Class 10 Science Notes Chapter 9 (Heredity and evolution) – Science Book
Detailed Notes with MCQs of Chapter 9: Heredity and Evolution. This is a crucial chapter, not just for your board exams but also forms the basis for many questions in competitive government exams. Pay close attention to the concepts, definitions, and Mendel's experiments.
Chapter 9: Heredity and Evolution - Detailed Notes for Government Exam Preparation
(Note: While the 'Evolution' part has been significantly reduced in the latest NCERT rationalized syllabus for Class 10 boards, some basic concepts might still appear in competitive exams drawing from general science or older syllabus patterns. We will cover the core 'Heredity' part in detail and touch upon essential related concepts.)
1. Introduction to Heredity and Variation
- Heredity: The transmission of characteristics (traits) from parents to their offspring. It is the reason why offspring resemble their parents.
- Genetics: The branch of biology that deals with heredity and variation.
- Variation: The differences in characteristics shown by individuals of a species and also by offspring of the same parents.
- Importance of Variation: Variation is essential for the survival of a species. It allows organisms to adapt to changing environments. Individuals with favourable variations are better suited to survive and reproduce (basis of natural selection).
- Sources of Variation:
- During Reproduction: Variations arise during the process of DNA copying (replication). While DNA copying mechanisms are highly accurate, minor errors can occur, leading to variations.
- Sexual Reproduction: Leads to greater variation compared to asexual reproduction because it involves the combination of genetic material from two different parents (through meiosis and fertilization), creating unique combinations of genes.
- Asexual Reproduction: Variations are fewer and arise mainly due to inaccuracies in DNA replication.
2. Mendel's Contributions to Inheritance
- Gregor Johann Mendel (1822-1884): Known as the 'Father of Genetics'. He conducted experiments on garden pea plants (Pisum sativum) to understand the principles of inheritance.
- Why Pea Plants?
- Easy to grow and maintain.
- Short life cycle (allowing multiple generations to be studied quickly).
- Have easily observable, contrasting characteristics (e.g., tall/dwarf height, round/wrinkled seeds, yellow/green seeds).
- Naturally self-pollinating but can be easily cross-pollinated manually.
- Key Terms Used in Genetics:
- Gene: A segment of DNA that codes for a specific protein, which in turn controls a specific trait. Mendel called them 'factors'.
- Alleles: Different forms of the same gene (e.g., the gene for height has alleles 'T' for tallness and 't' for dwarfness).
- Dominant Allele: An allele that expresses itself in the presence of another allele (represented by a capital letter, e.g., 'T').
- Recessive Allele: An allele that expresses itself only in the absence of the dominant allele (represented by a small letter, e.g., 't').
- Genotype: The genetic makeup of an individual for a particular trait (e.g., TT, Tt, tt).
- Phenotype: The observable physical characteristic of an individual resulting from their genotype (e.g., Tall, Dwarf).
- Homozygous: Having two identical alleles for a trait (e.g., TT or tt). Also called pure-bred.
- Heterozygous: Having two different alleles for a trait (e.g., Tt). Also called hybrid.
- F1 Generation (First Filial Generation): The first generation of offspring obtained by crossing two parent organisms.
- F2 Generation (Second Filial Generation): The generation obtained by self-pollinating or interbreeding individuals of the F1 generation.
3. Mendel's Experiments and Laws of Inheritance
-
Monohybrid Cross: A cross involving the inheritance of a single pair of contrasting characteristics.
- Experiment: Mendel crossed pure-bred tall pea plants (TT) with pure-bred dwarf pea plants (tt).
- F1 Generation: All plants were tall (Genotype Tt). This showed that 'Tallness' (T) is dominant over 'dwarfness' (t).
- F2 Generation: When F1 plants (Tt) were self-pollinated, the offspring were both tall and dwarf.
- Phenotypic Ratio: Tall : Dwarf = 3 : 1
- Genotypic Ratio: TT : Tt : tt = 1 : 2 : 1
- Conclusion (Law of Segregation): During gamete formation, the two alleles for a trait separate (segregate) from each other, so that each gamete receives only one allele.
-
Dihybrid Cross: A cross involving the inheritance of two pairs of contrasting characteristics simultaneously.
- Experiment: Mendel crossed pea plants having round yellow seeds (RRYY) with plants having wrinkled green seeds (rryy). (R=Round, r=wrinkled; Y=Yellow, y=green).
- F1 Generation: All plants had round yellow seeds (Genotype RrYy). This showed Round (R) is dominant over wrinkled (r), and Yellow (Y) is dominant over green (y).
- F2 Generation: When F1 plants (RrYy) were self-pollinated, four types of phenotypes were observed: Round Yellow, Round Green, Wrinkled Yellow, and Wrinkled Green.
- Phenotypic Ratio: Round Yellow : Round Green : Wrinkled Yellow : Wrinkled Green = 9 : 3 : 3 : 1
- Conclusion (Law of Independent Assortment): When two or more pairs of traits are inherited together, the alleles for each trait assort independently of the alleles for other traits during gamete formation. (This applies to genes located on different chromosomes).
