Mendel’s Inheritance Patterns
Mendel’s approach in explaining genetic inheritance was different from the common genetic knowledge then, in that he explained that each trait was as a result of a pair of genes commonly referred to as alleles (O’Neil, 2011). The accepted knowledge back then was then traits (phenotypes) in living things were a blend from the parents. However, the Mendel’s approach contradicted with this notion arguing that there could never be a blend of traits in an offspring but existence of phenotypes being as either recessive or dominant. “An individual is said to be heterozygous when two alleles differ and homozygous when they are the same (Stewart, 2008, p. 25)”. Therefore blending of two alleles of homozygous nature resulted in pure breeds.
Mendel proved that breeding of two different pure genotypes resulted in recessive and dominant genes where the latter are genes that can be physically observed while the former are those that are absent physically but can be transmitted to the offspring. He proved this using garden Peas (Pisium Satirum) where he noted that blending two peas having white or purple flowers resulted in offspring that had either purple or white colours and not a blend.
The success of the approach depended on Mendel choosing suitable traits that could allow tracking of the characteristics with exactly two alleles—one dominant and the other recessive. Where each characteristic is controlled by one of two alleles therefore it was proved through the pea experiment that it was possible to acquire independent inheritance of various traits even when the alleles responsible for them are found on different chromosomes (O’Neil, 2011). This managed to even explain why certain traits and genetic diseases are obtained by individuals yet their parents did not exhibit them.
In choosing the garden peas to study inheritance of characteristics, Mendel enjoyed advantages such as their quick growth to numerous numbers and easily manipulation of their reproduction (O’Neil, 2011). The pea plants contained both sexual organs and therefore they could self pollinate or cross pollinate. Due to the advantage of having both sexes, Mendel could specifically cross-pollinate pure breeds having certain traits and in turn observe them over many generations.
Three Major Principles
From the garden peas experiment Mendel came up with six major ideologies that theorized his work. The principles include: principle of segregation; principle of assortment; principle of unit characters; principle of individual traits; principle of dominance; principle of allele pairs per genes; and principle of allele separation before reproduction (Stewart, 2008).
The principle of dominance or uniformity was discovered by Mendel when he cross-pollinated two pea pure breeds. One pure breed had wrinkled seeds and the other pure breed had smooth seeds. From his analysis he discovered that the offspring did not have any semi-wrinkly seeds but rather all of them had smooth seeds (O’Neil, 2008). This proved that a cross-breed of pure breeds resulted in an offspring that had a trait of only one of the parent. Therefore from the experiment the hypothesis can be stipulated that the progeny of two homozygous individuals resulted in a single trait that is characteristic of one of the homozygous individuals. Exceptions of the principle are in sex-linkage, penetrance, and expressivity that were discovered later after Mendel (Stewart, 2008)
The segregation principle stipulates that heterozygous individuals could equally pass one of their alleles to their offspring. This means that the two alleles in the genes separate before reproduction and unite randomly with other alleles to form other breeds. From these hypothesis rules of probability can be used to ascertain the likelihood of a specific allele can be passed on. The Punnet square is the best tool that can be used to demonstrate this law.
The assortment principle governs hereditability where allele pairs split autonomously during gamete formation (Stewart, 2008). The principle means that characteristics are transmitted to offspring autonomously from each other. For instance, according to Mendel, “the ability of a pea plant to produce wrinkled seeds as opposed to smooth seeds does not increase the probability of inheriting purple flowers as opposed to white ones (O’Neil, 2011)”. This explains why human inheritance of a particular hair colour does not lower or increases the probability of six toes on each foot.
Dr. O’Neil, D. (2011). Basic Principles of Genetics: An introduction to Mendelian genetics. California: Palorama College. Available at:
Stewart, N. (2008). Plant Biotechnology and Genetics: principles, techniques, and applications. New Jersey: Wiley & Sons. Pp. 25-32