How do Mendel’s experiments show that traits may be dominant or recessive?

Gregor Mendel’s experiments with pea plants laid the foundation for our understanding of inheritance and the principles of genetics. Through his work, Mendel demonstrated the existence of dominant and recessive traits. Here’s a brief overview of Mendel’s experiments and how they illustrate the concept of dominance and recessiveness:

1. Choice of Traits:

» Mendel selected traits that exhibited clear and easily distinguishable variations in the pea plants, such as seed color (yellow or green), seed shape (round or wrinkled), flower color (purple or white), etc.

2. Purity of Parental Lines:

» Mendel ensured the purity of his experimental plants by using true-breeding lines. True-breeding means that when plants with a particular trait are self-fertilized or cross-fertilized, they consistently produce offspring with the same trait.

3. Crossbreeding Experiments:

» Mendel performed controlled crosses between plants with contrasting traits. For example, he crossed plants with yellow seeds (dominant trait) with those having green seeds (recessive trait).

4. Observation of Offspring (First Filial Generation – F1):

» Mendel observed that the offspring (F1 generation) of these crosses consistently displayed the dominant trait. In the case of seed color, all the F1 plants had yellow seeds.

5. Observation of Offspring (Second Filial Generation – F2):

» Mendel then allowed the F1 plants to self-fertilize or cross-fertilize. In the resulting F2 generation, he observed a 3:1 ratio of dominant to recessive traits. For example, in the case of seed color, approximately three-fourths of the F2 plants had yellow seeds, and one-fourth had green seeds.

6. Law of Segregation:

» Mendel proposed the Law of Segregation, which states that the two alleles (gene variants) for a trait segregate (separate) during the formation of gametes, and each gamete receives only one allele. This segregation explains the 3:1 ratio observed in the F2 generation.

7. Dominance and Recessiveness:

» The dominant trait, which is expressed in the phenotype of the organism, masks the expression of the recessive trait in heterozygous individuals (those carrying both dominant and recessive alleles).
Mendel’s experiments demonstrated that traits are controlled by discrete units (now known as genes) and that these units come in pairs. Dominant traits are expressed in the presence of at least one dominant allele, whereas recessive traits are only expressed when an individual carries two recessive alleles.

Mendel’s findings laid the groundwork for the understanding of inheritance patterns and genetics, and his principles continue to be fundamental in the study of genetics today.