Unit 3
Genetics
Chapter 3.2
Mendelian GeneticsClass 12 Biology – Mendelian Genetics Notes PDF
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Introduction
Mendel studied contrasting characters, performed controlled crosses and analyzed offspring ratios. His results led to the principles of dominance, segregation and independent assortment.
1. Basic Genetic Terms
| Term | Meaning |
|---|---|
| Gene | Unit of heredity controlling a character or contributing to a trait. |
| Alleles | Alternative forms of the same gene. |
| Genotype | Genetic constitution of an organism for a character. |
| Phenotype | Observable expression of a character. |
| Homozygous | Having identical alleles, e.g. TT or tt. |
| Heterozygous | Having different alleles, e.g. Tt. |
| Dominant | Allele expressed in the heterozygous condition. |
| Recessive | Allele masked by a dominant allele in a heterozygote. |
Fig. 1 – Gene, Alleles, Genotype and Phenotype
2. Mendel’s Experimental Work
- Used garden pea because it had clear contrasting characters, short generation time and naturally self-pollinating flowers.
- Established pure-breeding parental lines.
- Performed controlled cross-pollination.
- Studied one pair of contrasting traits in a monohybrid cross and two pairs in a dihybrid cross.
- Counted large numbers of offspring and analyzed characteristic ratios.
Fig. 2 – Mendel’s Basic Crossing Scheme
3. Monohybrid Cross
A monohybrid cross follows inheritance of one pair of contrasting characters.
When F1 plants self:
Fig. 3 – Punnett Square for Monohybrid Cross
4. Dihybrid Cross
A dihybrid cross follows two pairs of contrasting characters simultaneously. For example, seed shape and seed colour can be represented as R/r and Y/y.
Under independent assortment, selfing the F1 produces the classic phenotypic ratio:
Fig. 4 – Gamete Formation in a Dihybrid
Fig. 5 – Dihybrid Phenotypic Ratio
5. Mendel’s Laws
Law of Dominance
In a heterozygote, one allele may express itself while the other is masked.
Law of Segregation
The two alleles of a gene separate during gamete formation, so each gamete receives one allele.
Law of Independent Assortment
Alleles of different genes assort independently during gamete formation when the genes are not linked in a way that prevents independent assortment.
Key Exam Point
Segregation explains the 3:1 monohybrid phenotype; independent assortment explains the 9:3:3:1 dihybrid phenotype under ideal Mendelian conditions.
6. Test Cross and Back Cross
Back cross: crossing an F1 hybrid with either of its parents.
Test cross: crossing an individual showing the dominant phenotype with a homozygous recessive individual to determine the unknown genotype.
Fig. 6 – Test Cross
7. Quick Revision & Exam Points
- Why did Mendel select pea plants?
- Define genotype, phenotype, homozygous and heterozygous.
- Explain a monohybrid cross with a Punnett square.
- State the genotypic and phenotypic ratios of F2.
- Explain a dihybrid cross.
- State Mendel’s laws.
- Differentiate test cross and back cross.
- Monohybrid phenotype = 3:1.
- Monohybrid genotype = 1:2:1.
- Dihybrid phenotype = 9:3:3:1.
- Segregation = allele pair separates.
- Independent assortment = different gene pairs assort independently under Mendelian conditions.
- Test cross uses homozygous recessive parent.
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