The one of the best ways to learn about genetics is to understand heredity first hand, by examining actual examples of the dominant and recessive traits that you possess. Here we will explore the characteristics of facial dimples, chin cleft and free earlobes. Before delving into these examples, it is helpful to first review some associated genetic terminology.
* Alleles: The Alternate Forms of Genes *
Alleles are variations of a gene. A diploid organism gets one set of their alleles (genes) from one parent and the other set of alleles from the other parent. Some of the traits that we have are based on simple inheritance, where one version of a gene (dominant allele) masks the expression of the other version of that gene (recessive allele).
In writing, we represented dominant and recessive alleles with letters that distinguish the different types of alleles. A capital letter is used to represent the dominant allele and a lower-case letter is used to represent the recessive allele (example: dominant allele = P; recessive allele = p)
* Homozygous and Heterozygous *
When an organism has a pair of identical alleles for a character, they are said to be homozygous for that characteristic (PP, pp) When an organism has two different alleles for a gene (Pp) they are said to be heterozygous for that characteristic.
* Dominant or Recessive Phenotypes *
In cases if simple inheritance, where a characteristic is controlled by one pair of alleles and one allele is dominant over the other, this is called complete dominance. For that characteristic, an organism will have one of two phenotypes.
Recessive Phenotype: When an organism has the recessive phenotype, this means that both recessive alleles must be present (bb). With recessive traits, you know what the genotype is if the phenotype is recessive.
Dominant Phenotype: An organism showing the dominant phenotype can be homozygous or heterozygous for dominant allele (PP, Pp). It is more difficult to know genotype when dominant allele is involved.
* Examples of Simple Inheritance of Traits that Show Complete Dominance *
The characteristics listed below are products of simple inheritance. One can either possess the dominant phenotype or the recessive phenotype. All of the following are dominant traits, meaning that if the trait described is present (dimples, cleft chin, or free earlobes) it is the dominant phenotype and the corresponding genotype would be either homozygous dominant or heterozygous. If the trait is absent (no dimple, no cleft chin, attached earlobes) the corresponding genotype is recessive.
~ Facial Dimples ~
If you aren't sure if you have them, smile! Dimples are easiest to see when smiling. With dominant phenotype, you may have a dimple only on one side, or on both.
Alleles: D, d
Dominant phenotype: dimples present
Dominant genotype: D- (the dash means the genotype may be either DD or Dd)
Recessive genotype: dd
~ Chin Cleft ~
A prominent cleft in the chin is due to the bond structure which underlies the Y-shaped fissure of the chin. Females appear to be less conspicuously affected than males.
Dominant phenotype: cleft present
Dominant genotype: C-
Recessive genotype: cc
~ Free Earlobe ~
The dominant trait is for lobes to hang free, a bit of lobe hanging down prior to the point where the bottom of the ear attaches to the head. With the recessive phenotype, the lobes are attached directly to the head.
Alleles: F. f
Dominant phenotype: free lobes
Dominant genotype: F-
Recessive genotype: ff
* Sources *
Thorpe ed. (2007) Biology 120 Lab Manual. Grand Valley State University.
Doezema, B. (2006) Biology 101 Lab Manual. Grand Rapids Community College.