Heredity Class 10 ||Science|| Chapter 8 Notes
1. Introduction to Heredity
Heredity is the process by which traits and characteristics are passed from parents to their offspring through genes. These traits can be physical (e.g., eye color, height) or behavioral. The study of heredity and how traits are passed down from one generation to the next is called genetics.
2. Mendel’s Experiments
Gregor Johann Mendel is known as the "Father of Genetics." He conducted experiments on pea plants to understand how traits are inherited. Mendel chose pea plants because they had easily observable traits, such as flower color and seed shape.
Key Concepts from Mendel’s Experiments:
Monohybrid Cross: Mendel studied inheritance of a single trait (e.g., flower color) and crossed pure-bred pea plants (one with white flowers and one with purple flowers).
- F1 Generation: All plants had purple flowers (dominant trait).
- F2 Generation: Plants showed a 3:1 ratio of purple to white flowers, revealing that traits are inherited in specific proportions.
Dihybrid Cross: Mendel studied the inheritance of two traits at the same time (e.g., seed color and seed shape). The results showed a 9:3:3:1 ratio in the F2 generation, demonstrating that traits are inherited independently of each other (Law of Independent Assortment).
Monohybrid Cross: Mendel studied inheritance of a single trait (e.g., flower color) and crossed pure-bred pea plants (one with white flowers and one with purple flowers).
- F1 Generation: All plants had purple flowers (dominant trait).
- F2 Generation: Plants showed a 3:1 ratio of purple to white flowers, revealing that traits are inherited in specific proportions.
Dihybrid Cross: Mendel studied the inheritance of two traits at the same time (e.g., seed color and seed shape). The results showed a 9:3:3:1 ratio in the F2 generation, demonstrating that traits are inherited independently of each other (Law of Independent Assortment).
Mendel’s Laws of Inheritance:
- Law of Dominance: In a cross between pure organisms, only one trait (the dominant one) appears in the offspring.
- Law of Segregation: Each individual carries two alleles for a trait, but only one is passed to the offspring.
- Law of Independent Assortment: Different traits are inherited independently of one another.
3. Genetics and Heredity
Genes are units of heredity that carry instructions for traits. They are located on chromosomes inside the nucleus of cells. Each individual has two copies (alleles) of each gene, one inherited from each parent.
- Dominant Allele: The trait expressed in the presence of another allele (e.g., purple flowers).
- Recessive Allele: The trait masked by a dominant allele (e.g., white flowers).
- Homozygous: When both alleles for a trait are the same (e.g., TT or tt).
- Heterozygous: When the alleles for a trait are different (e.g., Tt).
Genotype: The genetic makeup of an organism (e.g., TT, Tt, or tt). Phenotype: The physical expression of the trait (e.g., tall or short).
4. Sex Determination
The process by which the sex of an organism is determined is known as sex determination. In humans, sex is determined by specific chromosomes:
- Females have two X chromosomes (XX).
- Males have one X and one Y chromosome (XY).
During reproduction, the male sperm determines the sex of the offspring:
- If an X-bearing sperm fertilizes the egg, the child will be female (XX).
- If a Y-bearing sperm fertilizes the egg, the child will be male (XY).
5. Inheritance of Traits
Traits can be classified as:
- Acquired Traits: Traits that are developed during an individual’s lifetime due to environmental factors or behavior (e.g., learning a skill). These traits are not passed to the next generation.
- Inherited Traits: Traits that are passed from parents to offspring through genes (e.g., eye color, height).
6. Variation and Evolution
Variation refers to the differences among individuals of the same species. Variations are caused by:
- Genetic factors: Changes or mutations in DNA.
- Environmental factors: Influence of surroundings.
Variations are important for evolution because they provide raw material for natural selection. Favorable variations increase an organism’s chance of survival and reproduction.
7. Mendelian and Non-Mendelian Inheritance
- Mendelian Inheritance: Follows the basic principles of inheritance as established by Mendel’s laws (dominant-recessive patterns).
- Non-Mendelian Inheritance: Includes incomplete dominance, codominance, and multiple alleles:
- Incomplete Dominance: Neither allele is completely dominant, leading to a blend of traits (e.g., red and white flowers producing pink flowers).
- Codominance: Both alleles are expressed equally in the phenotype (e.g., blood type AB, where both A and B alleles are expressed).
- Multiple Alleles: More than two alleles control a trait (e.g., ABO blood group system in humans).
- Incomplete Dominance: Neither allele is completely dominant, leading to a blend of traits (e.g., red and white flowers producing pink flowers).
- Codominance: Both alleles are expressed equally in the phenotype (e.g., blood type AB, where both A and B alleles are expressed).
- Multiple Alleles: More than two alleles control a trait (e.g., ABO blood group system in humans).
Summary:
- Heredity is the transmission of traits from parents to offspring through genes.
- Mendel’s Laws of Inheritance (Dominance, Segregation, and Independent Assortment) explain how traits are passed down.
- Traits can be dominant or recessive, and the combination of alleles determines the phenotype of an organism.
- Sex determination in humans is based on X and Y chromosomes.
- Acquired traits are not inherited, while inherited traits are passed through generations.
- Variation within species is essential for evolution and adaptation to changing environments.
These principles of heredity explain how genetic traits are passed down and how variation within species contributes to evolution.