Mendel's Experimental Organism: The Garden Pea
• Mendel's success attributed to choice of experimental test subjects.
• Garden peas easily grown in gardens or green houses.
• Pea flowers contain both male and female organs. The male sex organ, anthers, produce sperm
containing pollen, and the female organ, the ovary, produces eggs.
• Flower petals close down, preventing pollen from entering or leaving the flower.
• This causes self fertilization, pollen and eggs from the same flower fertilize to produce seeds.
• Pea strains are highly inbred, displaying little genetic variation from generation to generation:
• Each true-breeding pea strain was distinguished by one specific characteristic, such as size or
• Focusing on single differences allowed him to study 1 trait at a time.
• Mendel succeeded where other biologists failed because they studied multiple traits at once.
Mono-hybrid Crosses: The Principles of Dominance and Segregation
• Mendel cross fertilized tall and dwarf pea plants to see how the height trait was inherited.
• He removed the anthers of one variety, and pollinated that variety with anthers of the other
• The seeds of the cross fertilization were sown,he revealing hybrid plants that were all tall.
• The same results were obtained regardless of which trait came from either male or female.
• However, when all the tall plants were allowed to self fertilize, the yield was a 3:1 ratio of tall
to dwarf plants.
• Mendel inferred that the hybrids contained a latent, recessive genetic factor for dwarfism, and a
dominant expressed factor for tallness.
• He also inferred that the factors split when the hybrids self fertilized, explaining the re-
appearance of the dwarfed plants.
• Each cross fertilization involving a single trait is called a mono-hybrid cross.
• Multiple experiments revealed only one of the contrasting traits appeared in the hybrid
generation, but both appeared in the self fertilized generation, always in a 3:1 ratio.
• Mendel's conclusion was that each trait was controlled by heritable genetic factor (now called a
gene) that existed in two alternative forms, one recessive and one dominant (now called alleles).
• Mendel's other conclusion was that the genes came in pairs. Each of the parental strains in the
cross carried two identical versions of the gene, therefore diploid and homozygous. However,
during gamete production, only one version of the gene is obtained, therefore haploid.
• The diploid gene number would be restored when the pollen and egg fertilized. The resulting
zygote would be diploid, and heterozygous as it would receive an allele from each parent,
assuming the plants were genetically different in the cross.
• When the heterozygous hybrid reproduces, any gamete has an equal chance of obtaining either
the dominant or recessive allele.
• The re-appearance of the dwarf plants would be explained by the fact that some zygotes from
the progeny of the hybrid plants would inherit both recessive alleles.
• Both the true breeding parental varieties are homozygous for alleles controlling height.
• Lowercase d represents the allele for dwarf, and uppercase D represents the allele for tall. The
letter denoting the alleles for a gene comes from the word corresponding the recessive trait (d
• Tall will be symbolized by DD, and dwarf by dd.
• The allelic constitution of the genes of the strain is called the genotype. The physical characteristic that is the result of the allelic constitution is called the phenotype of the gene.
• The parental strains form the P generation.
• The hybrid progeny are called the first filial generation, or F1.
• Each parental strain can only produce either the dominant gamete or recessive gamete, and they