formation of gametes.
Law of dominance- It states that for a pair of inherited traits, the dominant allele will be exclusively expressed in the offspring instead of the recessive allele unless both alleles are recessive.
Monohybrid vs. Dihybrid Cross
A monohybrid cross is a cross between P generation organisms (parents) that differ in a single contrasting trait. Here’s how you can use a Punnett square to predict the probable genetic outcomes of a monohybrid cross.
The experiment to observe the inheritance of one pair of contrasting characters was conducted as follows. Mendel chose a pure green pod (genotype GG) and pure yellow pod (genotype gg) pea plants and crossbred them to achieve a first (filial) generation. As shown in the figure below, he obtained only greed pods. This indicated that only a single parental trait was observed instead of a mixture of two.
In the next experiment, the plants of the F generation were self pollinated to get F2 generation. In this case, Mendel observed distribution of 75% green pod plants and 25% yellow pod plants (phenotypic ratio 3:1). This indicated that in the F generation, green pods were expressed as a dominant trait and yellow pods as recessive. However, in the F2 generation, the genotype ratio is 1:2:1 i.e Pure green pod : Hybrid green pod : Pure yellow pod.
Monohybrid cross with homozygous parents:
Monohybrid cross with heterozygous parents:
A dihybrid cross is a breeding experiment where two pairs of contrasting characters are crossed between two plants. Let’s understand this in more detail.
For this experiment, Mendel crossbred pea plants that produced round green seeds and yellow wrinkled seeds. He observed that in the Fx generation, he got round and yellow seed which indicated that yellow and round are dominant traits while green and wrinkled are recessive. He then proceeded to self pollinate the plants in Fx generation to achieve F2 generation. The results of this dihybrid cross as follows:
9 – round and yellow seeds
3- round and green seeds
3- wrinkled and yellow seeds
1- wrinkled and green seeds
Sex Determination in Humans
Before we understand how sex is determined in humans, let’s take a look at some important terms and concepts in this chapter of heredity and evolution class 10.
Sex chromosomes– These are chromosomes that play an active role in determining the sex of the newborn. There can be two possibilities here:
- Female offspring- when a sperm carrying an X chromosome fertilizes an ovum with an X chromosome, the baby will have XX chromosomes (or female).
- Male offspring- when a sperm carrying a Y chromosome fertilizes an ovum with an X chromosome, the baby will have XY chromosome (or male)
In humans, chromosomes are not paired evenly. While males have 22 paired chromosomes with one odd pair (sex chromosomes), females have a perfect pair which is represented by XX. Consequently, the sex of the baby depends on how the ova is fertilized:
Sex determination is an imporatant feature of the chapter heredity and evolution class 10. While we are on this topic, let us quickly go through some unique ways in which sex is determined in other animals:
- In certain animals like reptiles, the temperature at which the egg fertilizes decides whether the offspring will be male or female.
- Some species of animals show no sex-determination. For instance, some species of snails start our as male and change their sex to female later.
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When we talk about evolution, we are talking about the genetic characteristics inherited and passed on from one generation to the other. The process of evolution rises at all levels, including species organisms and molecules. In this chapter heredity and evolution class 10 notes, we will discuss Darwin’s theory of evolution, classification, and its stages.
Here’s a classic thought experiment to understand how evolution occurs. Consider a group of twelve red beetles that live in green bushes. They are able to sexually reproduce and are therefore expected to develop variations. Here are the possible outcomes:
Situation 1: Crows can eat these beetles by easily picking them out by color. As a result of color variations (due to sexual reproduction), green beetles start to appear and their population eventually increases. Crows are unable to see them so their population increases while that of red beetles decreases. This kind of variation gives a better chance of surviving.
Situation 2: A few blue beetles appear due to color variation. Crows can pick out the red and blue beetles by color and eat them. While there are more red beetles than blue ones initially, a natural calamity like an elephant stomping on mostly red ones. This allows the blue beetles to grow in population. This scenario does not provide any survival advantage but does improve diversity without adaptation.
Situation 3: An increase in the population of beetles leads to a disease in the bushes which eventually reduces the population of beetles in the absence of food. However, when the bushes are free of diseases in a few years, the beetle population grows back due to availability of food. This kind of change is not inherited.
Acquired Traits vs Inherited Traits
Acquired traits: Acquired traits are traits that cannot be inherited over generations or passed on the DNA as they are a result of changes in non-reproductive tissues. For example, situation 3, callus on fingers, increase in muscle mass to evade predators, etc.
Inherited traits: Inherited traits are those which are passed on from generation to generation and are caused due to changes in the DNA. For example, situation 1, eye color, hair color, etc.
Darwin’s Theory of Evolution
Also known as “Theory of natural selection.”
Postulates of Darwin theory
- Speciation (formation of species)– When Variations occur within a species, it can only be passed to the next generation.
