CBSE Class 10 Biology Revision Notes for Heredity and Evolution
Biology is the study of life. It is the study of living organisms and how they interact with the environment. Biology recognizes the cell as the basic unit of life, genes as the unit of heredity and evolution as an engine which boosts the formation of new species. The study of life has helped in shaping the world. It has credible answers to why things happen in a scientific manner.
In CBSE Class10 Science, Biology is one of the scoring subjects. Through the CBSE Class 10 Biology Syllabus, you get to learn about life processes, natural resources, environment, control and coordination and reproduction. As your Biology Guide for Class 10 CBSE exams, we provide you with CBSE Class 10 Biology notes to help you understand the concepts.
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Heredity and Evolution
Heredity and Variation
- Living organisms have certain recognisable heritable features such as height, complexion, colour of hair and eyes, shape of nose and chin etc. These are called characters.
- The alternative forms of a character are called traits. The inheritable characteristics or traits may be morphological, anatomical, physiological or reproductive.
- The transmission or passing of genetically based characters or traits from the parents to their offspring is called heredity.
- The occurrence of small differences or changes among the individuals of a species is called variation. Hereditary variations are of great importance in the process of evolution of a new species.
- Asexual reproduction results in a small amount of variation as compared to sexual reproduction.
- Genes are the specific parts of chromosomes or deoxyribonucleic acid (DNA) segments which determine hereditary characteristics.
- Every gene has two alternative forms for a character, each of which produces different effects in an organism. These alternative forms are called alleles. Example: In case of pea plants, the stem height is controlled by two alleles—one for tallness and the other for dwarfness.
- Of the two alleles of a gene, one is dominant, i.e. super ruling and the other is recessive, i.e. subordinate or submissive. A dominant allele is the allele which hides or masks the expression of its corresponding allele, which in turn becomes recessive.
- A contrasting pair of alleles constitutes an allelomorph.
- The genetic constitution of an organism is called its genotype. It is the description of genes present in an organism. The genotype of a tall plant could be TT or Tt, while that of a dwarf plant is tt.
- Phenotype refers to the observable characteristics or the expressed shown character of an organism. Example: Tall and dwarf are the phenotypes of a plant because these traits are visible to us.
- When two parents are crossed to produce progeny, their progeny is called the first filial generation or F1 generation.
- When the first generation progeny or F1 progeny is crossed amongst themselves to produce a second generation progeny, this progeny is called the second filial generation or F2 generation.
- A new form of plant resulting from a cross of different varieties of a plant is known as a hybrid.
Rules for Inheritance of Traits
- Mendel conducted experiments on pea plants (Pisum sativum) and studied the inheritance of certain traits.
- A cross which involves only a single pair of contrasting characters is called a monohybrid cross.
Example: A cross between a tall pea plant (TT) and a dwarf pea plant (tt).
Phenotypic ratio: 3 : 1
Genotypic ratio: 1 : 2 : 1
- The results of the monohybrid cross enabled Mendel to formulate his first law of inheritance, which is called the law of segregation. It states that- ‘The characteristics or traits of an organism are determined by internal factors, which occur in pairs. Only one of a pair of such factors can be present in a single gamete’.
- A cross which involves plants with two pairs of contrasting characters is called a dihybrid cross. Example: A cross of pea plants having round and yellow seeds (RRYY) and plants with wrinkled and green seeds (rryy).
Phenotypic ratio: 9 : 3 : 3 : 1
Genotypic ratio: 1 : 4 : 1 : 1 : 1 :2 : 2 : 2 : 2
- The results of the dihybrid cross enabled Mendel to formulate his second law of inheritance, which is called the law of independent assortment. It states that- ‘In the inheritance of more than one pair of traits in a cross simultaneously, the factors responsible for each pair of traits are distributed independently to the gametes’.
- DNA (Deoxyribonucleic acid) is a highly complex molecule with a spirally coiled, double helical structure which appears like a ladder.
