Learning Objectives
- Understand the rules of inheritance (Mendel's laws)
- Learn about sex determination in humans
- Understand the concepts of evolution and speciation
- Distinguish between homologous and analogous organs
- Learn about evidences of evolution
Key Concepts
Heredity: Mendel's Contribution
Gregor Mendel studied inheritance using pea plants. He identified dominant and recessive traits.
Monohybrid cross (one trait): TT × tt → F₁ = Tt (all tall). F₁ × F₁ → F₂ = TT : Tt : tt = 1:2:1 (genotypic); Tall : Dwarf = 3:1 (phenotypic).
Dihybrid cross (two traits): F₂ phenotypic ratio = 9:3:3:1. This shows independent assortment of traits.
Mendel's Laws:
- Law of Dominance: In a pair of contrasting traits, one dominates the other.
- Law of Segregation: Alleles separate during gamete formation; each gamete receives one allele.
- Law of Independent Assortment: Genes for different traits are inherited independently.
Sex Determination
In humans, sex is determined by sex chromosomes. Females: XX; Males: XY.
All eggs carry X chromosome. Sperm can carry X or Y. If sperm carries X → girl (XX); if sperm carries Y → boy (XY). Thus, the father determines the sex of the child.
Evolution
Variation: Differences among individuals of the same species. Arises through mutations and sexual reproduction. Variations drive natural selection.
Natural Selection (Darwin): Organisms with favourable variations survive and reproduce more successfully. Over time, this leads to evolution of new species.
Evidence of Evolution
- Homologous organs: Same structural origin but different functions. Example: Forelimbs of whale, bat, horse, and human. Indicate common ancestry (divergent evolution).
- Analogous organs: Different structural origin but similar function. Example: Wings of birds and insects. Indicate convergent evolution.
- Fossils: Preserved remains or impressions of ancient organisms. Show gradual changes over time.
Speciation
Formation of new species due to geographical isolation, genetic drift, natural selection, and reproductive isolation. Over time, separated populations accumulate enough differences to become distinct species.
Summary
Heredity explains how traits are passed from parents to offspring through genes. Mendel's experiments established the laws of inheritance. Sex is determined by X and Y chromosomes. Evolution is the gradual change in species over generations driven by variation and natural selection. Fossils and comparative anatomy provide evidence for evolution.
Important Terms
- Gene
- A unit of heredity; a segment of DNA that codes for a specific protein/trait
- Allele
- Different forms of a gene (e.g., T for tall, t for dwarf)
- Genotype
- The genetic makeup of an organism (e.g., Tt)
- Phenotype
- The observable characteristic of an organism (e.g., tall)
- Speciation
- The formation of new and distinct species through evolution
Quick Revision
- Monohybrid cross F₂ ratio: 3:1 (phenotypic), 1:2:1 (genotypic)
- Dihybrid cross F₂ ratio: 9:3:3:1 (phenotypic)
- Sex determination: XX = female, XY = male; father determines sex
- Homologous organs = common origin, different function (divergent evolution)
- Analogous organs = different origin, similar function (convergent evolution)