CONCEPTS IN BIOLOGY

PART III. MOLECULAR BIOLOGY, CELL DIVISION, AND GENETICS

 

10. Patterns of Inheritance

 

10.4. The First Geneticist: Gregor Mendel

The inheritance patterns discussed in the section “Probability vs. Possibility” were initially described by Gregor Mendel—a member of the religious order of Augustinian monks. Mendel’s (1822-1884) work was not generally accepted until the 1900s, when three men, working independently, rediscovered some of the ideas that Mendel had formulated more than 30 years earlier. Because of his early work, the study of the pattern of inheritance that follows the laws formulated by Gregor Mendel is often called Mendelian genetics (figure 10.3).

FIGURE 10.3. Gregor Mendel and His Pea Plant Garden

(a) Gregor Mendel was an Augustinian monk who used statistics to describe the inheritance patterns he observed in pea plants. (b) He carried out his investigations in this small garden of his monastery in Brno, Czech Republic where Mendel did his experiments.

Mendel’s work established basic principles that allowed him and others to solve heredity problems. Heredity problems are concerned with determining which alleles are passed from parents to offspring and how likely it is that various types of offspring will be produced. Mendel performed experiments concerning the inheritance of certain characteristics in garden pea plants (Pisum sativum). From his work, Mendel developed the ideas of a genetic characteristic being dominant or recessive and categorized the inheritance patterns for a number of garden pea alleles by using rules of probability. Some of the phenotypes he used in his experiments are shown in table 10.1.

TABLE 10.1. Dominant and Recessive Traits in Pea Plants

Dominant Allele

Recessive Allele

Gene

Phenotype

Phenotype

Plant height

Tall

Dwarf

Pod shape

Full

Constricted

Pod color

Green

Yellow

Seed surface texture

Round

Wrinkled

Seed color

Yellow

Green

Flower color

Purple

White

What made Mendel’s work unique was that he initially studied only one trait at a time. In addition, he grouped the offspring by phenotype and counted them. Previous investigators tried to follow numerous traits at the same time. This made it very difficult to follow characteristics, and they did not determine the frequency of phenotypic groups. Therefore, they were unable to see any patterns in their data.

Mendel was very lucky to have chosen pea plants in his study because they naturally self-pollinate. This means that pea plants produce both pollen and eggs and that the eggs can be fertilized by haploid nuclei from their own pollen. When self-pollination occurs in pea plants over many generations, it is easier to develop a population of plants that is homozygous for a number of characteristics. Such a population is known as a pure line.

The following gene key organizes some of Mendel’s findings. Remember that Mendel didn’t know about DNA or even chromosomes! Mendel developed this way of thinking about genetics to explain the data he collected. He did this from the perspective of a mathematician—not a biologist.

Gene Key

Gene or Condition: flower color

Allele Symbols

Possible Genotypes

Phenotype

Possible Sex Cells

C = Purple

CC = homozygous

Cc = heterozygous

Purple

Purple

All sex cells have C (pure line)

Half of sex cells have C and half have c

c = white

cc = homozygous

White

All sex cells have c (pure line)

In one experiment, Mendel took a pure line of pea plants having purple flower color, removed the male parts (anthers), and discarded them, so that the plants could not self-pollinate. He then took anthers from a pure-breeding white-flowered plant and pollinated the antherless purple flower. These plants are called the parent generation, or P0. When the pollinated flowers produced seeds, Mendel collected, labeled, and planted them. When these seeds germinated and grew, they eventually produced flowers. The offspring of the P0 generation are called the F1 generation. The F stands for filial, which is Latin for relating to a son or daughter. F1 is read as the “F-one” generation or the “first filial” generation.

Mendel's First Cross

All the F1 plants resulting from this cross had purple flowers. One of the popular ideas of Mendel’s day would have predicted that the purple and white colors would have blended, resulting in flowers that were lighter than the purple parent flowers. Another hypothesis would have predicted that the offspring would have had a mixture of white and purple flowers. Neither of these two hypotheses was supported by Mendel’s results. He observed only purple flowers from this cross.

Mendel then crossed the F1 pea plants (all of which had purple flowers) with each other to see what the next generation would be like. Had the white-flowered characteristic been lost completely? The seeds from this mating were collected and grown. When these plants flowered, three-fourths of them produced purple flowers and one-fourth produced white flowers. This generation is called the F2generation.

Mendel's Second Cross

His experiments used similar strategies to investigate other traits. Pure-breeding tall pea plants were crossed with pure-breeding dwarf plants. Pure-breeding plants with yellow pods were crossed with pure-breeding plants with green pods. Mendel recognized the same pattern for each characteristic in the F1 generation: All the offspring showed the characteristics of one parent and not the other with no blending.

After analyzing these data, Mendel identified several genetic principles:

1. Organisms have two pieces of genetic information for each trait. It is now recognized that these are different alleles for each characteristic.

2. Because organisms have two pieces of genetic information for each characteristic, the alleles can be different. Mendel’s Law of Dominance states that some alleles interact with each other in a dominant and recessive manner whereby the dominant allele masks the recessive allele.

3. Gametes fertilize randomly.

4. Mendel’s Law of Segregation states that, when a diploid organism forms gametes, the two alleles for a characteristic separate from one another. In doing this, they move to different gametes and retain their individuality.

The application of Mendel’s Law of Segregation may not be as apparent as the application of the Law of Dominance. The movements of chromosomes during meiosis separate the four copies (one on each chromatid) of each allele into four different sex cells. This causes only 1 allele of each gene to be present in each sex cell. We first observed this law in this chapter when we discussed sex cells and how alleles separate in both homozygous and heterozygous organisms. Finally, this law is the basis for the Punnett square, in which the possible alleles from each parent are placed in a separate row or column.

10.4. CONCEPT REVIEW

10. In your own words, describe Mendel’s Law of Segregation.

11. Define self-pollination.

12. What is the “F1 generation”?