1.4 One Locus on a Chromosome – Segregation – Monohybrid

Not only did Mendel solve the mystery of inheritance as units (genes), he also invented several testing and analysis techniques still used today. Classical genetics is the science of examining biological questions using controlled matings of model organisms. It began with Mendel in 1865, but did not attain widespread usage until Mendel’s work was rediscovered in 1903 by four researchers (E. von Tschermak, H. de Vries, C. Correns, and W. J. Spillman). Then Thomas Morgan began working with fruit flies in 1908 and used this work. Later, starting with Watson and Crick’s structure of DNA in 1953, classical genetics was joined by molecular genetics, the science of solving biological problems using DNA, RNA, and proteins. The genetics of DNA cloning began in 1970 with the discovery of restriction enzymes and plasmids as cloning vectors.

Knowing what we now know about the process of meiosis, we can better understand the mechanisms underlying Mendel’s First Law. The Law of Segregation states that every individual contains a pair of alleles for each gene, which segregate during the formation of gametes, and so for every gene pair, each parent passes on a random allele to its offspring. The series of experiments that led to the formulation of Mendel’s First Law where based on the process of Monohybrid crosses, which we will discuss.

Take a look at the following video on the Law of Segregation (Mendel’s First Law of Inheritance) by FL-Genetics/03 (2018).



A specific position, region, or segment along a chromosome is called a locus. Each gene occupies a specific locus (so the terms locus and gene are often used interchangeably). Each locus will have an allelic form (allele). The complete set of alleles (at all loci of interest) in an individual is its genotype. Typically, when writing out a genotype, only the alleles at the locus (loci) of interest are considered – all the others are present and assumed to be wild type but are normally not written in the genotype. The observable or detectable effect of these alleles on the structure or function of that individual is called its phenotype. The phenotype studied in any particular genetic experiment may range from simple, visible traits such as hair color, to more complex phenotypes including disease susceptibility or behavior. If two alleles are present in an individual, then various interactions between them may influence their expression in the phenotype.

True-Breeding Lines

Geneticists make use of true-breeding lines just as Mendel did (Figure 1.4.1). These are in-bred populations of plants or animals in which all parents and their offspring (over many generations) have the same phenotypes with respect to a particular trait. True-breeding lines are useful, because they are typically assumed to be homozygous for the alleles that affect the trait of interest.

When two individuals that are homozygous for the same alleles are crossed, all of their offspring will all also be homozygous. The continuation of such crosses constitutes a true breeding line or strain. A large variety of different strains, each with a different, true breeding character, can be collected and maintained for genetic research.


Crosses of purple flowers with purple flowers and purple flowers with white flowers
Figure 1.4.1: (a) A True-Breeding Line (b) A Monohybrid Cross Produced by Mating Two Different Pure-Breeding Lines. [Long description]

File:Figure 12 01 02.jpg - Wikimedia Commons Figure 1.4.2 Experimental Cross Between Two True Breeding Plants [Long description]


Monohybrid Crosses

A monohybrid cross is one in which both parents are heterozygous (or a hybrid) for a single (mono) trait. The trait might be petal colour in pea plants (Figures 1.4.1 and 1.4.2). Recall that the generations in a cross are named P (parental), F1 (first filial), F2 (second filial), and so on.

By using monohybrid crosses, Mendel discovered that genes were discrete units that separated in the creation of offspring. Previous ideas of blending inheritance would mean that a cross between a white flower and a purple flower would create a ‘blended’ phenotype. Instead what Mendel saw was distinct parental colours in the hybrids, that when crossed would produce in specific ratios the purple and white seen in the parents. These traits were not blended when the true-breeding lines were crossed, but instead those parental alleles were carried on through the offspring. Through the monohybrid cross he was able to discern the dominant and recessive alleles of each gene he studied in the pea plants. In further crosses (F3, F4, etc.), these traits were continuously transmitted and not lost, though they may be hidden as seen in the F1 generation. Figure 1.4.3 demonstrates a monohybrid cross and progeny produced.

Cross of purple flower plant with white flower plant to give 3 to 1 ratio of purple to white in F2 generation
Figure 1.4.3 A Monohybrid Cross Showing Parental, F1 and F2 Generations [Long description]

Media Attributions


Deyholos, M. (2017). Figure 8. (a) A true-breeding line (b) A monohybrid cross produced…[digital file]. In Locke, J., Harrington, M., Canham, L. and Min Ku Kang (Eds.), Open Genetics Lectures, Fall 2017 (Chapter 16, p. 8). Dataverse/ BCcampus. http://solr.bccampus.ca:8001/bcc/file/7a7b00f9-fb56-4c49-81a9-cfa3ad80e6d8/1/OpenGeneticsLectures_Fall2017.pdf

Frank Lectures. (2018, May 12). Law of segregation (Mendel’s First Law of Inheritance) (FL-Genetics/03) [Video file]. YouTube.https://www.youtube.com/watch?v=4tVgmYlHPVM

Rye, C., Wise, R., Jurukovski, V.,  DeSaix, J., Choi, J. & Avissar, Y. (2016, October 21). Figure 12.3. In one of his experiments… [digital file]. CNX OpenStax Biology (Chapter 12). https://openstax.org/books/biology/pages/12-1-mendels-experiments-and-the-laws-of-probability

Long Descriptions

  • Figure 1.4.1 Two parts, showing purple and white flowers. The resulting F1 and F2 generations after breeding two pure line purple flowers (genotype AA) are: all offspring in the F1 and F2 contain purple flowers (all have genotype AA). The resulting F1 and F2 generations after breeding a pure line purple flower (AA) and a pure line white flower (aa) are: the F1 contains all purple flowers, but are heterozygous (Aa) and the F2 contains both purple (homozygous and heterozygous) as well as white flowers. [Back to Figure 1.4.1]
  • Figure 1.4.2 Plants containing either white or violet coloured flowers: Parental (P) and F1 and F2 generations, result from the breeding of a pure line violet flowered plant and a pure line white flowered plant. The F1 generation are hybrids, all containing violet flowered plants and the F2 generation produce a mixture of violet flowered plants and white flowered plants in a 3:1 ratio. [Back to Figure 1.4.2]
  • Figure 1.4.3 The parental pure breeding lines are represented by purple flowers of genotype PP and white flowers of genotype pp. Gamete formation is shown to produce the F1 generation, all of which are purple flowers of the heterozygous genotype, Pp. A punnett square outlines the production of the F2 generation from self-fertilization of the F1; three purple flowers are produced, with one white. The typical monohybrid ratio of 3:1 in the F2 generation is shown. [Back to Figure 1.4.3]


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