This topic covers the basic principles of genetics, including the history of genetics, Mendelian inheritance, and key terms such as alleles, genotype, and phenotype.
Introduction to Genetics – Study Notes
1. What Is Genetics?
- Definition: The branch of biology that studies heredity, variation, and the mechanisms by which traits are passed from one generation to the next.
- Key Questions:
- How are traits transmitted?
- Why do individuals of the same species differ?
- What molecular structures carry hereditary information?
2. Historical Milestones
| Year |
Scientist |
Contribution |
| 1865 |
Gregor Mendel |
Established the laws of inheritance using pea plants (dominant/recessive traits, segregation, independent assortment). |
| 1902 |
Walter Sutton & Theodor Boveri |
Proposed the chromosome theory of inheritance – genes reside on chromosomes. |
| 1953 |
James Watson & Francis Crick |
Described the double‑helix structure of DNA. |
| 1961 |
Marshall Nirenberg & Johann Matthaei |
Deciphered the first codon, linking DNA to protein synthesis. |
| 1970s‑80s |
Recombinant DNA technology |
Enabled gene cloning, sequencing, and manipulation. |
| 2003 |
Human Genome Project |
Completed the first draft of the human genome (~3 billion base pairs). |
3. Core Concepts
3.1 Genes and Alleles
- Gene: A functional unit of DNA that encodes a product (usually a protein or functional RNA).
- Allele: Different versions of the same gene that arise by mutation.
3.2 Chromosomes
- Linear DNA molecules packaged with proteins (histones).
- Humans: 46 chromosomes (23 pairs) – 22 autosomes + 1 pair of sex chromosomes (XX or XY).
3.3 DNA → RNA → Protein (Central Dogma)
- Replication: DNA copies itself (semi‑conservative).
- Transcription: DNA → messenger RNA (mRNA).
- Translation: mRNA → polypeptide chain (protein) via ribosomes and tRNA.
3.4 Mendelian Inheritance
| Law |
Principle |
Typical Ratio (monohybrid cross) |
| Segregation |
Two alleles for a trait separate during gamete formation. |
3 (dominant) : 1 (recessive) |
| Independent Assortment |
Genes for different traits segregate independently (if on different chromosomes). |
9 : 3 : 3 : 1 (dihybrid cross) |
| Dominance |
Dominant allele masks recessive allele in heterozygote. |
— |
3.5 Non‑Mendelian Patterns
- Incomplete dominance: Heterozygote phenotype intermediate (e.g., pink flowers from red × white).
- Codominance: Both alleles expressed (e.g., AB blood type).
- Multiple alleles: More than two alleles in a population (e.g., ABO blood groups).
- Polygenic inheritance: Many genes contribute to a trait (e.g., height, skin color).
- Sex‑linked inheritance: Genes located on sex chromosomes (e.g., hemophilia, red‑green color blindness).
- Mitochondrial inheritance: Maternal transmission of mitochondrial DNA.
4. Molecular Basis of Variation
| Source of Variation |
Mechanism |
Example |
| Point mutation |
Single‑base substitution, insertion, or deletion. |
Sickle |