🧬 Basic Genetics Terms
Key Definitions
- Heredity: Transmission of characters from parents to offspring
- Variations: Differences between individuals of same species
- Genetics: Science dealing with heredity and variations
- Gene: Basic unit of biological information (DNA sequence)
- Locus: Position of a gene on chromosome
- Allele: Alternative forms of a gene at same locus
- Gene Pool: All genes/alleles in a breeding population
Genetic Concepts
- Homozygous: Same alleles (RR or rr)
- Heterozygous: Different alleles (Rr)
- Phenotype: Physical appearance (Round seeds)
- Genotype: Genetic makeup (RR, Rr, rr)
- Dominant: Masks effect of other allele
- Recessive: Effect masked by dominant allele
🌟 Did You Know? Gregor Mendel is called the “Father of Genetics” for his pioneering work with pea plants in the 1860s!
🌱 Mendel’s Laws of Inheritance
Law of Segregation
- Alleles segregate during gamete formation
- Each gamete receives only one allele
- Alleles unite randomly at fertilization
- F₂ phenotypic ratio: 3:1
- F₂ genotypic ratio: 1:2:1
Law of Independent Assortment
- Alleles of different traits assort independently
- Applies to genes on different chromosomes
- Dihybrid cross F₂ ratio: 9:3:3:1
- Major source of genetic variation
| Cross Type | Parental Genotypes | F₁ Generation | F₂ Phenotypic Ratio |
|---|---|---|---|
| Monohybrid | RR × rr | All Rr (Round) | 3 Round : 1 Wrinkled |
| Dihybrid | RRYY × rryy | All RrYy | 9:3:3:1 |
🎭 Dominance Relations
| Type | Description | Heterozygote Phenotype | Example |
|---|---|---|---|
| Complete Dominance | One allele completely masks other | Resembles dominant homozygote | Mendel’s pea traits |
| Incomplete Dominance | Both alleles partially expressed | Intermediate between parents | 4 O’clock flower color |
| Co-dominance | Both alleles fully expressed | Shows both traits simultaneously | AB blood group |
| Over Dominance | Heterozygote exceeds both homozygotes | Superior to both parents | Drosophila eye pigments |
💉 Blood Group Systems
A
B
AB
O
ABO Blood Group System
- Discovered by Karl Landsteiner (1901)
- Gene ‘I’ on chromosome 9 with 3 alleles
- Iᴬ and Iᴮ are co-dominant
- Allele ‘i’ is recessive to both
| Blood Group | Genotype | Antigen on RBCs | Antibodies in Plasma | Can Donate To |
|---|---|---|---|---|
| A | IᴬIᴬ, Iᴬi | A | Anti-B | A, AB |
| B | IᴮIᴮ, Iᴮi | B | Anti-A | B, AB |
| AB | IᴬIᴮ | A & B | None | AB only |
| O | ii | None | Anti-A & Anti-B | All groups |
Rh Blood Group & Erythroblastosis Fetalis
- Rh factor discovered in Rhesus monkeys
- Rh⁺: DD or Dd, Rh⁻: dd
- Erythroblastosis fetalis: Maternal-fetal Rh incompatibility
- Controlled by Rh-antiserum injection
🔗 Gene Linkage & Crossing Over
Gene Linkage
- Genes on same chromosome stay together
- Forms linkage groups
- Number of linkage groups = haploid chromosome number
- Humans: 23 linkage groups
- Linked genes don’t assort independently
Crossing Over
- Exchange between non-sister chromatids
- Occurs during meiosis
- Separates linked genes
- Frequency proportional to distance between genes
- Used for gene mapping
🔍 Detection: Test cross with 1:1:1:1 ratio indicates no linkage. Deviation indicates linkage.
⚤ Sex Linkage & Inheritance
Morgan’s Drosophila Experiments
- Discovered sex linkage (1910)
- White eye mutation in male Drosophila
- Eye color gene on X chromosome
- Y chromosome lacks corresponding allele
Human Sex-Linked Disorders
- Hemophilia: X-linked recessive bleeding disorder
- Color Blindness: X-linked recessive
- Duchenne Muscular Dystrophy: X-linked recessive
- Vitamin D Resistant Rickets: X-linked dominant
| Trait Type | Inheritance Pattern | Example | Characteristics |
|---|---|---|---|
| X-linked Recessive | Zigzag inheritance | Hemophilia, Color blindness | More common in males |
| X-linked Dominant | Affected father → all daughters affected | Vitamin D resistant rickets | More common in females |
| Y-linked (Holandric) | Father to all sons | Hypertrichosis of ear | Only in males |
| Sex-limited | Confined to one sex | Beard growth, Milk yield | Anatomical differences |
| Sex-influenced | More common in one sex | Pattern baldness | Hormonal influence |
🚀 Genetics Study Strategies
1
Master Genetic Crosses
Practice Punnett squares for monohybrid (3:1), dihybrid (9:3:3:1), and test crosses. Understand phenotypic vs genotypic ratios.
2
Blood Group Genetics
Create tables for ABO and Rh systems. Understand transfusion compatibility and Rh incompatibility prevention.
3
Pedigree Analysis
Learn to identify autosomal/X-linked, dominant/recessive patterns from pedigree charts. Practice zigzag inheritance patterns.
4
Gene Linkage Problems
Calculate recombination frequencies. Understand that linked genes deviate from Mendelian ratios.