Mendel performed many experiments in which he tracked the inheritance of characters in pea plants, such as flower color and seed shape (pea shape). The results led him to formulate several hypotheses about inheritance. Take a look at some of his experiments and follow the reasoning that led to his hypotheses.
Mendel's Principle of Segregation
In one experiment, Mendel crossed purple-flowered pea plants with white-flowered pea plants (Figure 10-3). This is an example of a monohybrid cross, a pairing in which the parent plants differ in only one (mono) character. Mendel saw that the F1 hybrid plants were not a blend of purple and white. The F1 hybrids all had purple flowers, the same color as the purple-flowered parent. Was the factor for white flowers now lost as a result of the crossing? By allowing the F1 plants to self-fertilize, Mendel found the answer to be no. About one fourth of the F2 plants had white flowers. Mendel concluded that the factor for white flowers did not disappear in the F1 plants. Instead, only the purple flower factor was affecting F1 flower color. He reasoned that the F1 plants must have carried two factors for the flower color character, one for purple and one for white. Today, Mendel's "factors" are called genes.
In addition to studying the inheritance patterns of flower color, Mendel used monohybrid crosses to investigate six other pea plant characters (Figure 10-4). Each cross produced the same pattern. One of the two parent traits disappeared in the F1 generation, but then reappeared in about one fourth of the F2 offspring. From these results, Mendel developed four hypotheses. Using modern terms (such as gene instead of factor), these hypotheses are as follows:
Probability and Punnett Squares
As the F1 plants fertilize each other, gametes combine randomly and form zygotes with pairs of alleles. The likelihood of each specific pair forming is key to the inheritance pattern seen in the F2 generation. Consider the analogy of being handed two pennies (Figure 10-5, left). As you look at the two pennies, you will see 2 heads or 1 head and 1 tail or 2 tails. (Note that there are two different ways to get the outcome of 1 head and 1 tail.) The side shown by one coin is unaffected by the side shown by the other coin. But what is the probability of a particular combination occurring? For example, what is the probability that you will see 2 heads? You can build a table that shows the probability of each combination. List the probabilities for the first coin along the top of a piece of paper, and the probabilities for the second coin along the edge of the paper. Create a grid as shown in Figure 10-5. The probability of a particular combination is the product of the separate probabilities for each coin. For example, the probability of 2 heads showing is 1/2 x 1/2 = 1/4.
In the same way, you can calculate the probabilities for different combinations of alleles resulting from a genetic cross. The gametes of the purple F1 flowers pair randomly, making the allele combinations in the F2 generation PP or Pp or pp (Figure 10-5, right). (Again, note that there are two ways to get the heterozygousPpoutcome.) This type of diagram that shows all possible outcomes of a genetic cross is called a Punnett square. You can use a Punnett square to predict probabilities of particular outcomes if you know the genetic makeup of both parents.
Genotype and Phenotype
An observable trait (such as purple flowers) is called the phenotype (FEE noh type). The genetic makeup, or combination of alleles (such as PP), is called the genotype (JEE noh type). For the F2 plants, the ratio of plants with purple flowers to those with white flowers (3 purple : 1 white) is called the phenotypic ratio. The genotypic ratio is 1 PP : 2 Pp : 1 pp.
The appearance of the offspring resulting from the testcross will reveal the genotype of the mystery plant. Because the homozygous recessive parent can only contribute a recessive allele to the offspring, the phenotype will indicate the allele contributed by the mystery plant. If the purple-flowered parent is homozygous, you would expect all of the offspring to be purple-flowered since the mystery plant can only contribute a P allele. Thus, all offspring would be Pp (Figure 10-6, left). However, if the purple-flowered parent is heterozygous, you would expect both purple-flowered (Pp) and white-flowered (pp) offspring (Figure 10-6, right). Figure 10-6 also shows that the white-flowered and purple-flowered offspring of a Pp X pp testcross are predicted to have a 1 : 1 (1 purple to 1 white) ratio of phenotypes.
Mendel's Principle of Independent Assortment
The hybrid peas grew into F1 plants, which Mendel allowed to self-fertilize. This produced four phenotypes of peas. Assuming there are four equally likely combinations of alleles in the gametes produced by the F1 generationRY, rY, Ry, and rya Punnett square predicts a phenotypic ratio of 9 : 3 : 3 : 1 (Figure 10-8). And in fact, Mendel counted 315 round yellow, 108 round green, 101 wrinkled yellow, and 32 wrinkled green peas, a ratio that is approximately 9 : 3 : 3 : 1.
The Punnett square in Figure 10-8 also reveals that a dihybrid cross has the same outcome as two monohybrid crosses occurring at the same time. If you just look at seed shape, there are 12 plants with round seeds to every 4 with wrinkled seeds. If you look at just seed color, there are 12 yellow-seeded plants to every 4 green-seeded ones. A 12 : 4 ratio is the same as a 3 : 1 ratio, the phenotypic ratio for the F2 generation of a monohybrid cross.
Mendel tested his seven pea characters in various dihybrid combinations. The ratio of phenotypes in the F2 generation was always very close to the predicted ratio of 9 : 3 : 3 : 1. Based on these results Mendel proposed his principle of independent assortment. This principle states that during gamete formation in an F2 cross, a particular allele for one character can be paired with either allele of another character. For instance, in the above example, R can end up with either Y or y, and r can end up with either Y or y. The alleles for different genes are sorted into the gametes independently of one another.
Mendel's principle of independent assortment accurately described the seven pea plant characters he studied. But you will learn later in this chapter that there are certain exceptions to this principle.
Concept Check 10.2