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

sex-linked Sex linkage describes the sex-specific patterns of inheritance and expression when a gene is present on a sex chromosome (allosome) rather than a non-sex chromosome (autosome). Genes situated on the X-chromosome are thus termed X-linked, and a ...

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 or viral replication, ...

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

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

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 i ...

from a

population Population is a set of humans or other organisms in a given region or area. Governments conduct a census to quantify the resident population size within a given jurisdiction. The term is also applied to non-human animals, microorganisms, and pl ...

of ''Drosophila melanogaster'' fruit flies, which usually have dark brick red compound eyes. 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, " ...

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 linkage describes the sex-specific patterns of inheritance and expression when a gene is present on a sex chromosome (allosome) rather than a non-sex chromosome ( autosome). Genes situated on the X-chromosome are thus termed X-linked, and ...

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

sex chromosome Sex chromosomes (also referred to as allosomes, heterotypical chromosome, gonosomes, heterochromosomes, or idiochromosomes) are chromosomes that carry the genes that determine the sex of an individual. The human sex chromosomes are a typical pair ...

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

allele An allele is a variant of the sequence of nucleotides at a particular location, or Locus (genetics), locus, on a DNA molecule. Alleles can differ at a single position through Single-nucleotide polymorphism, single nucleotide polymorphisms (SNP), ...

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 nucleotide bases 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 ...

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 aromat ...

, 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 Cholesterol is the principal sterol of all higher animals, distributed in body Tissue (biology), tissues, especially the brain and spinal cord, and in Animal fat, animal fats and oils. Cholesterol is biosynthesis, biosynthesized by all anima ...

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