Introduction

Gregor Mendel is considered to be the ‘Father’ of modern genetics.  It is important to realize that Mendel performed his research long before science had discovered DNA, chromosomes, or the process of Meiosis.  How did Mendel make his discoveries without the benefit of this knowledge? Let us review his experiments to understand how he arrived at his conclusions.

Learning

Mendel chose the pea plant for his studies for several reasons.  First, pea plants have several distinct traits that are easily visible, such as flower color or stem length.  Second, due to the anatomy of the pea flower, it was relatively easy to control the pollination in these plants.

Like humans, plants produce gametes (sperm, egg) and have different reproductive structures responsible for producing these cells (Stamens produce sperm; Carpel produce eggs).

Unlike humans, however, a single pea flower contains both of these reproductive structures and the pea plant naturally undergoes the process of ‘self-fertilization’ since all of the reproductive structures are physically enclosed within the petals of the pea flower.

Mendel took advantage of these properties to control fertilization between pea plants by opening the pea flower and removing the stamens. He could then use the pollen obtained from the stamens of one plant to fertilize the carpel of another plant to perform true cross-fertilization. By keeping precise records of the plants he crossed and the traits exhibited by the parents and their offspring, he was able to identify several key patterns which lead to his ‘rules’ of inheritance.

Mendel’s Laws

Law of Segregation

Mendel’s Law of Segregation states that a diploid organism passes a randomly selected allele for a trait to its offspring, such that the offspring receives one allele from each parent.

Mendel hypothesized that each trait must be controlled by a pair of ‘factors’ (which we now know as ‘genes’) present in each plant.  He also determined that the two ‘factors’ controlling a physical trait in the pea plant must segregate from one other during the production of gametes (sperm, egg).

If we consider that the flower color trait is controlled by two factors (or genes), in this example we can designate the purple-flower gene with a single letter = ‘P’. Therefore, each true-breeding purple-flowered plants contain two copies of the ‘P’ gene and can be designated as ‘PP’. Since PP represents the set of genes (or alleles) present in a plant, we refer to this as an organism’s ‘genotype’ (the combination of physical alleles carried by an organism). It is important to understand that the genotype of an organism determines the appearance of that particular trait, referred to as the organism’s ‘phenotype’ (the observable characteristic caused by the genotype). In this instance, the phenotype of the plant would be purple flowers. One way to remember these terms is Genotype = Genes and Phenotype = Physical appearance.

An example of this using the yellow and green peas: A true breeding yellow pea plant produces yellow pea color gametes. Likewise, a true breeding green pea plant produces green pea color gametes.

Let us now consider what happens when we cross-fertilize two different true-breeding plants:

In Mendel’s model, parents pass along heritable “factors,” which we now call genes, that determine the traits of the offspring. Each individual has two copies of a given gene, such as the gene for seed color (Y gene) shown. If these copies represent different versions, or alleles, of the gene, one allele—the dominant one—may hide the other allele—the recessive one. For seed color, the dominant yellow allele Y hides the recessive green allele y.

In this instance, the resulting plant has two different alleles (‘Yy’) and therefore would be considered to have a heterozygous genotype with a phenotype of yellow peas. This clearly demonstrates the concept of dominance.  In this case, the yellow pea color trait is dominant over the green pea color trait (which, consequently, would be considered a ‘recessive’ trait). In Mendel’s experiments where he cross-pollinated two true-breeding plants, a trait that disappeared in the F1 generation, but reappeared in the F2 generation would be considered recessive.

How would we predict this?

These gamete combinations can be aligned into a grid to predict the outcome of subsequent crosses as shown below where the gametes from one parent are placed at the top, those of the other parent on the left. This grid is called a ‘Punnett square’ after the geneticist R.C. Punnett who was the first to develop and use the tool.

Mendel’s Law of Independent Assortment states that the alleles of two (or more) different genes get sorted into gametes independently of one another. In other words, the allele a gamete receives for one gene does not influence the allele received for another gene.

Mendel proposed that pairs of factors in a parental generation seemed to segregate into different gametes that when combined together form a new genotype for the offspring.  This is the basis of Mendel’s Law of Segregation

Summary

• In this section, we have learned the following:
• Inherited traits in the pea plants seemed to be controlled by a pair of ‘factors’, which are now called genes or alleles.
• Genes come in different versions called alleles. A dominant allele hides a recessive allele and determines the organism’s appearance.
• The genotype of a plant represents the combination of alleles/genes present (i.e. ‘Pp’)
• The phenotype of the plant is the physical/observable result from the plant’s genotype.
• A plant that is homozygous for a gene has two identical copies of the same allele present.
• A plant that is heterozygous for a gene has two different copies of the alleles for that particular trait. 
• When Mendel cross-pollinated two true-breeding plants that are different in one trait, he found that the resulting plants resembled only one of the parent plants. The trait found in this F1 generation is considered dominant.
• Mendel proposed that pairs of factors in a parental generation seemed to segregate into different gametes that when combined together form a new genotype for the offspring.  This is the basis of Mendel’s Law of Segregation
• Mendel also proposed that if you examined two traits in a pea plant, the alleles for the two traits are separated into gametes completely independently of each other during the process of Meiosis.  This principle provides the basis for Mendel’s Law of Independent Assortment.
• A Punnett square can be used to predict genotypes (allele combinations) and phenotypes (observable traits) of offspring from genetic crosses.

---

Sources:

“Mendel’s Experiments and the Laws of Probability. (2021, March 6). Retrieved May 21, 2021, from https://bio.libretexts.org/@go/page/1882

“Mendel’s experiments and the laws of probability,” by OpenStax College, Biology. Retrieved from http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@9.85 / Licensed under: CC-BY: Attribution

“The Law of Segregation.” By Kahn Academy. Retrieved from https://www.khanacademy.org/science/high-school-biology/hs-classical-genetics/hs-introduction-to-heredity/a/the-law-of-segregation/ Licensed under: CC-BY: Attribution