White, abbreviated w, was the first sex-linked mutation

Mutation

In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...

 ever discovered in the fruit fly Drosophila melanogaster

Drosophila melanogaster

Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...

. In 1910 Thomas Hunt Morgan

Thomas Hunt Morgan

Thomas Hunt Morgan was an American evolutionary biologist, geneticist and embryologist and science author who won the Nobel Prize in Physiology or Medicine in 1933 for discoveries relating the role the chromosome plays in heredity.Morgan received his PhD from Johns Hopkins University in zoology...

, (more accurately, his wife) collected a single male

Male

Male refers to the biological sex of an organism, or part of an organism, which produces small mobile gametes, called spermatozoa. Each spermatozoon can fuse with a larger female gamete or ovum, in the process of fertilization...

 white-eyed mutant

Mutant

In biology and especially genetics, a mutant is an individual, organism, or new genetic character, arising or resulting from an instance of mutation, which is a base-pair sequence change within the DNA of a gene or chromosome of an organism resulting in the creation of a new character or trait not...

 from a population

Population

A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...

 of Drosophila melanogaster fruit flies, which usually have bright red eyes. Upon crossing this male with wild-type female

Female

Female is the sex of an organism, or a part of an organism, which produces non-mobile ova .- Defining characteristics :The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon, is produced by the male...

 flies he found that the offspring did not conform to the expectations of Mendelian inheritance

Mendelian inheritance

Mendelian inheritance is a scientific description of how hereditary characteristics are passed from parent organisms to their offspring; it underlies much of genetics...

. The first generation (the F1) produced 1,237 red-eyed offspring and three white-eyed flies, all males. 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 Morgan to the conclusion that this mutation was somehow physically connected to the "factor" that determined gender

Gender

Gender is a range of characteristics used to distinguish between males and females, particularly in the cases of men and women and the masculine and feminine attributes assigned to them. Depending on the context, the discriminating characteristics vary from sex to social role to gender identity...

 in Drosophila. Morgan named this trait white, now abbreviated w. Flies carrying the white allele

Allele

An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...

 are frequently used to introduce high school and college students to genetics.

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

Guanine

Guanine is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine . In DNA, guanine is paired with cytosine. With the formula C5H5N5O, guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with...

 and tryptophan

Tryptophan

Tryptophan is one of the 20 standard amino acids, as well as an essential amino acid in the human diet. It is encoded in the standard genetic code as the codon UGG...

, into the developing eyes during pupation. The human version of white is ABCG1

ABCG1

ATP-binding cassette sub-family G member 1 is a protein that in humans is encoded by the ABCG1 gene. It is a homolog of the well-known Drosophila gene white.- External links :...

, and is involved in transporting lipids and cholesterol into cells.