Mendel's First Law

Variations to Mendel's First Law

Pedigree Analysis

Mendel's Second Law

Chi-Square Test



Modifier Genes

Penetrance and Expressivity

Study Questions

Mendelian Genetics Overheads

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Genetic Topics

Variations to Mendel's First Law of Genetics

The true test of any theory of science is its ability to explain results that at first glance appear to be a clear exception to the theory. But, if the exception can be explained by the theory, then the theory is further validated. One such genetic example that challenged Mendel's first law was the relationship between two alleles that do not express a typical dominance/recessive relationship. That is, the F1 does not exhibit the genotype of one of the two pure line parents. This type of allelic relationship was termed codominance.

Codominance - a relationship among alleles where both alleles contribute to the phenotype of the heterozygote

Example: Codominance

Species: Four o'clock plants

Trait: Flower color

Pure line phenotypes: red or white flower

Parental cross: Red x White

F1: We would expect red or white flowers in this generation, depending upon which allele is dominant. But, the F1 plants produced pink flowers. As with any experiment of this sort, the F1 plants are selfed. The results that were obtained were:

F2 phenotypic ratio: 1/4 Red : 1/2 Pink : 1/4 White

Snapdragon Flower Colors

It appears as if the red and white alleles are interacting in the heterozygote to generate the pink flowers. Another example of codominance can be seen by looking at a biochemical phenotype.

Biochemical phenotype - a phenotype that is revealed by biochemical experimentation; examples are DNA markers (RFLPs); protein-size markers (isozymes); quantity of a metabolite; immunological reaction

As an example, let assume that a gene of interest resides on a DNA fragment that is 3.0 kb in size in parent one and 2.0 kb in size in a second parent. (See figure below.) When we cross the two parents, one chromosome carrying the gene comes from each parent. Since our technique recognizes both copies in the parents, the parental signal will be twice as strong as the F1 signal that contains one chromosome, and thus one copy of the particular DNA size, from each parent. The F2 generation will segregate for the three different genotypes in a 1:2:1 ratio.

Genotypic designations could be given for each of the alleles. For example, if the 3.0 kb fragment is designated as allele A1, the genotype of parent 1 will be A1A1. Allele designation A2 will be used for the 2.0 kb fragment and the genotype of parent 2 will be A2A2. Because the F1 is heterozygous, its genotype will be A1A2. Finally, the genotypes of the F2 generation will segregate 1A1A1:2A1A2:1A2A2.

Often expression levels in an individual can only reach a certain intensity regardless of whether the individual is a homozygote or heterozygote. For example, pea plants heterozygote for the tall/dwarf allelic pair are the same size as the homozygous tall parent. But expression is not considered with DNA markers because we are monitoring the presence or absence of a specific DNA fragment. Therefore, DNA fragments are a true example of codominance where each allele is equally expressed in the F1 individual.

Incomplete dominance - the F1 produces a phenotype quantitatively intermediate between the two homozygous parents; if the product is exactly intermediate between the two homozygous parents the relationship is termed no dominance (although some have tried to substitute the term no dominance for codominance, it has not been widely accepted)

Diagrammatic Representation of Allelic Relationships

Copyright © 2000. Phillip McClean