1.2 Dominant and Recessive Alleles

What is the Phenotype of a Heterozygote?

The dominant/recessive character is a relationship between two alleles and must be determined by observation of the heterozygous phenotype.

An example of a simple phenotype is the flower colour in Mendel’s peas. One allele as a homozygote produces purple flowers, while the other allele as a homozygote produces white flowers. But what about a heterozygous individual that has one purple allele and one white allele? What is the phenotype of a heterozygote?

This can only be determined by experimental observation. We know from observation that individuals heterozygous for the purple and white alleles of the flower colour gene have purple flowers. Thus, the allele associated with purple color is therefore said to be dominant to the allele that produces the white colour. The white allele, whose phenotype is masked by the purple allele in a heterozygote, is recessive to the purple allele.

Look at the video, Dominant Alleles vs Recessive Alleles | Understanding Inheritance by 2 Minute Classroom (2017) on YouTube, which gives an overview of dominant and recessive alleles.

Remember, alleles are different versions of a gene. The relationship of different alleles of a gene can be described as complete dominance, incomplete dominance, or co-dominance. The traits Mendel studied with his peas were all completely dominant, and therefore will only be briefly reviewed here.

In a diploid organism, if an allele is dominant, only one copy of that allele is necessary to express the dominant phenotype. If an allele is recessive, then the gene needs to have two copies (or be homozygous) to express the recessive phenotype. If an organism is a heterozygote, or has one copy of each allele type, then it will show the dominant phenotype. When representing these in written form, a dominant allele is written as a capital letter (e.g., A), while a recessive allele will be written in lower case (e.g., a). If these are alleles of the same gene, they should be written with the same letter. This is the most common way of writing genotypes (Table 1.2.1), but there are many different systems that often deviate from these general rules. Note that genes and alleles are usually written in italics and chromosomes and proteins are not. Proteins are often written in all capitals. For example, the white gene (w) in Drosophila melanogaster on the X chromosome encodes a protein called WHITE.

Table 1.2.1 Examples of Symbols Used to Represent Genes and Alleles
Alleles Meanings
A and a Uppercase letters represent dominant alleles and lowercase letters indicate recessive alleles. Mendel invented this system but it is not commonly used because not all alleles show complete dominance and many genes have more than two alleles.
a+ and a1 Superscripts or subscripts are used to indicate alleles. For wild type alleles the symbol is a superscript +.
AA or A/A Sometimes a forward slash is used to indicate that the two symbols are alleles of the same gene, but on homologous chromosomes.

References

2 Minute Classroom. (2017, February 4). Dominant Alleles vs Recessive Alleles | Understanding Inheritance [Video file]. YouTube. https://www.youtube.com/watch?v=G-_fwABa2BU&feature=youtu.be

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Introduction to 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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