|Athro, Limited Biology Genetics Eye Color Genes|
|How are human eye colors inherited?|
(An explanation for our interactive eye color genetics calculator)
Genes are structures that are carried on larger structures called chromosomes. The genes for each characteristic come in pairs, and the two genes together produce a given characteristic. One of the genes of the pair came from the father and the other came from the mother.
An individual with brown and blue alleles of the bey2 gene
on chromosome 15. There are two copies of chromosome 15.
Each copy has the bey2 gene. On one copy the bey2 gene is
in the brown flavor, in the other the bey2 gene is in the blue
flavor. The difference between the brown and blue alleles is
due to some difference in the genetic code for each gene (the DNA
sequence for the bey2 gene isn't yet known).
Genes are particles that get inherited.
Humans have several eye color genes. Two of these genes are named bey2 (brown eye) and gey (green eye).
Genes come in flavors called alleles.
The bey2 gene has two flavors - brown and blue.
The gey gene also has two flavors - green and blue.
Genes are on chromosomes. There is one copy of the gene on each chromosome (some genes also come in many copies).
Chromosomes come in pairs.
Thus each individual has two copies of each gene. These two copies can be the same flavor (allele) or different flavors.
Genes are used to produce proteins. A gene that comes in two flavors might come in one flavor where the protein works correctly and another flavor in which the protein does not work. Thus an individual with one copy of the good flavor and one copy of the defective flavor for a gene could still produce a protein that worked.
This is the simplest case of dominance. The allele that produced the working protein would be dominant over the allele that produced the defective protein. Individuals with two copies of the working allele and individuals with one copy of the working and one copy of the defective allele would both be able to produce working protein. Only individuals with two copies of the defective allele would be unable to produce working protein. The allele that produces this defective protein would be recessive to the allele that produces the functional protein.
Dominance can also be produced by other differences between the protein products of alleles - a recessive allele does not mean a defective gene product. Dominant and recessive should be taken only as descriptions of the expression of alleles, and not given any value judgement. Blue eyes are not worse than green or brown eyes.
Dominant and recessive flavors of genes and the presence of genes on chromosomes that come in pairs can explain the eye color inheritance patterns described here (but remember that there is more to human eye color inheritance than the simple two gene model described here).
The bey2 gene has two flavors - brown is dominant over blue. Each individual has two copies of this gene, each can be one of the flavors. The possible allele combinations for the bey2 gene are: brown-brown, brown-blue, and blue-blue. Of these three, the brown-brown and brown-blue combinations will both produce brown eyes, as brown is dominant over blue. Only the blue-blue combination will produce blue eyes.
But there is a second common gene for eye color - the gey gene. It also has two flavors - green is dominant over blue. In addition, a green allele of gey is dominant over a blue allele of bey2 and recessive to a brown allele of bey. Thus the alleles of the two genes have a dominance hierarchy - bey2-brown is dominant over everything else, gey-green is dominant over bey2-blue and gey-blue but recessive to bey2-brown, and both of the blues are recessive to everything else. Thus the bey2 (brown/blue) gene might be better thought of as the brown/non-brown gene.
The final piece of the story (and remember, it is just a story, there are more than just these two genes involved in eye color inheritance) is that a child gets one chromosome of each pair from each parent. The bey2 gene is on chromosome 15. A parent with brown-blue alleles of the bey2 gene has a pair of chromosome 15s, with the brown allele on one and the blue allele on the other. This parent could give either the chromosome bearing the brown allele or the chromosome bearing the blue allele to a child. A child with brown-blue alleles of the bey2 gene got the brown allele (and one copy of chromosome 15) from one parent, and the blue allele (and the other copy of chromosome 15) from the other parent.
Thus we can figure out the possible flavors of genes that we could find in the children of a particular pair of parents (the genotype) and what eye color those children would have (the phenotype).
For Example: A parent with bey2 alleles of brown-brown and gey alleles of blue-blue would have brown eyes. If this parent mated with a blue eyed partner (bey2=blue-blue, gey=blue-blue) then all of their children would have brown eyes and a genotype of (bey2=brown-blue, gey=blue-blue) These offspring would carry blue alleles and themselves be capable of having children with blue eyes. More Examples
|Eye color is more complex than two genes|
In humans three genes involved in eye color are known. They explain typical patterns of inheritance of brown, green, and blue eye colors. However, they don't explain everything. Grey eye color, Hazel eye color, and multiple shades of blue, brown, green, and grey are not explained. The molecular basis of these genes is not known. What proteins they produce and how these proteins produce eye color is not known. Eye color at birth is often blue, and later turns to a darker color. Why eye color can change over time is not known. An additional gene for green is also postulated, and there are reports of blue eyed parents producing brown eyed children (which the three known genes can't easily explain [mutations, modifier genes that supress brown, and additional brown genes are all potential explanations]).
The known Human Eye color genes are: EYCL1 (also called gey), the Green/blue eye color gene, located on chromosome 19 (though there is also evidence that another gene with similar activity exists but is not on chromosome 19). EYCL2 (also called bey1), the central brown eye color gene, possibly located on chromosome 15. EYCL3 (also called bey2), the Brown/blue eye color gene located on chromosome 15. EYCL3 probably involves mutations in the regulatory region just before the OCA2 gene (which produces a protein that is expressed in melanocytes). A second gene for green has also been postulated. Other eye colors including grey and hazel are not yet explained. We do not yet know what these genes make, or how they produce eye colors. The two gene model (EYCL1 and EYCL3) used above explains only a portion of human eye color inheritance. Both additional eye color genes and modifier genes are almost certainly involved.