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Heredity and Evolution Class 10 Notes Science Chapter 8

When living things make new individuals, they end up being a bit similar but also a little different. We've talked about how even without sexual reproduction, there's still some variety during reproduction. But when it comes to sexual reproduction, where two parents are involved, we get even more diverse and unique variations. For example, look at a sugarcane field; the plants are quite similar. But in animals, like humans, which reproduce through sexual reproduction, individuals can be very different from one another.

Accumulation of Variation During Reproduction

  • Reproduction is how new beings are made, and they're somewhat like their parents but not exactly due to DNA copying. In asexual reproduction, like in plants, there are small differences. But in sexual reproduction, where two different gametes come together, there are more significant variations, creating diversity in each generation.
    Heredity and Evolution Class 10 Notes Science Chapter 8
  • These differences can be good or bad for individuals, helping or not helping them deal with changing environments. 
  • For instance, bacteria that can handle high temperatures survive heat waves. These variations add up over generations because of the environment, leading to new species and helping them survive.

Question for Chapter Notes: Heredity
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Why is there more diversity in individuals produced through sexual reproduction compared to asexual reproduction?
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Heredity is the process that passes on traits from parents to their kids, like skin color, hair, eyes, and height. It's what keeps differences going from one generation to another, contributing to the evolution of species over time.

1. Inherited Traits

  • Traits are features passed down from parents, such as eye or skin color in people.
  • If a parent has brown and black eyes, the child might inherit brown, black, or a mix of both, based on the responsible genes.
  • These inherited traits cause variations in the population while everyone still shares some basic features.

2. Rules for the Inheritance of Traits- Mendel’s Contributions

  • The rules of inheritance come from the fact that both parents contribute equally to the development of the traits in the offspring. 
  • Gregor Johann Mendel, often called the 'Father of genetics,' used his knowledge of science and math to establish laws of inheritance.
    Heredity and Evolution Class 10 Notes Science Chapter 8
  • He conducted experiments with pea plants due to their ease of cultivation and clear characteristics like tall/short, white/violet flowers, and round/wrinkled seeds.
  • Mendel discovered that traits were controlled by factors, now known as genes, which exist in pairs called alleles.
  • Genes can be dominant or recessive; for instance, tallness ('T') is dominant, while shortness ('t') is recessive. A plant will be tall if it has 'TT' or 'Tt' alleles.
  • Homozygous means having the same alleles for a trait (e.g., TT or tt), while heterozygous means having different alleles (e.g., Tt).
  • The physical appearance of a trait, like tallness or shortness, is the phenotype. The genetic makeup is the genotype, such as Tt or TT.
  • Mendel, being a mathematician, used statistical methods like Punnett squares to predict possible genotypes and phenotypes in each generation of offspring.
  • He conducted his experiments to find two types of inheritance namely:

(i) Monohybrid Cross

  • Mendel experimented by crossing a tall pea plant (TT) with a short one (tt), resulting in all tall offspring (F1 generation). 
  • When he self-pollinated the parents and F1 plants, the F2 generation showed both tall and short plants. The tallness trait dominated (expressed with TT or Tt), while shortness required a recessive genotype (tt). 
  • The ratio of genotypes in F2 was 1:2:1, and the ratio of physical appearances (phenotypes) was 3:1. This type of inheritance is called monohybrid inheritance, involving a pair of single alleles for a gene.

Heredity and Evolution Class 10 Notes Science Chapter 8

(ii) Dihybrid Inheritance

  • When a pea plant with round green seeds (RRyy) is crossed with a wrinkled yellow seed plant (rrYY), the F1 generation has round yellow seeds (RrYy), showing dominance in the traits of round shape and yellow color.
  • Upon self-pollination of the F1 generation, the F2 generation displays greater variations and introduces new combinations. The resulting genotypes have 9 different combinations, and the physical appearances (phenotypes) follow a ratio of 9:3:3:1.
  • This type of inheritance, involving two pairs of alleles for different traits, is termed dihybrid inheritance. In this case, the traits of round/wrinkled seeds and green/yellow color are inherited independently.
    Heredity and Evolution Class 10 Notes Science Chapter 8

Question for Chapter Notes: Heredity
Try yourself:
Which term describes the process of passing on traits from parents to their offspring?
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3. How do These Traits Get Expressed? 

