Before discussing the genetics of eye pigment and myopia, it helps to first review some of the terminology related to these aspects of heredity.
* Alleles: Variations of a Gene *
Alleles are variations of a gene. A diploid organism gets one set of 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 or Heterozygous Genotype *
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. 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. 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 (non-blue eyes, nearsightedness) it is the dominant phenotype and the corresponding genotype would be either homozygous dominant or heterozygous. If the trait is absent (blue eyes, not nearsighted) the corresponding genotype is recessive.
~ Eye Color ~
We're kind of cheating here. Eye color, as well as hair and skin color, is a complex trait. Not a case of simple inheritance. The main pigment is melanin, and the more melanin, the darker the color.
Although the genetics of eye color is complex, alleles for the production of melanin dominate those for lack of melanin. So if we evaluate eye color as being blue (recessive) or non-blue (dominant) we can treat it as a characteristic of simple inheritance.
Alleles: E, e
Dominant phenotype: non-blue eyes
Dominant genotype: E- (the dash indicates dominant phenotype, a genotype of either EE or Ee)
Recessive genotype: ee
~ Early Onset Myopia (childhood) ~
Nearsightedness, or myopia, is a complex trait with at least 4 gene loci involved, however the heritability of myopia is very high and shows a dominant pattern.
Alleles: M, m
Dominant phenotype: nearsightedness
Dominant genotype: M-
Recessive genotype: mm
* Sources *
Thorpe ed. (2007) Biology 120 Lab Manual. Grand Valley State University.
Doezema, B. (2006) Biology 101 Lab Manual. Grand Rapids Community College.
ions in Microbiology.