Differences between Inherited and Acquired Traits
|1. Characteristics inherited from the previous generation.||1. Characteristics which develop in response to the environment and cannot be inherited.|
|2. Occur due to a change in genes or DNA.||2. No change in genes or DNA is involved.|
|3. Pass on from one generation to another.||3. Cannot pass on from one generation to another.|
|4. Examples: Red curly hair, brown eyes||4. Examples: Cycling, swimming|
- The phenomenon or process which determines whether a developing embryo will be a male or a female is known as sex determination.
- In most organisms, environmental and genetic or chromosomal mechanisms are mainly responsible for the determination of sex of an individual.
- Humans have 22 pairs of autosomes and 1 pair of sex chromosome.
- Females have similar sex chromosomes XX, whereas males have a dissimilar pair, i.e. XY. All eggs carry the X chromosome, while sperms may either carry an X or a Y chromosome.
- The sex of a child depends on whether the egg fuses with the sperm carrying an X chromosome (resulting in a female) or with the sperm carrying a Y chromosome (resulting in a male).
- Evolution can be defined as the formation of more complex organisms from pre-existing simpler organisms over a certain period. It is a slow, but progressive, natural, sequential development or transformation of animals and plants from ancestors of different forms and functions.
- • Variation and heredity are the two basic factors of evolution. The selection of variants by environmental factors forms the basis of evolutionary processes.
Evidences for Evolution
- A large amount of information has been collected over the last 200 years to support the theory of organic evolution. Such supporting information which helps us in accepting the theory is called evidence.
Study of Fossils
Darwin's Theory of Evolution
- According to Darwin’s Theory of Natural Selection, organisms produce more offspring than they need for their existence. They compete among themselves and fight with the environmental factors for their various needs in life. In the struggle for existence, those with favourable variations continue to exist and those with unfavourable variations die out. Thus, a new species is formed by natural selection.
- A species is a population of organisms consisting of similar individuals which can breed together and produce fertile offspring.
- The process by which a new species develops from the existing species is known as speciation.
Important Factors which Contribute to Speciation
Evolution by Stages
- The great variety of organisms existing on the Earth is due to changes which have occurred gradually in stages and have resulted in the evolution of a new species.
- The occurrence of different stages of evolution in a species is not because of a single DNA change.
Evolution of Eyes
Evolution of Feathers
Evolution by Artificial Selection
Evolution and Classification
- The principles of classification help us trace the evolutionary relationships of the species around us.
- In 1859, Charles Darwin first described this concept of evolution in his book The Origin of Species.
- Certain groups of organisms have ancient body designs and are referred to as primitive or lower organisms. Some organisms have acquired their body designs relatively recently and are called advanced or higher organisms.
- There is a strong possibility that complexity within organisms increases with an increase in evolutionary time. Hence, we can say that older organisms are relatively simpler, while younger organisms are more complex.
Evolution Should Not Be Equated With Progress
- Evolution has resulted in the generation of new varieties of species. It results in the production of diverse life forms subjected to environmental selection. The only progress which has occurred due to evolution is the emergence of more complex body designs of organisms.
- When we consider the evolutionary history of man, we often say that human beings evolved from chimpanzees. However, this is not the case. In fact, both chimpanzees and human beings had a common ancestor a long time ago. The two offspring of that common ancestor evolved differently to form the modern day chimpanzees and human beings.
- Human evolution has been studied using various tools of tracing evolutionary relationships such as excavating, carbon-dating, studying fossils and determining DNA sequences.
- Research reveals that the early members of Homo sapiens came from Africa. About hundred years ago, some of our ancestors left Africa, while others stayed back. So irrespective of where we live, all human species are natives of Africa. The earliest fossils of human beings include the genus Australopithecus, followed by Homo habilis, Homo erectus, Homo heidelbergensis and finally modern day man Homo sapiens.
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