Usually no other sort of male is produced throughout the experiment except these two, but occasionally there is produced a male both yellow-bodied and white-eyed, or one which is gray-bodied and redeyed, like wild flies. How do these arise? If in F, females the paired X's were to exchange loads in part, so that G and R came to be attached to the same X and g and r to the other X, and if each of the

FIG. 88. Reciprocal cross to that shown in Figure 87. Parents, red-eyed male and white-eyed female; F., white-eyed males and red-eyed females ("crisscross inheritance"-Morgan); F2 equal numbers of red-eyed and white-eyed individuals of both sexes. The distribution of the sex chromosomes is shown at the right, as in Figure 87. (From Conklin, after Morgan.)

eggs having such a constitution were to be fertilized with a sperm which lacked X (male determining sperm), this would make possible the production of F, males possessing both dominant characters and others possessing both recessive characters or gray-red and yellowwhite respectively, as actually observed in about one case in a hundred by Morgan.

It may add interest to the case to state parenthetically that in man occur a number of sex-linked variations which are inherited in this same curious fashion. Among them may be mentioned color blindness and

bleeding (haemophilia), which occur chiefly in males, but are never transmitted by males to their sons but only through their daughters to their grandsons.

FIG. 89.-Drawing showing the four pairs of chromosomes seen in the dividing egg of Drosophila. (After Dr. C. E. V. Metz.)

Morgan and his pupils have described between forty and fifty characters in Drosophila which are sex-linked in heredity; they also have discovered a large number of other Mendelizing characters in Drosophila which are not sex-linked but which nevertheless are inherited in groups, characters in the same group showing coupling when introduced in a cross from the same parent, and repulsion when introduced from different parents. The number of these groups exactly corresponds with the number of the chromosomes and Morgan believes that their genes are located in the chromosomes, an hypothesis which seems reasonable but which would be severely strained if an additional group of characters should be discovered. There are three groups of the non-sex-linked characters. (See Fig. 90.) In one of these referred to as Group II (the sex-linked group being called Group I), are found variations known as black body and vestigial wings respectively, together with some thirty-five other variations. In Group III are found the variations known as pink eye, spread wings, and ebony body, together with some twenty other variations. In Group IV 'are included as yet only two characters, bent wings and eyeless, which however show linkage with each other. No inherited characters have been discovered in Drosophila which are not inherited in one or another of the four linkage groups.

TABLE II

RECIPROCAL CROSSES OF WHITE-EYED AND YELLOW-BODIED FLIES

DROSOPHILA TYPE AND POULTRY TYPE OF SEX-LINKED INHERITANCE

1. Drosophila type.-The same type of sex-linked inheritance which is found in Drosophila is found also in man, in cats (inheritance of yellow color), and in the plants, Lychnis and Bryonia. The essential feature of the "Drosophila type" of inheritance is this. In a race

breeding true for a sex-linked character, the female is homozygous for the character in question while the male is heterozygous and incapable of becoming homozygous. Reciprocal crosses with such a race give unlike results, because the female transmits the character to all her offspring, but the male transmits it to only half his offspring, viz., the females.

FIG. 91.-Sex-linked inheritance of barred and unbarred (black) plumage in poultry. P, parents, barred male, unbarred female; F1, barred males and females; F2, males all barred, females in equal numbers barred and unbarred. (After Morgan.)

2. Poultry type.-Another type of sex-linked inheritance exists in which the sex relations are exactly reversed. This was first observed in the moth, Abraxas, but more familiar cases occur in poultry, for which reason it may be called the poultry type of sex-linked inheritance. Here the male is the homozygous sex, the female being heterozygous. This condition is found in moths and in certain birds, viz.,

in domestic fowls, pigeons, ducks, and canaries. As an example we may take the inheritance of the color pattern, barring, in crosses of barred Plymouth Rock fowls. In reciprocal crosses between purebred barred Plymouth Rocks and black Langshans (or another unbarred breed), the results are not identical. If the barred parent is the male (Fig. 91 and Table III), all F, offspring are barred and in F, all males are barred, but half the females are black and half are barred. If, however, the barred parent is the female (Fig. 92 and Table III),

F2 Barred 1 Barred: 1 Black 1 Barred: 1 Black I Barred: 1 Black

FIG. 92.-Reciprocal cross to that shown in Figure 91. P, parents, unbarred male, barred female; F1, barred males, unbarred females (crisscross inheritance); Fa, barred and unbarred birds equally numerous in both sexes. (From Castle.)