Mendel's Pea Hybridization Experiment (Part One)

Section 1: Mendel's Pea Hybridization Experiment (Part One) The view of fusion genetics was once prevalent in the latter half of the 19th century. However, in an Austrian monastery, Mendel broke free from this erroneous view and proposed a completely different theory.

Mendel had a childhood fascination with natural sciences. Due to his impoverished background, he became a monk at the age of 21. Later, he was sent to study natural sciences and mathematics at the University of Vienna. Upon returning to the monastery, he utilized a small garden plot to cultivate peas, hawkweed, corn, and other plants, conducting hybridization experiments and dedicating years to intensive research. Among these experiments, the pea hybridization was particularly successful. Through analyzing the results of his pea hybridization experiments, Mendel discovered the laws of biological inheritance.

Advantages of Peas as Genetic Experimental Materials Pea flowers are bisexual. Before opening, their pollen falls onto the stigma of the same flower, completing self-pollination, a process known as autogamy or self-crossing. Autogamy prevents interference from foreign pollen, so peas in their natural state are typically purebred. Using peas for artificial hybridization experiments yields reliable and easily analyzable results.

Pea plants also exhibit easily distinguishable traits. For instance, there are tall-stemmed (1.5-2.0m) and dwarf-stemmed (approximately 0.3m tall) pea plants, as well as round-seeded and wrinkled-seeded varieties. Such variations within the same biological trait are termed relative traits. These traits can be reliably inherited by offspring. Conducting hybridization experiments with plants exhibiting relative traits allows for straightforward observation and analysis of experimental outcomes.

Carefully observing, Mendel selected 7 pairs of relative traits from 34 varieties of peas for hybridization experiments, such as stem height, seed shape, cotyledon color, and flower position.

Mendel observed that different varieties of peas simultaneously exhibit multiple pairs of relative traits. For clarity in analysis, he initially studied the inheritance of one pair of relative traits.

Hybridization Experiment of One Pair of Relative Traits Mendel crossed purebred tall-stemmed peas with purebred dwarf-stemmed peas as parental (denoted as P). Surprisingly, regardless of whether tall-stemmed peas were used as the maternal parent (direct cross) or as the paternal parent (reciprocal cross), the first generation offspring (referred to as F1) always displayed tall-stemmed traits.

Why were all F1 offspring tall-stemmed without any dwarf-stemmed traits? Puzzled, Mendel self-crossed the F1 generation. In the second generation (F2), both tall-stemmed and dwarf-stemmed plants appeared. It seemed that the dwarf-stemmed trait was present but not expressed in the F1 generation. Mendel termed the traits expressed in F2 as dominant traits (e.g., tall stem) and those not expressed as recessive traits (e.g., dwarf stem). Later, the phenomenon of both dominant and recessive traits appearing in hybrid offspring was termed trait segregation.

F2 Mendel did not merely observe and describe experimental phenomena but statistically analyzed individuals with different pairs of relative traits in F2. The results showed that among 1064 pea plants obtained, 787 were tall-stemmed and 277 were dwarf-stemmed, approximating a 3:1 ratio of segregation between tall and dwarf traits.

Is the 3:1 segregation ratio in F2 a random occurrence? Mendel also conducted hybridization experiments on six other pairs of relative traits in peas, with results as shown in Table 1-1.

It appears that the 3:1 segregation ratio observed in F2 is not coincidental. What causes the inheritance traits to segregate in hybrid offspring according to a specific ratio?

Explanation of Segregation Phenomenon Based on observations and statistical analysis, Mendel decisively rejected previous fusion genetics views. Through rigorous reasoning and bold imagination, he proposed the following hypotheses (Figure 1-4) to explain the segregation phenomenon:

1.      

Biological traits are determined by hereditary factors (hereditary factor). These factors act like independent particles that neither fuse with each other nor disappear during transmission. Each factor determines a specific trait. Factors determining dominant traits are dominant hereditary factors, represented by capital letters (e.g., D); factors determining recessive traits are recessive hereditary factors, represented by lowercase letters (e.g., d).

2.      

3.      

In somatic cells, hereditary factors exist in pairs. For example, the somatic cells of purebred tall-stemmed peas contain paired hereditary factors DD, and those of purebred dwarf-stemmed peas contain paired hereditary factors dd. Such individuals with identical hereditary factors are called homozygotes. Because the F, progeny of self-crossing exhibit recessive traits, the somatic cells of F1 must necessarily contain recessive hereditary factors; whereas F, exhibits dominant traits, so the somatic cells of F, contain hereditary factors Dd. Such individuals with different hereditary factors are called heterozygotes.

4.      

5.      

When organisms form reproductive cells gametes at,the contain.