  • The DNA in a cell is in charge of creating proteins. A specific part of this DNA that gives instructions for a particular protein is called a gene. 
  • These proteins play a crucial role in many biochemical reactions, influencing the expression of traits, and their activities are regulated by specific enzymes.
    Heredity and Evolution Class 10 Notes Science Chapter 8
  • Changes in proteins can cause variations in traits, and genes control these traits. For traits to be inherited independently from both parents, they must be present separately. This separation is in the form of chromosomes, with each cell having two sets—one from each parent. 
  • When the germ cells combine during reproduction, they restore the number of chromosomes and DNA, ensuring there are two genes for the expression of every trait.

4. Sex Determination

  • The sex determination in newborn organisms involves various mechanisms. In some reptiles, it relies on the temperature of the fertilized eggs, while in snails, it can change. For humans, however, the sex of a newborn is primarily determined by the genes inherited from the parents.

Question for Chapter Notes: Heredity
Try yourself:
What is the term for traits that are genetically inherited and passed down from one generation to the next?
View Solution

  • Human chromosomes come in pairs, with generally 22 pairs determining various traits, each from one parent. Among these, there is a special pair known as sex chromosomes that differs between males and females. Females have a matching pair of X chromosomes (XX), while males have an X and a Y chromosome (XY).

Heredity and Evolution Class 10 Notes Science Chapter 8

  • In the inheritance pattern, a child inherits an X chromosome from their mother by default. The sex of the child depends on which sex chromosome pair is inherited from the father. If it's an X, the pair becomes XX, resulting in a girl. If it's a Y, the pair becomes XY, resulting in a boy.

Topics from Old NCERT

1. Evolution

  • Evolution is the gradual development of organisms over millions of years, passing down traits through generations. This process leads to the emergence of new species. According to J.B.S. Haldane, life on Earth may have originated as a simple organic molecule and evolved into complex organisms due to changing conditions.
  • Charles Robert Darwin, an English naturalist, introduced the concept of "evolution of species by natural selection," even though he didn't fully understand the mechanisms behind variations. Natural selection involves favoring traits in a population that enhances survival and adaptation to the environment.
  • Mendel proposed the mechanism for trait inheritance, working independently from Darwin. These inherited traits can also be acquired over time.
    Heredity and Evolution Class 10 Notes Science Chapter 8

Acquired and Inherited Traits

  • Traits that an organism develops during its lifetime due to external conditions, like the way a person speaks or their skills, are known as acquired traits. Importantly, these changes don't impact germ cells and thus cannot be passed on to the next generation.
  • In contrast, traits that are genetically inherited and passed down from one generation to the next are termed as inherited traits. Examples include the color of eyes or skin.

2. Speciation 

Species are groups of organisms sharing similar traits, capable of breeding within their own group. The process through which a new species emerges from an existing one due to various factors is called speciation. This results in the formation of distinct species within a population that cannot interbreed.

Several factors contribute to speciation:

  1. Population Splitting:

    • Favorable environments lead to rapid population growth, making interactions difficult. Geographical barriers like mountains or rivers further isolate groups, reducing gene flow and potentially leading to the development of new species.
  2. Genetic Drift:

    • Unforeseen events can cause changes in specific traits, impacting the genetic makeup of a population. For instance, if a forest fire eliminates green beetles, only red beetles remain, altering the genetic composition of the population.
  3. Natural Selection:

    • Populations that possess variations enabling them to adapt better are favored over others. For example, birds with average wings survive a storm compared to those with long or short wings. This natural selection process aids in the evolution of species over time.

The combined effects of genetic drift and natural selection can lead to the emergence of isolated subspecies, eventually forming an entirely new species. This process defines speciation, where distinct species evolve from pre-existing ones.

3. Evolution and Classification

  • Every species undergoes evolution, and similarities among organisms help group them based on shared characteristics. While most organisms share fundamental features like cells being the basic unit of life, further classifications vary, such as the presence of a nucleus or being single-celled or multicellular. 
  • This hierarchical classification aids in understanding the evolutionary relationships between groups. The closer the shared characteristics between two species, the more closely related they are, suggesting recent common ancestors. 
  • This creates a hierarchy, from closely related species with recent common ancestors to those with more distant common ancestors, tracing backward in the evolutionary process.

(i) Tracing Evolutionary Relationships

In understanding evolutionary relationships, the presence of shared characteristics is fundamental for classification. These common features fall into two types:

  • Homologous Characteristics: These are traits found in different organisms that appear similar and share a common ancestor. While they may have comparable basic organ structures, these structures often serve different functions in various organisms. For instance, mammals, birds, reptiles, and amphibians all have four limbs, but each limb is modified to fulfill a distinct purpose.
  • Analogous Characteristics: These are traits with similar functions in different organisms, evolving independently from different ancestors. For example, the wings of bats and birds may look alike, serving the common function of flying, but the wings of bats are actually a fold of skin between their fingers.

