Introduction
Mendelian genetics forms the foundation of our understanding of inheritance, explaining how traits are passed from parents to offspring. This guide explores the principles established by Gregor Mendel, the father of genetics, and how they apply to biological inheritance.
Key Concepts
1. Gregor Mendel
- Gregor Mendel was an Austrian monk and scientist who conducted experiments on pea plants in the mid-19th century.
- His work revealed that traits are inherited according to specific patterns, laying the groundwork for modern genetics.
2. Laws of Inheritance
Law of Segregation
- Each individual has two alleles for a given gene, one inherited from each parent.
- These alleles separate (segregate) during the formation of gametes (sperm and egg), meaning each gamete carries only one allele for each gene.
Law of Independent Assortment
- Genes for different traits are distributed to gametes independently of one another.
- This means the inheritance of one trait does not affect the inheritance of another, though this law applies mainly to genes on different chromosomes or those that are far apart on the same chromosome.
Genotype vs. Phenotype
Genotype
- The genotype refers to the genetic makeup of an organism—the specific alleles it carries.
- Example: A pea plant may have a genotype of "TT" or "Tt" for the trait of height.
Phenotype
- The phenotype is the physical expression or appearance of a trait, determined by the genotype.
- Example: A pea plant with the genotype "TT" (homozygous dominant) or "Tt" (heterozygous) will have a tall phenotype.
| Genotype | Phenotype |
|---|---|
| TT | Tall |
| Tt | Tall |
| tt | Short |
Punnett Squares
A Punnett square is a visual tool used to predict the possible genotypes of offspring from a genetic cross. It helps illustrate how alleles are distributed during gamete formation and fertilization.
Example: Monohybrid Cross
Consider a cross between two heterozygous pea plants for the trait of height (T = tall, t = short):
- Parent Genotypes: Tt x Tt
Punnett Square
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
Genotypic Ratio
- 1 TT : 2 Tt : 1 tt
Phenotypic Ratio
- 3 Tall : 1 Short
Conclusion
Mendelian genetics provides a systematic way to understand how traits are inherited from one generation to the next. By analyzing genotypes and predicting phenotypes with tools like Punnett squares, we can grasp the fundamental principles of genetic inheritance.