The Organization Of The Experiments

In his lengthy paper of 1866,[1] Mendel described the design and the results of his experiments. He chose the ordinary garden pea as his primary experimental material. Peas had the virtues of having several simple, easily separated traits, were naturally self-fertilizing although they could be crossed, and true-breeding varieties could be established. They were also very prolific so that large experimental populations could be developed quickly. The basic design of the experiments is described in many basic textbooks of genetics.[2-5] Some definitions are appropriate:

Phenotype observable properties of an organism Genotype genetic makeup of an organism Gene determinant of a characteristic of an organism Allele alternative form of a gene

Homozygous individual that received the same allele from each parents for a particular gene Heterozygous individual that received different alleles from its two parents for a particular gene Dominant allele that is expressed either in the homozy-

gous or the heterozygous state Recessive allele that is expressed only in the homozy-gous state

True-breeding parents and offspring consistently display the same phenotype generation after generation Parental generation experimental generation that starts with true-breeding parents Fj generation offspring experimental generation resulting from mating of parental strains F2 generation offspring experimental generation resulting from mating of members of the F1 generation Hybrid cross between true-breeding parents Monohybrid cross between true-breeding parents that differ for one characteristic Gamete reproductive cell that contains one member of each gene pair in the parent

Mendel chose seven characteristics, each with a clear dominant recessive relationship. These were (dominant allele listed first):

Seed shape smooth vs. wrinkled Seed color yellow vs. green Flower color purple vs. white Pod shape inflated vs. constricted Pod color green vs. yellow Flower position axial vs. terminal Plant height tall vs. short

Mendel's basic experiments started with true-breeding parents. These were mated in hybrid crosses and Mendel carefully counted the offspring. Several of the experiments resulted in thousands of observations.

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