Genetics of the white Locus

An example chosen to illustrate the fundamental nature of dominance

by relating genotype and phenotype

 

from Wikipedia.org

Background

 

Drosophila melanogaster, the fruit fly, is one of the "model systems" in which many of the rules of genetics were worked out.  The rules have proven to be valid for plants and animals in general, including humans.

 

Drosophila collected from the wild have dark red eyes.  Most populations maintained in laboratories do too.  However, occasionally mutations occur, giving rise to eye colors that are different from normal.  One of the first mutants that was discovered had white eyes.  Therefore, the mutation was called white, and the genetic element responsible for the white-eyed trait was also called white.

 

Since the discovery of the first mutant allele of white, literally hundreds of additional mutations in the white gene have been discovered.  Some alleles cause the eyes to be white; some cause the eyes to be pale orange; some cause the eyes to be white with red spots; some cause the eyes to be darker than the original allele from the first wild population that was studied.  With so many different alleles, how can we keep track of the alleles in genetic crosses?

 

We might be tempted to choose the symbol w to refer to the white gene.  This is what Drosophila geneticists have done.  We might also be tempted to use a lower-case w to refer to a recessive allele, and an upper-case W to refer to a dominant allele, but this would not work.  There are hundreds of alleles, not merely two.  (Besides, Drosophila geneticists use W to refer to the Wrinkled gene!)  We need a separate, unique symbol for each allele.  The solution to this dilemma is illustrated below:

 

 

3 Alleles of white

Phenotypes of flies homozygous for each allele

Description of phenotype

wildtype

white

light-orange

Name of allele

white+

white1118

whiteapricot

Symbol of allele

w+

w1118

wa

 

 

Each mutant allele is named white (to indicate what gene it is), followed by an allele designation.  The symbols all use w to indicate the white gene, with the allele designations as superscripts.  Some allele designations are descriptive, such as whiteapricot , whitecherry, and whitespotted.  Others are simply numbers (white1118 may indicate that it was the 1,118th allele found by a particular research group.)  The original allele found in the type of flies collected from the wild (wildtype) is usually designated merely by a + to indicate the presence of the normal version of the gene.

 

Phenotypes of Heterozygotes

 

By crossing flies of various eye colors, we can construct flies that are heterozygous for these different alleles, in any combination we like.  The following table illustrates what we find:

 

 

Genotype

Phenotype

w+

__________

w1118

w+

_______

wa

wa

__________

w1118

 

 

Here is your challenge:

 

1.  Describe the relationships of these alleles.  For each genotype, which allele is dominant and which is recessive?

 

2.  Is dominance or recessiveness a property of an allele?  Or, is it a relationship between alleles?  As you answer this question, consider the wa allele in particular.

 

3.  What ideas can you come up with to explain what's going on?hint

 

Hint: Connect Genotype with Phenotype

 

The protein produced by the white gene builds pigment granules.  If the amount of this protein is normal (say, 50% - 150% of wildtype), the eyes accumulate a normal amount of pigment.  If the protein cannot function at all due to mutation, then no pigment can accumulate.  If a mutation in the gene alters the protein so that it cannot function very well--but can still function a little--then the eyes can accumulate only a small amount of pigment.

 

According to the data available to you, how well do the proteins produced from these three different alleles function?  Do some have "lots of activity," and others have "very little activity" or "no activity"?  Based on your assessment of how well the proteins function, what do you predict for the eye colors?  It may help you to fill in the following table:

 

Genotype

Degree of Protein Activity (for each allele)*

Total Protein Activity in Eye*

Amount of Pigment Expected*

 

w+

_____________

w1118

 

w+ activity

____________**

 

w1118 activity

 

 

 

w+

_____________

wa

 

w+ activity

_____________

 

wa activity

 

 

 

wa

_____________

w1118

 

wa activity

_____________

 

w1118 activity

 

 

* We've merely looked at the eye colors, so we can't be more quantitative than to use a scale of activity that goes from "none" to "lots."

** Put your assessment of one allele above the line, and your assessment of the other allele below the line, the way the genotypes are represented.

 

[Note what you have just done.  You proposed a tentative explanation for eye color--an hypothesis that each allele produces a protein with more or less activity.  You then used this hypothesis to predict the eye colors of heterozygotes.  If your predictions are consistent with what we find when we perform the genetic crosses, then your hypothesis might be a good model for what's going on in fly eyes.  You'd need more tests before you could feel fairly certain about your model--such as determining the precise nature of each mutation and how it affects the protein--but you've made a good start.]

 

The Last Two Questions:

 

4. For the white locus, how does genotype control phenotype?

 

5. Use the same principles you have uncovered here to describe how genotype might control phenotype for other pigment-based phenotypes.

Some possible examples:

- human hair color (black vs dark brown vs light brown vs blonde)

- one aspect of human eye color (dark brown vs medium brown vs light brown vs blue [blue = lack of brown pigment])

- human skin color (dark brown vs medium brown vs light brown vs pale)

- snapdragon flower color (red vs pink vs white)

 


Sex Linkage

 

The white gene is on the X chromosome of Drosophila, and therefore shows an X-linked pattern of inheritance.  There are many good descriptions of sex linkage, so we will not address it here.  Test your understanding by describing how you would construct flies with the genotypes mentioned above.