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.

1. 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.”)
2. 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.
3. The chromosomes and their alleles for each trait segregate independently, so all possible combinations are present in the gametes.
4. 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.
5. 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.
6. 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.
7. 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 were pure 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.