Quick Summary
Inheritance explains resemblance between parents and offspring; variation explains differences. Mendel's experiments on pea plants laid the foundation for understanding genes, alleles, dominance, segregation, and independent assortment.
Core Terms
- Gene
- Allele
- Genotype
- Phenotype
- Homozygous
- Heterozygous
- Dominant
- Recessive
Mendel's Laws
Law of Dominance
In a heterozygote, one allele may express while the other remains masked.
Law of Segregation
The two alleles of a gene separate during gamete formation.
Law of Independent Assortment
Alleles of different genes assort independently during gamete formation when genes are not linked.
Monohybrid and Dihybrid Crosses
| Cross | Main Focus | Typical Ratio |
|---|---|---|
| Monohybrid | One pair of contrasting traits | 3:1 phenotype in F2 |
| Dihybrid | Two pairs of contrasting traits | 9:3:3:1 phenotype in F2 |
Beyond Simple Dominance
- Incomplete dominance
- Codominance
- Multiple alleles
- Pleiotropy
- Polygenic inheritance
Sex Determination
- XX-XY type in humans
- Male is heterogametic
- Female is homogametic
Important Questions
One Mark
1. Define heredity. 2. What is genotype? 3. State the F2 phenotypic ratio of a monohybrid cross.
Short Answer
1. Distinguish between genotype and phenotype. 2. Explain law of segregation. 3. Compare incomplete dominance and codominance.
Long Answer
1. Explain Mendel's monohybrid cross. 2. Explain dihybrid cross and law of independent assortment. 3. Explain sex determination in humans.
Common Mistakes
- Mixing genotype ratio with phenotype ratio.
- Writing dominance as destruction of the recessive allele.
- Forgetting gamete formation in law of segregation.
- Confusing incomplete dominance with codominance.
Revision Checklist
- [ ] Key definitions
- [ ] Three Mendelian laws
- [ ] Monohybrid and dihybrid ratios
- [ ] Incomplete dominance vs codominance
- [ ] Human sex determination