These distinct types of characteristics play a significant role in tracing the evolutionary relationships between species.

(ii) Fossils

To explore evolutionary relationships, it's crucial to study both existing and extinct species. When organisms die, their bodies typically decompose; however, under certain conditions like hot mud or lava, they can be preserved as fossils. Fossils are the preserved traces of past living organisms, providing insights into various evolutionary stages. The process of turning an organism into a fossil is called fossilization, and the study of these remnants is known as paleontology.

Determining the age of fossils involves two methods:

  1. Relative dating: This technique involves excavating fossils from rocks by digging into the earth. Generally, more recent fossils are found closer to the earth's surface.

  2. Radiometric dating: This method relies on dating fossils based on the radioactive elements present in rocks. It involves detecting the ratios of different isotopes of the same element in the material of the fossil.
    Heredity and Evolution Class 10 Notes Science Chapter 8

(iii) Evolution by Stages

  • Evolution is a gradual process occurring over thousands of years, shaping complex organs through numerous DNA changes. Distinct features, like octopus and vertebrate eyes, suggest independent evolution. Changes initially serving one purpose may transform into entirely different functions over time, such as feathers initially for insulation in reptiles like dinosaurs, later adapted by birds for flight.
  • Dissimilar structures can evolve from common ancestors, illustrated by various vegetables derived from wild cabbage through artificial selection. This highlights the importance of DNA changes in understanding evolutionary relationships, as comparing DNA among species provides insights into their evolutionary processes.

4. Evolution Should Not be Equated With Progress (Old NCERT)

  • The evolution of a new species doesn't mean the old one is inferior; it signifies adaptation to changing environmental conditions. Evolution, driven by natural selection and genetic drift, creates diverse populations unable to reproduce with the original species. For instance, humans and chimpanzees, though sharing common ancestors, have evolved separately.
  • Evolution doesn't imply progress; each species diversifies for reproduction and survival, adapting to its environment. Complex organs develop, but simpler species like bacteria still exist in diverse conditions. Humans, considered evolved, are just another species in this ongoing process.
  • In human evolution, diverse features and colors exist, but grouping based on skin color lacks biological justification. All humans are part of the same species, Homo sapiens, with a vast gene pool generating variations in appearance, abilities, etc. Despite global dispersion, humans originated in Africa, migrating worldwide over time, creating a diverse gene pool as the population expanded.

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FAQs on Heredity and Evolution Class 10 Notes Science Chapter 8

1. What is meant by the accumulation of variation during reproduction?
Ans. The accumulation of variation during reproduction refers to the process in which offspring inherit traits from their parents, resulting in a combination of genetic information that is different from both parents. This variation is due to the mixing and recombination of genes during sexual reproduction, leading to the production of genetically diverse individuals within a population.
2. How does heredity play a role in the accumulation of variation during reproduction?
Ans. Heredity is the transmission of traits from parents to their offspring. It plays a crucial role in the accumulation of variation during reproduction. Offspring inherit a combination of genetic information from both parents, which results in new combinations of traits. This variation is essential for the survival and adaptation of species to changing environments.
3. Why is the accumulation of variation important for the survival of species?
Ans. The accumulation of variation is important for the survival of species because it provides the raw material for natural selection to act upon. In a changing environment, individuals with advantageous traits have a higher chance of survival and reproduction. The accumulation of variation ensures that there is a diverse pool of genetic information from which natural selection can select individuals with traits that are better suited for survival and reproduction.
4. How does sexual reproduction contribute to the accumulation of variation during reproduction?
Ans. Sexual reproduction contributes to the accumulation of variation by combining genetic information from two parents. During sexual reproduction, gametes (sperm and egg cells) from each parent unite, resulting in the formation of a new individual with a unique combination of genes. This recombination and mixing of genes create genetic variation in offspring, leading to the accumulation of variation within a population.
5. What are some factors that can influence the accumulation of variation during reproduction?
Ans. Several factors can influence the accumulation of variation during reproduction. These include genetic mutations, genetic recombination during meiosis, environmental factors, and natural selection. Genetic mutations introduce new variations into the gene pool, while genetic recombination shuffles existing genetic information. Environmental factors can also influence the expression of certain traits, leading to variation. Finally, natural selection acts upon this variation to favor individuals with advantageous traits, leading to the accumulation of beneficial variations over time.
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