BackMendelian Genetics: Mendel's Experiments and Laws
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Mendel's Experiments
Introduction to Mendel's Work
Gregor Mendel was an Austrian monk who pioneered the study of genetics through experiments with pea plants. His systematic approach laid the foundation for the principles of inheritance.
Pure lines: Mendel used pure lines, meaning all offspring produced by pure line mating are genetically identical for the trait in question.
Example: Yellow-seeded pure line mating produces only yellow-seeded offspring.
Generational Terminology
Mendel labeled each generation in a specific way to track inheritance patterns:
Parental (P) Generation: The initial mating that occurs.
First Filial (F1) Generation: Offspring produced from parental mating.
Self-mating: F1 plants can self-fertilize (one plant's pollen fertilizes its own ovules).
Cross-fertilization: Pollen from one plant fertilizes another plant's ovules.
Second Filial (F2) Generation: Offspring produced from F1 self-mating.
Example: Mendel's Crosses
One of Mendel's classic experiments involved crossing yellow-seeded and green-seeded pea plants:
Parental: Yellow (pure) × Green (pure)
F1 Generation: All offspring were yellow-seeded.
F2 Generation: After self-mating F1 plants, offspring included both yellow and green-seeded plants.
Generation | Seed Color | Number of Plants |
|---|---|---|
F2 | Yellow | 6022 |
F2 | Green | 2001 |
Total F2 plants | 8023 | |
Additional info: The observed ratio of yellow to green seeds in F2 is approximately 3:1, a classic Mendelian ratio.
F3 Generation Analysis
F3 from yellow F2: 3/4 yellow, 1/4 green
F3 from green F2: 100% green
These results demonstrate the segregation and reappearance of traits across generations.
Key Observations from Mendel's Crosses
Yellow-seeded plants always produced at least some yellow-seeded offspring.
Selfed green-seeded plants only produced green-seeded offspring.
The green-seeded trait could skip generations, reappearing after being absent.
Mendel's Laws
Properties of Heredity
Mendel deduced that a hereditary factor (now known as a gene) is necessary for producing a trait. Each gene comes in two forms, called alleles, and one allele can be dominant over the other.
Gene: A unit of heredity responsible for a trait.
Allele: Different forms of a gene.
Dominant allele: Expressed trait when present.
Recessive allele: Masked by dominant allele in heterozygotes.
Mendel's Three Laws
Law of Segregation: Alleles separate during meiosis so each gamete contains a single allele for each trait.
Each parent contributes one allele to offspring.
Law of Dominance: Some alleles are dominant and mask the effect of recessive alleles.
Law of Independent Assortment: Genes for different traits segregate independently during gamete formation.
Example: Flower Color Cross
A cross between white (W) and red (R) flowers demonstrates Mendel's laws:
Parent 1 | Parent 2 | F1 Offspring |
|---|---|---|
White (WW) | Red (RR) | All Red (WR) |
When F1 offspring are crossed, the F2 generation shows a 3:1 ratio of red to white flowers, illustrating segregation and dominance.
Key Terms and Concepts
Genotype: Genetic makeup of an organism (e.g., YY, Yy, yy).
Phenotype: Observable trait (e.g., yellow or green seeds).
Monohybrid cross: A cross involving one trait.
Test cross: Crossing an individual with a homozygous recessive to determine genotype.
Formulas and Ratios
Mendelian Ratio (Monohybrid F2):
Genotypic Ratio (Monohybrid F2):
Additional info: These ratios are foundational for predicting outcomes in genetic crosses and understanding inheritance patterns.
