Gregor Mendel (1822–1884), an Austrian monk, conducted systematic breeding experiments on pea plants (Pisum sativum) and discovered the fundamental principles of heredity. His work, published in 1866, forms the foundation of classical genetics.
Mendel chose pea plants because they:
| Term | Definition |
|---|---|
| Gene | A sequence of nucleotides in DNA that codes for a polypeptide/trait |
| Allele | Alternative forms of a gene (e.g., for round, for wrinkled) |
| Dominant allele | Allele that masks the effect of the other (written in uppercase, e.g., ) |
| Recessive allele | Allele whose effect is masked by the dominant allele (lowercase, e.g., ) |
| Genotype | The genetic makeup of an organism (e.g., , , ) |
| Phenotype | The observable physical trait (e.g., round seeds, wrinkled seeds) |
| Homozygous | Both alleles are identical (e.g., or ) |
| Heterozygous | Two different alleles (e.g., ) |
| True-breeding | Organisms that produce only the same variety as the parent upon self-pollination (homozygous) |
A monohybrid cross tracks the inheritance of a single trait.
Mendel crossed true-breeding round-seeded plants () with true-breeding wrinkled-seeded plants ():
P generation:
generation: All → all round seeds (dominant phenotype expressed)
:
generation:
The two alleles for a heritable character segregate during gamete formation and end up in different gametes, so each gamete carries only one allele for each gene.
This law is explained by the separation of homologous chromosomes during Meiosis I.
A test cross is used to determine the unknown genotype of an organism showing a dominant phenotype.
A dihybrid cross tracks the inheritance of two traits simultaneously.
P generation:
generation: All (round, yellow)
:
Each parent produces 4 types of gametes: , , ,
phenotypic ratio: Round Yellow Round Green Wrinkled Yellow Wrinkled Green
Each pair of alleles segregates independently of every other pair of alleles during gamete formation.
This law applies to genes located on different (non-homologous) chromosomes.
During Meiosis I, homologous chromosome pairs align independently at the metaphase plate. The orientation of one pair does not influence the orientation of another pair. This random orientation is the physical basis of independent assortment.
Independent assortment does not apply when:
Despite these limitations, independent assortment is useful because:
Independent assortment is a major source of genetic variation in sexually reproducing organisms:
Mendelian inheritance follows the rules of probability:
The probability of two independent events both occurring = product of their individual probabilities.
Example: In , the probability of round yellow offspring:
The probability of either of two mutually exclusive events = sum of their individual probabilities.
Example: Probability of or from :
| Law | Statement | Meiotic Basis |
|---|---|---|
| Segregation | Allele pairs separate during gamete formation | Separation of homologs in Meiosis I |
| Independent Assortment | Different gene pairs assort independently | Random orientation of bivalents at metaphase I |