- The struggle of existence – An increase in the number of organisms, limited space, and food creates competition between the organisms.
- Survival of the fittest or Natural selection- It is a process governed by nature in which organisms inherent characteristics that are fittest for their survival, such that they are adaptive to prevailing conditions.
Classification and Evolution are both linked to each other. Nature provides different pieces of evidence for evolution. These can be classified as:
- Homologous organs: These organs have evolved from the same ancestors, but their functions are different. For example, human hands and wings of bats.
- Analogous organs: These types of organs have evolved from different ancestors but have the same functions. Some examples are the wings of birds, wings of insects, wings of bats.
- Paleontological (fossil) evidence: The remains of dead organisms are known as fossils. In the digging process, it is considered that the deeper the fossil, the older it is. For example, Archaeopteryx consists of features of reptiles and birds. This means that birds have evolved from dinosaurs.
- Vestigial organs: These are rudimentary such that they have lost their functions through evolution—for example, Wisdom tooth, muscles of ears, etc.
Evolution by Stages
The evolution of feathers is an example of evolution by stages such that earlier dinosaurs had feathers but couldn’t fly. However, later, birds used them for flying.
- Molecular Phylogeny: The changes in DNA during reproduction are also considered to be basic events of evolution. Organisms distantly related to each other carry a greater difference in their DNA.
- Evolution by artificial selection: This selects special phenotype characters to create organisms with enhanced characteristics, for example, producing different cabbage-like varieties such as red cabbage, broccoli, etc.
- Human evolution: Excavation, time dating, determination of DNA sequence are used to study the human evolutionary relationship. The study shows that we all belong to a single species that was evolved in Africa that spread across the world.
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According to the latest CBSE class 10 syllabus, heredity and evolution class 10 is an important foundation topic. A proper understanding of these will help you quickly grasp the concepts in subsequent chapters. You can also refer to this blog for class 10 ncert science solutions.
Gregor Mendle was a supremely talented scientist, working in a monastery of Austria on pea plants. He chooses garden pea (Pisum sativum) for his experiments. He was working on seven different characters, namely, stem height, flower color, flower position, pod shape, pod color, seed shape, and seed color for contrasting traits based on alleles.
While doing his research on a single character at a time (Monohybrid cross), he explained two laws, the Law of Dominance and Law of Segregation. Later, he worked with two characters of pea plants at a time (Dihybrid cross) and explained the Law of Independent Assortment. These three laws were represented as Laws of Heredity. The reason for choosing pea plants for research: pea plants have large varieties with sharp contrasting characters; self-pollination potential to obtain pure lines; it is the annual plant and so completes its life cycle in one season; pea plants are producing large varieties of flowers and seeds of adequate size and easy to handle.
Monohybrid cross in which only one pair of contrasting characters is taken under consideration at a time. For example, Mendle has crossed a tall height plant with a dwarf height plant (here the height of a plant is a character). He found all tall plants in the first consistent generation as Gene of tallness was dominant over the dwarf. Thus, F1 always resembles one of the parents but having genotype Tt. Now, he self-pollinated F1 plants (both Tt genotype) and found three tall and one dwarf plants. Thus, the phenotypic ratios are 3:1 (tall: dwarf), and the genotypic ratio is 1:2:1 (TT: Tt: tt).
Based on the above observation, Mendle proposed that something has been stably passed down unchanged from parents to offspring through gametes over successive generations. These are called factors later identified as genes – serve as a unit of inheritance. There are two alleles of each gene; here they are T- for tallness and t- for dwarfness. He described two laws based on the monohybrid cross:
Law of Dominance: Out of one pair of contrasting characters brought together in an offspring, one always dominates the other and gets expressed while another remains masked and called recessive.
Law of Segregation: Also called the Law of Purity of Gametes. When two members of a pair get separated during the formation of gametes, they result in the purity of gametes, and both will further express independently.
Graphical representation to calculate all the probabilities of genotypes and phenotypes of offsprings/ progenies in a genetic cross are called a punette square.
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Dihybrid is a cross in which two pairs of contrasting characters are taken into consideration at a time. Both the members of a pair/ character called alleles to get separated during the formation of gametes without any blending between them. Thus, they result in the purity of gametes. In other words, this was represented as the law of Independent assortment. Each pair of the character may combine with either of the other pair.
For example, Mendle crossed pea plants with yellow color seeds that are round in shape (both characters are dominant- YYRR) with green color seeds that are wrinkled in shape (both characters are recessive- yyrr). Here, Y is a gene for dominant yellow seed color; y is a gene for recessive green seed color; R- the gene for the round shapes of seed while r is the gene for the recessive wrinkled shape of the seed. In the F1 generation, all the plants are showing seeds which are yellow in color and round in shape due to the dominance of yellow color and round shape gene with genotype as YyRr. But, when he crossed the F1 generation plants (YyRr x YyRr – selfing of F1).
