More Elaboration About Mendel’s Complete Dominance 
Ben G. Bareja, Nov. 15, 2013 

Complete dominance is a type of dominance in which one allele in a single gene pair manifests itself fully in the heterozygote to the complete exlusion of the other, contrasting allele. It was discovered by Gregor Mendel in his monohybrid study with garden pea.

Thus in the beginning of his report (Experiments on Plant Hybridisation, Mendel 1865), he introduced the terms dominant and recessive. Here’s a quotation: 

Henceforth in this paper those characters which are transmitted entire, or almost unchanged in the hybridization, and therefore in themselves constitute the characters of the hybrid, are termed the dominant, and those which become latent in the process recessive. The expression ‘recessive’ has been chosen because the characters thereby designated withdraw or entirely disappear in the hybrids, but nevertheless reappear unchanged in their progeny, as will be demonstrated later. 

Mendel reported that he crossed two pureline parents of garden pea, producing a ‘hybrid’ which exhibited the dominant character from one parent. As to seed form (or shape), he cross-pollinated the flowers of smooth-seeded parent plants by dusting the stigma with pollen from wrinkled-seeded parents.  He found that the seeds that formed were smooth, or round in shape.

This phenomenon is called complete dominance in which a heterozygote (the hybrid) only shows the dominant character. Stated another way,  the dominant character manifests itself in the heterozygote by masking (hiding or concealing) the recessive character.

To elucidate further, consider the contrasting characters in Mendel’s cross smooth-seed x wrinkled-seed. Although the parent plants may differ in other characteristics, let them be disregarded and we focus only on seed form. Each character is carried by a gene (Mendel’s ‘factor’) or, in reference to its position in the chromosome, or locus, an allele

In this cross there are two parents, one we call P1, and the other we call P2. Using letters and number subscripts, let us designate by the symbol a1 the allele for the smooth seed and a2 the allele for the wrinkled seed character. Both parents being homozygous, P1’s genotype will be a1a1 and that of P2 will be a2a2. This representation shows that the character under consideration is governed by the nature of the two alleles composing the gene pair under a diallelic gene system. The resulting F1, therefore, is a heterozygote, meaning that it is an individual having a genotype consisting of a gene pair in which both alternative alleles, a1 and a2, are present. This heterozygous genotype is denoted as a1a2. The cross is as illustrated below:

a1a1 (P1) x a2a2 (P2)

a1a2 (F1)

With complete dominance, the genotype a1a2 will be expressed as smooth seeds. This is because the smooth character is dominant over the wrinkled-seed character. It does not matter that the allele a2 for the wrinkled seed is present in the F1. Its expression is completely suppressed by the a1 allele.

By analogy, it can be said that the dominant allele is so powerful that it asserts itself and completely renders powerless the recessive allele in the heterozygote. 

Consequently, where it is established that one character is completely dominant over another in a cross, it has become of common usage to designate the dominant allele by a capital letter and the recessive by a small letter. It is thus common to designate the genotype of the smooth seeded heterozygous pea as Ss or, in describing the same character as round, as Rr

But what if the observed character was the other way around? What if it was the wrinkled-seed character that was expressed, thereby excluding the smooth character? Is the dominance relation still complete dominance? 

I submit that the answer should be in the affirmative. It does not matter which allele is dominant. If such was the case, the symbol for the genotype of the heterozygote could probably be Ww where the allele W (capital letter) is for wrinkled seed and w (small letter) is for smooth or round seed. 

It’s not that there is an absolute rule that letters used to denote alleles should be based on the starting letter of the description of dominant characters. It is nonetheless the usual practice for convenience. By using capital letter of the beginning of the description of the dominant character, it will be easier to recall the character being referred to, as well as the recessive character which is then designated by the corresponding small letter. 

However, the phenotypic expression of any gene may be subject to certain environmental conditions. It is also possible that the type of dominance displayed may be modified when using other methods of analysis.

Now, if there is complete dominance, is there also incomplete dominance? The answer is Yes and there are other more (click here to continue reading).

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