Mendel's Experiments on Pea Hybridization (Part 2)

Section 2 Mendel's Experiments on Pea Hybridization (Part 2)

Impact of the Separation of One Pair of Contrasting Traits on Other Contrasting Traits After discovering the inheritance pattern of one pair of contrasting traits, Mendel further studied the inheritance of two pairs of contrasting traits in peas.

Hybridization Experiment with Two Pairs of Contrasting Traits Mendel crossed purebred yellow round peas with purebred green wrinkled peas. Regardless of whether it was a reciprocal cross or a direct cross, the resulting seeds (F1) were all yellow and round. This indicates that yellow color and round shape are dominant traits, while green color and wrinkled shape are recessive traits.

Mendel then allowed the F1 generation to self-pollinate. In the resulting F2 generation, yellow round peas and green wrinkled peas appeared, which was expected. Strangely, the F2 generation also showed combinations of traits not present in the parents—green round peas and yellow wrinkled peas (Figure 1-6).

Why did these new trait combinations appear? Is there a quantitative relationship between them? Mendel also conducted a quantitative analysis of the different trait types in the F2 generation: out of a total of 556 seeds obtained, the numbers of yellow round, green round, yellow wrinkled, and green wrinkled seeds were 315, 108, 101, and 32 respectively. Their ratios closely approximate 9:3:3:1, similar to the 3:1 ratio observed in the inheritance of one pair of contrasting traits. Mendel first individually analyzed each pair of contrasting traits, finding that the inheritance of each pair of contrasting traits followed the law of segregation (Figure 1-7).

The above analysis indicates that, looking at each pair of contrasting traits individually, whether it is the shape of pea seeds or the color of cotyledons, they still follow the law of segregation. This means that the inheritance of the factors controlling seed shape does not interfere with the inheritance of factors controlling cotyledon color. So, when considering the inheritance of two pairs of contrasting traits together and the occurrence of new combinations between different traits, do the factors controlling the inheritance of two pairs of contrasting traits also combine?

Explanation and Verification of the Law of Independent Assortment Assuming that round and wrinkled peas are controlled by the genetic factors R and r, respectively, and yellow and green colors are controlled by genetic factors Y and y, respectively, purebred yellow round and purebred green wrinkled peas have the genetic compositions YYRR and yyrr, respectively. The F1 generation produced has the genetic composition YyRr, exhibiting yellow round peas.

Mendel's explanation is as follows: in the F1 generation, during gamete formation, each pair of genetic factors segregates independently, and different pairs of genetic factors can freely combine. Thus, the female and male gametes produced in the F1 generation each have four types: YR, Yr, yR, and yr, with a ratio of 1:1:1:1. Fertilization occurs randomly between these male and female gametes. There are 16 possible combinations of male and female gamete unions; the genetic combinations have 9 forms: YYRR, YYRr, YyRR, YyRr, YYrr, Yyrr, yyRR, yyRr, yyrr; the phenotypes manifest as 4 types: yellow round, yellow wrinkled, green round, green wrinkled, with a ratio of 9:3:3:1 (Figure 1-8).

Is the above explanation correct? Mendel also designed test-cross experiments: Crossing the hybrid F1 generation (YyRr) with the homozygous recessive (yyrr).

Similarly, based on the hypothesis proposed, Mendel deduced the results of the test-cross experiment (Figure 1-9).

In Mendel's test-cross experiments, whether using the F generation as the female or male parent, the results matched the predictions (Table 1-2).

Law of Independent Assortment In the pea traits studied by Mendel, he selected two pairs of traits for hybridization experiments, yielding consistent results. This situation is often observed in other organisms as well. Later researchers named this genetic law Mendel's Second Law, also known as the Law of Independent Assortment: the segregation and combination of genetic factors controlling different traits do not interfere with each other; when gametes are formed, paired genetic factors determining the same trait segregate, while genetic factors determining different traits freely combine.

Insights from Mendel's Experimental Methods Before Mendel, many scholars had conducted hybridization experiments on animals and plants, observing the phenomenon of trait segregation in biological inheritance, but failed to summarize genetic laws. Why was Mendel successful?

Re-discovery of Mendel's Genetic Laws In 1866, Mendel compiled his research results into a paper, unfortunately, this important achievement did not attract attention and remained obscure for more than 30 years. In 1900, three scientists independently rediscovered Mendel's paper. They conducted many observations similar to Mendel's experiments and recognized the significance of Mendel's proposed theory.

In 1909, Danish biologist W.L. Johannsen (1857-1927) coined a new term "gene" for Mendel's "genetic factors," and introduced the concepts of phenotype (also known as appearance) and genotype. Phenotype refers to the traits expressed by individual organisms, such as tall and dwarf peas; the genetic composition related to the phenotype is called genotype, such as the genotype of tall peas being DD or Dd, and dwarf peas being dd. Alleles are the genetic factors controlling contrasting traits, such as D and d.

With the rediscovery of Mendel's genetic laws, the nature and principles of genes became central issues in genetic research. The study of these issues has brought people closer to the essence of life activities and laid the theoretical foundation for biotechnologies like genetic engineering. It is because of Mendel's outstanding contributions that he is recognized as the "father of genetics" by later generations.

Applications of Mendel's Genetic Laws The laws of segregation and independent assortment have universal applicability in biological inheritance. Mastering these laws not only helps people correctly interpret the common genetic phenomena in the biological world, but also predicts the types and probabilities of hybrid offspring, which is significant in areas such as animal and plant breeding and medical practices. In hybrid breeding, people purposefully hybridize two parents with different desirable traits to combine their favorable traits and select the desired superior varieties. For example, in wheat, resistance to lodging (D) is dominant over susceptibility (d), and resistance to rust (T) is dominant over susceptibility (t). Wheat susceptible to rust (Figure 1-10) or lodging can lead to reduced yields or even crop failures. If there are two different varieties of wheat, one resistant to lodging but susceptible to rust (DDTT); the other variety is susceptible to lodging but resistant to rust (ddtt). Crossing these two wheat varieties will result in new types in F (such as DDtt or Ddtt). Continuing to breed them, through selection and cultivation, can produce pure varieties that are both resistant to lodging and rust (DDtt).

In medical practice, people can make scientific inferences about the probability of certain genetic diseases in offspring based on the laws of segregation and independent assortment, providing a theoretical basis for genetic counseling. For example, human albinism is a genetic disease controlled by a recessive gene (a). If both parents of a patient have a normal phenotype, according to the law of segregation, it is known that both parents of the patient are heterozygous (Aa), and the probability of the disease occurring in the offspring of the parents is 1/4.