Quick Summary
Heredity explains how characters pass from parents to offspring. Focus on genes, chromosomes, Mendel's experiments, dominant and recessive traits, and simple inheritance crosses.
Key Definitions
- Heredity: Transmission of characters from parents to offspring.
- Variation: Differences among individuals of the same species.
- Gene: Unit of heredity.
- Chromosome: Thread-like structure carrying genes.
- DNA: Genetic material that carries hereditary information.
- Dominant trait: Trait expressed even when one copy is present.
- Recessive trait: Trait expressed only when both copies are recessive.
- Genotype: Genetic constitution of an organism.
- Phenotype: Observable character of an organism.
Mendel's Work
Mendel studied pea plants because they had clear contrasting characters and were easy to grow. His experiments showed how traits are inherited from one generation to the next.
Important Terms
| Term | Meaning |
|---|---|
| Homozygous | Two identical alleles |
| Heterozygous | Two different alleles |
| Allele | Alternative form of a gene |
| Monohybrid cross | Cross involving one character |
Monohybrid Cross Strategy
Use symbols carefully. For example, T may represent tall and t may represent dwarf. Write parent genotypes, gametes, offspring genotypes, and phenotype ratio.
Worked Monohybrid Cross (Tall × Dwarf)
Tall (TT) is dominant; dwarf (tt) is recessive.
This proves Law of Dominance (F1 all tall) and Law of Segregation (recessive trait reappears in F2).
Mendel's Laws
- Law of Dominance: in a pair, the dominant allele is expressed.
- Law of Segregation: the two alleles separate during gamete formation, one going to each gamete.
- Law of Independent Assortment: alleles of different traits are inherited independently (shown by the dihybrid 9:3:3:1 ratio).
Sex Determination
In humans, females have XX chromosomes and males have XY chromosomes. The sperm determines the sex of the child because it may carry either X or Y.
Textbook Solution Strategy
1. Define the genetic term clearly. 2. For crosses, write parent genotype first. 3. List gametes. 4. Use a Punnett square if needed. 5. Write genotype ratio and phenotype ratio separately.
Important Questions
One Mark
- Define heredity.
- What is a gene?
- Name the father of genetics.
Two Marks
- Differentiate between genotype and phenotype.
- What are dominant and recessive traits?
- Why did Mendel select pea plants?
Five Marks
- Explain Mendel's monohybrid cross.
- Describe sex determination in humans.
- Explain genes, DNA, and chromosomes.
Common Mistakes
- Mixing genotype ratio with phenotype ratio.
- Writing recessive trait as always absent.
- Forgetting that genes are located on chromosomes.
- Saying the mother determines sex of the child in humans.
Revision Checklist
- Genetic terms revised
- Mendel's pea plant work understood
- Monohybrid cross practised
- Sex determination chart revised
- Dominant and recessive traits memorised