6.1 Introduction

Learning Objectives

  • Discuss what is meant by “Extensions and Modifications of Basic Principles of Mendelian Genetics”.
  • Identify additional factors at a single locus and the effect on the results of genetic crosses.
  • Describe the various types of dominance, lethal alleles and pleiotropic genes.
  • State the molecular conditions required to produce “Muller’s Morphs”.


Mendelian inheritance describes inheritance patterns that obey two laws — the Law of Segregation and the Law of Independent Assortment. Simple Mendelian inheritance involves a single gene with two different alleles, and alleles which display a simple dominant/recessive relationship. We will examine traits that deviate from the simple dominant/recessive relationship — the inheritance patterns of these traits still obey Mendelian laws, however, they are more complex and interesting than Mendel had realized. In this chapter, we will investigate the interactions of alleles at a single locus. We will begin with the difference between somatic and germ line mutations, followed by the concept of Pleiotrophy. Then, we will look at the various types of dominance, followed by the Biochemical basis of dominance. Finally, we will end with more sophisticated interactions that can be described by “Muller’s Morphs”, which deal with the interrelationships of mutant and wild type alleles at a more detailed level.

Red flower crossed with white flower to result in pink flowers
Figure 6.1.1 Incomplete Dominance

A specific section of a chromosome is called a locus. Because each gene occupies a specific locus along a chromosome, the terms locus and gene are often used interchangeably. However, the term “gene” is a much more general term, while “locus” usually is limited to defining the position along a chromosome. Each locus will have an allelic form (allele); that is, a specific DNA sequence. In a population of individuals, there will be sequence variation so there will be different alleles. Some may be defined as wild type, some as variants, others as mutant. The complete set of alleles at all loci in an individual is its genotype. Typically, when writing out a genotype, only the alleles at the locus (or loci) of interest are considered and written down — all the others are still present and assumed to be wild type. So, usually only the alleles at the few mutant loci appear in the written genotype. All the many, many others that are wild type, are not. The visible or detectable effect of alleles on the structure or function of that individual is called its phenotype — what it looks like. 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 behaviour. If two alleles are present in an individual, as is the case with diploid organisms, then various interactions between them may influence their expression in the phenotype.

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Open Genetics by Natasha Ramroop Singh, Thompson Rivers University is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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