''white'', abbreviated ''w'', was the first

sex-linked Sex linked describes the sex-specific patterns of inheritance and presentation when a gene mutation (allele) is present on a sex chromosome (allosome) rather than a non-sex chromosome ( autosome). In humans, these are termed X-linked recessive, ...

mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA replication, DNA or viral repl ...

discovered, found in the fruit fly ''Drosophila melanogaster''. In 1910

Thomas Hunt Morgan Thomas Hunt Morgan (September 25, 1866 – December 4, 1945) was an American evolutionary biologist, geneticist, embryologist, and science author who won the Nobel Prize in Physiology or Medicine in 1933 for discoveries elucidating the role that ...

and Lilian Vaughan Morgan collected a single male white-eyed

mutant In biology, and especially in genetics, a mutant is an organism or a new genetic character arising or resulting from an instance of mutation, which is generally an alteration of the DNA sequence of the genome or chromosome of an organism. It ...

from a

population Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction using a ...

of ''Drosophila melanogaster'' fruit flies, which usually have dark brick red

compound eyes A compound eye is a visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidia, which are tiny independent photoreception units that consist of a cornea, lens, and photoreceptor cells which distin ...

. Upon crossing this male with

wild-type The wild type (WT) is the phenotype of the typical form of a species as it occurs in nature. Originally, the wild type was conceptualized as a product of the standard "normal" allele at a locus, in contrast to that produced by a non-standard, "m ...

female flies, they found that the offspring did not conform to the expectations of

Mendelian inheritance Mendelian inheritance (also known as Mendelism) is a type of biological inheritance following the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 by Hugo de Vries and Carl Correns, and later popularize ...

. The first generation (the F1) produced 1,237 red-eyed offspring and three white-eyed male flies. The second generation (the F2) produced 2,459 red-eyed females, 1,011 red-eyed males, and 782 white-eyed males. Further experimental crosses led them to the conclusion that this mutation was somehow physically connected to the "factor" that determined sex in ''Drosophila''. This led to the discovery of

sex linkage Sex linked describes the sex-specific patterns of inheritance and presentation when a gene mutation (allele) is present on a sex chromosome (allosome) rather than a non-sex chromosome (autosome). In humans, these are termed X-linked recessive, ...

, in which the gene for a trait is found on a

sex chromosome A sex chromosome (also referred to as an allosome, heterotypical chromosome, gonosome, heterochromosome, or idiochromosome) is a chromosome that differs from an ordinary autosome in form, size, and behavior. The human sex chromosomes, a typical ...

. Morgan named this trait ''white'', now abbreviated ''w''. Flies possessing the ''white''

allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...

are frequently used to introduce high school and college students to genetics. Sex-linked inheritance

Function

The protein coded by the ''white'' gene functions as an ATP-binding cassette (ABC) transporter. It carries the precursors of the red and brown eye color pigments,

guanine Guanine () ( symbol G or Gua) is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine (uracil in RNA). In DNA, guanine is paired with cytosine. The guanine nucleoside is c ...

and

tryptophan Tryptophan (symbol Trp or W) is an α-amino acid that is used in the biosynthesis of proteins. Tryptophan contains an α-amino group, an α-carboxylic acid group, and a side chain indole, making it a polar molecule with a non-polar aromatic ...

, into the developing eyes during pupation. White-eyed flies are not blind; instead they are easily temporarily blinded by bright light at certain frequencies because they lack the protection provided by the red and brown pigments. The human version of ''white'' is ABCG1, and is involved in transporting lipids and cholesterol into cells.

Effects mutation

''Drosophila melanogaster'' with the white eye mutation typically have shorter life spans than wildtype ''Drosophila''. They also experience many neurological deficiencies in addition to eye defects. Some of the deficiencies that they experience includes difficulty in mobility, and a low stress tolerance. ''Drosophila melanogaster'' with the white eye mutation often experience an increased sensitivity to light and a decrease in visual acuity. They have significantly less in the number of synaptic vesicles of photoreceptors. White eye mutants of ''Drosophila melanogaster'' experience a lower rate of reproduction than their wildtype counterparts because they experience a reduced rate of sexual arousal during daylight. Ectopic expression of ''white+'' induces male-male courtship in ''Drosophila''. ''White+'' controls the copulation success in ''Drosophila melanogaster''.

Notes and references

{{reflist Classical genetics Drosophila melanogaster genes