Some say Mendel was lucky, others that his reported results are too good to be true, that he (or someone else) must have fudged the data to make them “come out right.” His choice of garden peas was fortuitous. Peas are self-pollinated, and the seven traits he chose to measure are inherited as single factors, so Mendel could establish true-breeding lines for each trait. Thus, he was able to select the parent traits, pollinate the flowers, and count the results in the offspring with no complicating elements. He was mathematically trained, kept accurate records, and applied mathematical analyses (and was among the first to do so with biological materials).
Mendel's first law: Law of Segregation
Mendel did not formulate his conclusions as laws or principles of genetics, but later researchers have done so. Restating and using modern, standardized terminology, this is the information that developed and expanded from his early experiments.
Inherited traits are encoded in the DNA in segments called genes, which are located at particular sites ( loci, singular locus) in the chromosomes. (Genes are Mendel's “factors.”)
Genes occur in pairs called alleles, which occupy the same physical positions on homologous chromosomes; both homologous chromosomes and alleles segregate during meiosis, which results in haploid gametes.
The chromosomes and their alleles for each trait segregate independently, so all possible combinations are present in the gametes.
The expression of the trait that results in the physical appearance of an organism is called the phenotype in contrast to the genotype, which is the actual genetic constitution.
The alleles do not necessarily express themselves equally; one trait can mask the expression of the other. The masking factor is the dominant trait, the masked the recessive.
If both alleles for a trait are the same in an individual, the individual ishomozygous for the trait, and can be either homozygous dominant or homozygous recessive.
If the alleles are different—that is, one is dominant, the other recessive—the individual is heterozygous for the trait. (Animal and plant breeders often use the term “true-breeding” for homozygous individuals.)
Geneticists use a standard shorthand to express traits using letters of the alphabet, upper case for dominant, lower case for recessive. Red color, for example, might be Ror r so a homozygous dominant individual would be RR, a homozygous recessive individual, rr and a heterozygous individual Rr.
Crosses between parents that differ in a single gene pair (such as those that Mendel made) are called monohybrid crosses (usually TT and tt). Crosses that involve two traits are called dihybrid crosses. Symbols are used to depict the crosses and their offspring. The letter P is used for the parental generation and the letter F for the filial or offspring generation. F1 is the first filial generation, F2 the second, and so forth.
What kinds of crosses did Mendel make to conclude that factors/genes segregate? First of all, he made certain that the plants that he planned to use in the experiment werepure line for the trait—that is, that they bred true for the trait for two or more years. (Peas are self-pollinated so he simply grew the plants and examined their offspring.) Other experimenters omitted this step, which confounded their results. Mendel then made a series of monohybrid crosses for each of the seven traits he had identified using parents of opposite traits—tall (TT) vs. dwarf (tt), yellow seed (YY) vs. green (yy) seed, round seed (RR) vs. wrinkled (rr), and so forth. (He, of course, did not symbolize them with letters, but he did know that seeds from his tall pure-line plants would always produce tall plants, seeds from the dwarfs would always produce dwarf plants, and so on.)