Mendelian Genetics

The breeding experiments of the monk Gregor Mendel in the mid‐1800s laid the groundwork for the science of genetics. He published only two papers in his lifetime and died unheralded in 1884. The significance of his paper published in 1866 on inheritance in peas (which he grew in the monastery garden) apparently went unnoticed for the next 34 years until three separate botanists, who also were theorizing about heredity in plants, independently cited the work in 1900. During the next 30 years, the universality of his findings was confirmed, and breeding programs for better livestock and crop plants—and the science of genetics—were well under way.

At the time of Mendel's work, scientists widely believed that offspring blended the characteristics of their parents, but Mendel's painstaking experimentation suggested this was not so. Remember, no one had yet heard of genes, chromosomes, or meiosis, but Mendel concluded from his breeding experiments that particles or “factors” that passed from the parents to the offspring through the gametes were directly responsible for the physical traits he saw first lost in the offspring's generation, then repeated in the next. Closer still to the actual truth, Mendel even hypothesized that two factors, probably one from each parent, interacted to produce the results. His “factors” were, of course, the genes, which do, indeed, come in pairs or alleles for each trait.

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.)

Mendel then let the F1 plants self-pollinate: Tt × Tt and in the F2 generation counted the numbers of individuals with each of the traits. For the tall × dwarf crosses he got 787 tall plants and 277 dwarf plants (6,022 yellow seeds and 2,001 green seeds, and so forth).

An easy way to determine the possible gene combinations is to construct a Punnett square, a grid in which all the possible gametes from one parent are listed on one side and those from the second parent across the top. Combine the gametes from the side and the top in the squares, and all of the possible gamete combinations are diagrammed. The previous cross in a Punnett square would look like this:

You can see from the Punnett square that three of the four gamete combinations will contain at least one dominant allele (T) and that there is only one chance out of four that the recessive (t) can be expressed. Mendel's experimental results fit the phenotypic probability ratio of 3:1. The genotypic ratio, which Mendel didn't know about, is not 3:1, but 1:2:1. That is, 1 homozygous dominant (TT):2 heterozygous dominants (Tt):1 homozygous recessive (tt). The Punnett square shows only thepossible combinations, not the actual. It provides an easy way to visualize theprobabilities of a certain combination occurring. In some inherited traits, whether the allele comes from the male or the female parent can make a difference, but in most traits such information does not matter.

After making monohybrid crosses for all the traits and finding that the ratios always approximated 3:1, although the actual numbers of plants and offspring for each cross varied, Mendel concluded that the traits must be carried in pairs that segregate(separate) when gametes are formed. This conclusion is now known as Mendel's first law, the Law of Segregation.

To confirm his hypothesis, he made another kind of cross, a backcross, which mates an offspring with one of its parents. Mendel backcrossed his F2 tall plants to the dwarf parent and got half tall plants, half dwarf, a 1:1 ratio. If he had backcrossed to the tall parent, what would the ratio have been? Right, all tall; that's why breeders today maketest crosses back to the homozygous recessive parent to see if their phenotypically dominant individuals are homozygous or heterozygous.

Mendel's second law: Law of Independent Assortment

Mendel also worked with crosses involving two traits—this is where his luck really entered in. The traits he picked are on separate chromosomes (though, of course, he didn't know this). Had they been on the same chromosomes, the ratios he obtained would not have been possible because the traits would always go together in the same gamete unless some cellular tinkering took place.

The mechanisms for figuring out the possible gametes with two traits, filling out the Punnett square, and counting the possibilities are the same—only with more variations possible (see Table 1 for potential numbers). 

Here's what the cross looks like for two of Mendel's traits combined, flower color and pod characteristics. One allele for each goes in each gamete; purple color (P) is dominant over white (p) flowers, and inflated pods (I) are dominant over constricted (i).

Self pollinate the F1 purple flowered, inflated pod plants and what is the F2 ratio? Not 3:1 anymore. Fill out a Punnett square and see the possibilities. Each gamete gets one allele of each trait, so a dominant purple (P) can have either a dominant inflated pod (I) or a recessive constricted pod (i); ditto the white (p). Thus, four kinds of gametes are possible: PI, Pi, pI, pi and 4 × 4 combinations are possible from the two parents:

The phenotypic dihybrid ratio is 9:3:3:1–9 purple inflated, 3 purple constricted, 3 white inflated, and 1 white constricted. (Geneticists now test their results statistically to see if they approach the theoretical 9:3:3:1 and usually use the χ2 [chi-square] test.)

Mendel drew a conclusion on the basis of his dihybrid crosses that is now known asMendel's second law: the Law of Independent Assortment. It states that during gamete formation the segregation of the alleles of one allelic pair is independent of the segregation of the alleles of other genes.

Mendel confirmed this hypothesis further (as he did in the monohybrid crosses) by backcrossing the F1 dihybrid to the recessive parent.

The Punnett square for the backcross looks like this:

The phenotypic ratio for the testcross is: 1:1:1:1; that is, 1 purple inflated:1 purple constricted:1 white inflated:1 white constricted—which indicates that the traits have separated and recombined independently of one another.