R

P generation

P generation

Cross-pollination

Self-pollination

Ft generation All purple

Self-pollination

F2 generation 705 purple: 224 white

Ft generation All purple

F2 generation 705 purple: 224 white

TABLE 9-1 Mendel's Crosses and Results

Characteristic P generation

F, generation F2 generation

Observed ratio

Position of flowers along stem axial x terminal all axial

651 axial 207 terminal axial x terminal all axial

Predicted ratio

Height of plant

Height of plant tall x short all tall

787 tall 277 short

tall x short all tall

Pod appearance inflated x constricted all inflated

882 inflated 2.95:1 299 constricted inflated x constricted all inflated

Pod color green x yellow all green

428 green 152 yellow

green x yellow all green

Seed texture

5,474 round 2.96:1 1,850 wrinkled

round x wrinkled all round

Seed color yellow x green

6,022 yellow 3.01:1 2,001 green

yellow x green all yellow all yellow

purple x white all purple purple x white all purple

MENDEL'S RESULTS AND CONCLUSIONS

In one of his experiments, Mendel crossed a plant true-breeding for green pods with one true-breeding for yellow pods, as shown in Figure 9-4. The resulting seeds produced an F1 generation that had only green-podded plants. No yellow pods developed even though one parent had been true-breeding for yellow pods. Only one of the two traits found in the P generation appeared in the F1 generation.

Next, Mendel allowed the F1 plants to self-pollinate and planted the resulting seeds. When the F2 generation plants grew, he observed that about three-fourths of the F2 plants had green pods and about one-fourth had yellow pods.

His observations and careful records led Mendel to hypothesize that something within the pea plants controlled the characteristics observed. He called these controls factors. Mendel hypothesized that each trait was inherited by means of a separate factor. Because the characteristics studied had two alternative forms, he reasoned that a pair of factors must control each trait.

Recessive and Dominant Traits

Whenever Mendel crossed strains, one of the P traits failed to appear in the Fj plants. In every case, that trait reappeared in a ratio of about 3:1 in the F2 generation. This pattern emerged in thousands of crosses and led Mendel to conclude that one factor in a pair may prevent the other from having an effect. Mendel hypothesized that the trait appearing in the Fj generation was controlled by a dominant factor because it masked, or dominated, the factor for the other trait in the pair. He thought that the trait that did not appear in the F1 generation but reappeared in the F2 generation was controlled by a recessive factor.

Thus, a trait controlled by a recessive factor had no observable effect on an organism's appearance when that trait was paired with a trait controlled by a dominant factor.

The Law of Segregation

Mendel concluded that the paired factors separate during the formation of reproductive cells. That is, each reproductive cell, or gamete, receives one factor of each pair. When two gametes combine during fertilization, the offspring have two factors for each characteristic. The law of segregation states that a pair of factors is segregated, or separated, during the formation of gametes.

The Law of Independent Assortment

Mendel also crossed plants that differed in two characteristics, such as flower color and seed color. The data from these more-complex crosses showed that traits produced by dominant factors do not necessarily appear together. A green seed pod produced by a dominant factor could appear in a white-flowering pea plant.

Green-podded Yellow-podded plant plant

green-podded 1 yellow-podded plants plants figure 9-4

green-podded 1 yellow-podded plants plants figure 9-4

O True-breeding green-podded pea plants crossed with true-breeding yellow-podded pea plants produce only green-podded plants. © Yet when the F1 generation is permitted to self-pollinate, about one-fourth of the plants of the F2 generation are yellow-podded plants.

Word Roots and Origins recessive from the Latin recessus, meaning "to recede"

figure 9-5

Independent assortment of these two pairs of homologous chromosomes (Pp and Yy) would result in gametes that contain the allele combinations shown above. Pdenotes the dominant purple flower color, and p denotes the recessive white flower color. Ydenotes yellow seed color, and y denotes green seed color.

figure 9-5

Independent assortment of these two pairs of homologous chromosomes (Pp and Yy) would result in gametes that contain the allele combinations shown above. Pdenotes the dominant purple flower color, and p denotes the recessive white flower color. Ydenotes yellow seed color, and y denotes green seed color.

Mendel concluded that the factors for individual characteristics are not connected. Recall that the random separation of homologous chromosomes is called independent assortment. The law of independent assortment states that factors separate independently of one another during the formation of gametes.

SUPPORT FOR MENDEL'S CONCLUSIONS

Most of Mendel's findings agree with what biologists now know about molecular genetics. Molecular genetics is the study of the structure and function of chromosomes and genes. A chromosome is a threadlike structure made up of DNA. A gene is the segment of DNA on a chromosome that controls a particular hereditary trait. Because chromosomes occur in pairs, genes also occur in pairs. Each of two or more alternative forms of a gene is called an allele. Mendel's factors are now called alleles.

Letters are used to represent alleles. Capital letters refer to dominant alleles, and lowercase letters refer to recessive alleles. For example, the dominant allele for the trait of purple flower color may be represented by P, and the recessive allele for the trait of white flower color may be represented by p, as is shown in Figure 9-5. Whether a letter is capitalized or lowercased is important. The actual letter selected to represent an allele is typically the first letter of the dominant trait. During meiosis, gametes receive one chromosome from each homologous pair of chromosomes. Thus, when the gametes combine in fertilization, the offspring receives from each parent one allele for a given trait.

Mendel's law of independent assortment is supported by the independent segregation of chromosomes to gametes during meio-sis. Therefore, the law of independent assortment is observed only for genes located on separate chromosomes or located far apart on the same chromosome.

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