Unlike sex-linked hybrids, such as 'red sex-links' or 'black sex-links', the Legbar is an auto-sexing breed. Several other auto-sexing breeds or auto-sexing varieties of breeds exist, such as Plymouth Rock, Bielefelder Kennhuhn, Niederrheiner, and Norwegian Jærhøns. Most breeds that end with -bar, such as Welbar, Rhodebar, Brussbar or Wybar, are auto-sexing as well.

The importance that auto-sexing plays in the Legbar breed is also reflected in the fact that, next to a standards for the adult birds, the down colour and patterns are also standardised.^[5] Day-old male chicks can be distinguished from day-old female chicks by the down colour and the pattern they form. Female Legbar chicks in general have a broad very dark brown stripe extending over the head, neck and rump and a clear eye barring. The edges of the stripe are clearly defined and should not be blurred and blending with the ground colour, which is dark brown. A light head spot should be visible but is usually small. The male Legbar chicks in contrast have a much paler down shade and the pattern is blurred and washed out from head to rump.^[5]

The marked difference between male and female chicks is due to gene dosage of the sex-linked barring gene ('barring' (B), 'nonbarring' (b+)).^[5][6] This gene is located on the Z-Chromosome of birds. Birds have different sex-chromosomes (Z and w) and a different sex-determination system compared to mammals (X and Y). Male birds have therefore two Z-chromosomes while female birds have a Z- and a dwarfed w-chromosome. This means that phenotypically barred cocks can either have the B/B or the B/b+ genotype, while a barred hen always has to have a B/- genotype. The colour-sexing of Legbar chicks, however, is only possible because the male chicks have a double dose of the sex-linked barring gene (genotype B/B), while the female chicks only have a single dose (genotype B/-), resulting in the observed down colours.^[5][6][7]

The concept of an auto-sexing breed of chicken is due to the geneticist Reginald Punnett, who during the First World War had already proposed the technique of cross-breeding chickens carrying the barred gene (B) with others to produce sex-linked chicks with plumage differences that could easily be distinguished.^[3]: 317

Working at the Genetical Institute of Cambridge University, he and Michael Pease cross-bred Golden Campines with barred Plymouth Rocks, resulting in the creation of the Cambar in 1928.^[4]: 73 About ten years later they produced the Legbar by crossing brown Leghorns with barred Plymouth Rocks.^[3]: 318

Other "Cambridge" breeds later developed were:

• the Brockbar, created in 1940 from buff and barred Plymouth Rocks, became extinct by about 1950^[1]: 54
• the Brussbar, created in 1952 from light Sussex, brown Sussex and barred Plymouth Rocks^[1]: 55
• the Dorbar, an auto-sexing heavy meat breed from Dorkings and barred Plymouth Rocks, bred from 1941 to about 1949, when development stopped^[1]: 59
• the Rhodebar, from Rhode Island Reds and barred Plymouth Rocks, standardised in Britain in 1951; a similar cross-breed was developed in Canada^[1]: 63
• the Welbar, not created at Cambridge but by a Devon breeder, from Welsumer and barred Plymouth Rocks;^[1]: 65 and
• the Wybar, also not created at Cambridge but by an individual breeder, from Wyandotte, Brussbar and barred Plymouth Rocks.^[1]: 68

Many other breeds were created in the same way, all making use of barred Plymouth Rocks to impart the barred gene:

• The American California Grey was bred in the 1930s in Modesto, California.^[8]: 432
• In about 1936 Arend L. Hagedoorn introduced the barred gene to Barnevelders and to brown Leghorns.^[9]: 210
• In 1940 R. George Jaap produced the Oklabar by crossing dark Cornish with barred Plymouth Rocks.^{[9]: 210 [10]}
• By 1941 the Ancobar had been bred from mottled Anconas and barred Plymouth Rocks, by W. F. Lamoreux at Cornell University in Ithaca, New York.^{[9]: 211 [11]}
• The Polbar was created between 1946 and 1954 by Laura Kaufman, who crossed the native Polish Green-legged Partridge breed with barred Plymouth Rocks.^[12]: 556
• The German Bielefelder Kennhuhn, developed in the 1970s in the area of Bielefeld in Nordrhein-Westfalen from Malines, Welsumer and barred Plymouth Rocks.^[13]: 6