As pointed out, an interesting feature of color inheritance in poultry is the large number of cases of sex-linked inheritance. It might seem probable here, as in the case of _Drosophila_, that this is due to a well-recognized difference between sex-linked and autosomal characters, namely, that a recessive mutation in one of the sex chromosomes of a sperm-cell of the male bird will have a chance of showing its effect immediately if that sperm-cell unites with an egg without a Z to form a daughter, whereas it would not immediately show up in the offspring if the mutation were autosomal.[4] In consequence the recessive mutant would have a greater chance of being observed and selected if it appeared in a sex chromosome. But dominant sex-linked characters, however, have the same chance as dominant autosomal ones and the question turns therefore on the kinds of characters shown in the cross. The first indication of sex-linkage in fowls was furnished by evidence that Spillman published in 1903 on information supplied by poultry-men--information that has been proven subsequently to have been accurate. Spillman pointed out clearly the similarity between the facts he quoted and the then known cases of sex-linkage in the canary and in the currant moth. The case referred to by Spillman was a cross between Barred Plymouth Rock and Black Langshan. Goodale and I repeated the cross, using both Plymouth Rock and American Dominques, publishing the results in 1912. In addition to the F₁ results evidence was obtained for the F₂ generation. The theory was also tested by back-crossing. The results of such a cross that are typical for all cases of the sort are briefly as follows: Plymouth Rock cock by Langshan hen gives F₁ barred sons and barred daughters. These inbred give F₂ barred cocks and barred and black hens (2:1:1). In the following schemes the sex chromosomes are represented by Z and W, while the exponents stand for the factors involved, viz, B for barred and b for not-barred, which here means a black bird. Barred♂ Black♀ P₁ ZᴮZᴮ ZᵇbW \ / \ / ZᴮZᵇ ZᴮW F₁ Barred♂ Barred♀ \ / \ / ZᴮZᴮ ZᴮZᵇ ZᴮW ZᵇW Barred♂ Barred♂ Barred♀ Black♀ In the reciprocal cross, a black cock was mated to a barred hen. The sons were barred, the daughters black (F₁). These inbred gave (F₂) barred males and females, black males and females in the ratio of 1:1:1:1. The chromosome scheme of inheritance is as follows: Black♂ Barred♀ P₁ ZᵇZᵇ ZᴮW \ / \ / \ / ZᴮZᵇ ZᵇW F₁ Barred♂ Black♀ \ / \/ ZᴮZᵇ ZᵇZᵇ ZᴮW ZᵇW Barred♂ Black♂ Barred♀ Black♀