4. How are Traits Expressed? (Mechanism of Inheritance)
- Genes control traits: Genes located on chromosomes in the cell nucleus contain the information for protein synthesis.
- Proteins and Traits: Proteins (often enzymes) control various metabolic pathways and cellular processes, which ultimately determine the physical characteristics or traits of an organism.
- Example: A gene for 'Tallness' might code for an enzyme that produces a growth hormone efficiently, leading to a tall plant. A modified allele (for 'dwarfness') might code for a less efficient enzyme, resulting in less growth hormone and a dwarf plant.
5. Sex Determination
- The process by which the sex of a newborn individual is determined.
- Sex Chromosomes: Special chromosomes involved in determining sex.
- In Humans:
- Females have two similar sex chromosomes: XX.
- Males have two different sex chromosomes: XY.
- Gamete Formation:
- Females produce eggs, all containing one X chromosome.
- Males produce sperm, half containing an X chromosome and half containing a Y chromosome.
- Fertilization:
- If an X-carrying sperm fertilizes the egg (X), the zygote (XX) develops into a female.
- If a Y-carrying sperm fertilizes the egg (X), the zygote (XY) develops into a male.
- Conclusion: The father's sperm determines the sex of the child. There is always a 50% probability of having a male child and a 50% probability of having a female child.
6. Basic Concepts Related to Evolution (Relevant for General Science)
- Evolution: The gradual change in the inherited characteristics of biological populations over successive generations. It occurs due to the accumulation of variations over long periods.
- Acquired Traits vs. Inherited Traits:
- Acquired Traits: Characteristics developed by an individual during its lifetime due to environmental influences, use/disuse of organs, or learning (e.g., muscle development through exercise, learning to ride a bicycle). These traits affect somatic (body) cells and are not passed on to the next generation because they do not cause changes in the DNA of germ cells (sperm/egg).
- Inherited Traits: Characteristics passed from parents to offspring because they are controlled by genes present in the germ cells (e.g., eye colour, hair type, height potential). These are the traits subject to evolutionary change.
- Speciation: The formation of new and distinct species in the course of evolution. Key factors include geographical isolation, genetic drift, and natural selection acting on variations.
- Evidence for Evolution (Often asked in GK/Reasoning):
- Homologous Organs: Organs having a similar basic structure and origin but modified to perform different functions in different species (e.g., forelimbs of humans, bats, whales, cheetahs). Indicate common ancestry.
- Analogous Organs: Organs having different basic structures and origins but performing similar functions (e.g., wings of birds and wings of insects). Indicate convergent evolution (adaptation to similar environments).
- Fossils: Preserved remains or traces of organisms from the past. They provide direct evidence of past life forms and evolutionary changes.
Multiple Choice Questions (MCQs)
-
Which of the following is known as the 'Father of Genetics'?
a) Charles Darwin
b) Gregor Mendel
c) Hugo de Vries
d) Jean-Baptiste Lamarck -
In Mendel's monohybrid cross between tall (TT) and dwarf (tt) pea plants, the phenotypic ratio observed in the F2 generation was:
a) 1:1
b) 1:2:1
c) 3:1
d) 9:3:3:1 -
An organism's genetic constitution for a particular trait is called its:
a) Phenotype
b) Genotype
c) Allele
d) Genome -
Which of the following represents a heterozygous condition for a trait?
a) TT
b) tt
c) Tt
d) TTT -
In humans, the sex of the child is determined by:
a) The mother's egg cell
b) The father's sperm cell
c) The autosomes of the mother
d) The autosomes of the father -
The phenomenon where different genes independently separate from one another during gamete formation is known as:
a) Law of Dominance
b) Law of Segregation
c) Law of Independent Assortment
d) Linkage -
Which of the following is an example of an acquired trait in humans?
a) Eye colour
b) Hair texture
c) Blood group
d) Scar from an injury -
Wings of a bat and wings of an insect are examples of:
a) Homologous organs
b) Vestigial organs
c) Analogous organs
d) Fossilized organs -
Variations are more prominent in organisms reproducing through:
a) Asexual reproduction
b) Budding
c) Spore formation
d) Sexual reproduction -
If a round, green seeded pea plant (RRyy) is crossed with a wrinkled, yellow seeded pea plant (rrYY), what will be the phenotype of the F1 generation? (Assume R=Round, r=wrinkled; Y=Yellow, y=green)
a) Round, green
b) Wrinkled, yellow
c) Round, yellow
d) Wrinkled, green
Answer Key for MCQs:
- b) Gregor Mendel
- c) 3:1
- b) Genotype
- c) Tt
- b) The father's sperm cell
- c) Law of Independent Assortment
- d) Scar from an injury
- c) Analogous organs
- d) Sexual reproduction
- c) Round, yellow (Genotype will be RrYy, and both R and Y are dominant)
Study these notes thoroughly. Focus on understanding the terms, Mendel's experiments, the ratios, and the mechanism of sex determination. Remember the difference between acquired and inherited traits. Good luck with your preparation!