He observed the distribution of each pair individually in the form of gametes in the next generation, and so found the following ratios: Phenotypes: round yellow: round green: wrinkled yellow: wrinkled green = 9:3:3:1.
Henking in 1891 had traced a specific nuclear structure called X body or X chromosome involved in determining the sex of an individual. Sex determination is a biological process that identifies the development of sexual characters in an individual. We have various types of sex determination systems like the XY system for humans, XO system for insects, ZW for birds, and haplodiploidy in honeybees. We need to understand the concept of homozygous and heterozygous first.
The sex of the progeny always depends on the heterogamy condition, either shown by male or female. Look at the below example and try to understand the concept. For example, in humans, males have one X and one Y chromosomes, while females have both XX chromosomes. So, we have equal probabilities of X gamete (ova) fusing with X or Y gametes (sperms) and thus, there is 50-50 percent of the chances of male and female offspring production in the coming generations. Hence, the genetic makeup of the sperm will determine the sex of the child. Human females are homozygous XX, producing the same types of gametes, while human males are XY, Heteroygous and thus producing two types of gametes X and Y. So, the sex of the child always depends on the father.
In our society, many of us are blaming women on the arrival of the baby girl child. This has to be taken into consideration very seriously!
Traits are basically features or characteristics that an organism develops or inherited for the betterment of life. Acquired traits are gained during the life of an organism. For example, knowledge or any game that we play as per our likings like to learn badminton or table tennis. The genes for acquired characters are present on somatic cells, and so can’t be passed to the next generations.
Traits are basically features or characteristics that an organism develops or inherited for the betterment of life. Inherited traits are inherited from the parents. For example, hair color, eye color. The genes for inherited characters are present on sex chromosomes, and so can be passed on to the next generations.
Charles Darwin Theory
British naturalist Charles Darwin, born in England in 1809, was credited for the natural selection theory. The prime key features of Darwin’s Theory of Evolution are:
- Individuals of a species aren’t identical
- Traits are passed from generation to generation
- More offspring are born than can survive
- Only the survivors of the competition will go on reproducing new ones
Survival of the Fittest was the main concept featured by Charles Darwin as the species which are best adjusted to the environment and flexible enough to adjust, compete among other species, get survived and reproduce will survive, and the rest will die because they cannot adjust and bear pressure. While roaming on Galapagos Island, he found Finches (birds) with various kinds of beaks in different regions. He analyzed that the types of beak depend upon the types of grain size available in that respective area for food. The birds thus evolved their beaks based on the food for their better survival in the region, called adaptation for survival. Thus, those who have adapted were survived, and the rest died. These finches were later recognized as Darwin’s finches.
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Human evolution is an extraordinary process that led to the creation of anatomically modern humans. Scientific evidence states that behavioral development and physical developments have originated from the apelike ancestors and have evolved over nearly six million years. This evolution is the outcome of many factors: mutation, genetic recombination, chromosomal abnormalities, reproductive isolation, and natural selection.
These are the possibilities for producing variation at any level, either beneficial or harmful. One of the earliest humans was Homo habilis or handy man. The latest man is called Homo sapiens (wise man). Thus, this is known as the origination of a fully developed wise man from a handy man. Thus, we found this as a beneficial one! The following sequence is considered correct for human evolution.
Homohabilis →Homo neanderthalensis→ Homo erectus → Cro magnon → Homo sapiens
Human evolution, in other words, is characterized by a number of morphological, developmental, physiological, and behavioral changes. The most important adaptations are increased brain size, bipedalism, decreased sexual dimorphism, and lengthened ontogeny (gestation).
Tracing Evolutionary Relationship
Evolutionary relationships aid us to trace who we are closest to and who is our common ancestor. A group of things/ organisms is similar enough to be placed together based on specific characteristics. For example, mammals and reptiles, all of them have 4 limbs.
Human evolution can be studied and explored by using the fundamental tools and techniques like digging earth (excavating), carbon-dating, fossils study, and DNA sequencing. All the evolutionary investigations are based on morphological (structure and function) and genetic studies. The final structure is called a phylogenetic tree or evolutionary tree to identify the immediate common ancestors. Now, these evolutionary characteristics in different organisms would be similar because they are inherited from a common ancestor and can be studied through tracing evolutionary relationships.
Homologous Organs: The basic structure of the limbs is comparable though it has been modified to work different functions in various vertebrates. Such a homologous characteristic aids in identifying an evolutionary relationship between apparently different species. The arm of a human, the leg of a dog, the wing of a bat or a bird, and the flipper of a whale or dolphin are homologous structures.
Analogous Organs: When we observe the wings of bats, we understand that the wings of bats are skin folds stretched between elongated fingers, whereas they are feathery in birds covering all along the arm. The patterns of the two wings, their structure, and components are thus very different. They appear similar because they have a mutual utilization for flying, but their origins are not common. This makes analogous characteristics.
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