future problems of heredity will be solved by its aid." A problem arises at once in the attempt to explain what is known as blended inheritance where the hybrids of a cross do not split up into the original characters but preserve the composite character. This is said to be true of the cross between the border Leicester rams and Cheviot ewes in sheep. The cross of the white and the black human races is a mulatto who does not revert so far as known to either parent color. Nillson-Ehle found that a certain brown-chaffed wheat when crossed with a white-chaffed variety yielded a brown-chaffed hybrid which did not in their turn give the Mendelian ratio of 3 to 1 but instead a ratio of about 15 to 1. Careful investigation seems to show that the answer to these difficulties lies in the fact that color is not always fixed by one determiner but that there are several which may be separately inherited and hence the color of hybrids may vary. Thus Nillson-Ehle found two factors for the brown of the wheat and was able to explain the phenomenon on the basis of a dihybrid as illustrated by the chart of the round and wrinkled, green and yellow peas given on page 202. Davenport thinks he can prove that the black color of the pure Negro is due to two double factors. Other observers have reported that seven factors are involved in producing the color of the mouse, and eight for the rabbit. It may be that there is no true blend but that we are dealing with several or many determiners in all cases of blends, which combine in Mendelian fashion.

Of all the questions in the field of heredity that concerning the determination of sex has probably been most dis

• DARBISHIRE, A. D. Breeding and the Mendelian Discovery, pp. 215-216.

cussed and all sorts of theories have been suggested. The relative age of the parent has been considered significant or, perchance, the sex has been determined by the parent in best condition at the time of conception. Diet has often been thought important and several men have achieved considerable fame and fortune by their alleged ability to secure sons for royal families by treating pregnant women with a diet of sugar or some other essential. It now seems probable that sex is determined by the germ cells and that outside influences are of no avail.

As early as 1891 Henking discovered that two sorts of sperms occurred in certain insects, the difference depending upon the presence of an odd or "accessory" chromosome in some. It is now known that in many animals the ovum contains one pair of chromosomes which seems to be represented in the sperm by a single chromosome, or a pair, one of which does not function. In some animals the extra chromosome is in the sperm. In 1902 Professor C. E. McClung suggested that here might lie the explanation of sex. In the reduction division the ovum would always contain an accessory chromosome while half of the spermatozoa would have one, the other half would not. Hence, when fertilization took place, the zygotes containing the accessory pair would be female, those having only one accessory chromosome would be male. This suggestion has won pretty general acceptance though much remains to be done before all doubt is removed. In the best count we now have of the human chromosomes 47 are assigned to the sperm and 48 to the ovum, so the theory appears to hold true for human beings. The substantially equal division of the human race into male and female is thus shown, apparently, in accordance with Mendelian laws.

Additional evidence is offered by animals like the armadillo, where the offspring in any litter are always of the same sex. In this case it appears that only one egg is produced at a time and if this is fertilized the offspring are either all males or all females. Identical twins among human beings are likewise always of the same sex. It is believed that they result from the fertilization of a single egg, for they are inclosed in a single fetal membrane, whereas ordinary twins come from separate eggs and have separate fetal membranes.

It has been known for some time that certain characteristics though passed along generation after generation were not equally divided between the sexes. These characters seem to be in some way associated with the sexchromosomes and hence have been called "sex-linked." Morgan has made some interesting studies of the small fruit flies, and has found some 25 sex-linked characters. The eyes of this fly are normally red, but occasionally a male appears which has white eyes. Morgan found that if a white-eyed male were crossed with the red-eyed female all the next generation had red eyes but on interbreeding this second generation half of the resulting males would have white eyes, the other half red, while all the females would have red eyes. In order to get a female with white eyes it was necessary to cross a white-eyed male with a female also descended from a white-eyed male and then half of the females would have white eyes. Among themselves pure barred rock chickens breed

true. If the males are crossed with other breeds only barred offspring result; but if the females are mated with a non-barred breed, approximately one-half of the offspring are barred, the rest are not. Moreover the barred

ones will be found to be males, the non-barred females

without exception. In other words the males are homozygous, the females heterozygous.

"Sex-limited inheritance such as this finds its most probable explanation in the existence in the egg of an extra or plus element never found in the sperm, this element pairing with the sex-limited character in the reduction division. Thus, in the barred rock, calling barring B, the male of pure race is clearly BB and every sperm is B. But the female clearly contains only one B and cannot be made to contain two. Perhaps a second B is kept out by some structural element, X, the distinctive structural element of the female individual. Then the eggs will be of two sorts: B and X. Since the sperms are all B, the first type of egg when fertilized will contain BB, a homozygous barred individual and a male since it lacks X; the second type will contain BX, a bird heterozygous in barring, and a female, since it contains X." 7

DIAGRAM OF SEX-LIMITED INHERITANCE, WHEN THE FEMALE IS A HETEROZYGOTE, AS IN THE BARRED FOWLS X, FEMALE

SEX DETERMINER; B, BARRING.

Dorset horned sheep crossed with the hornless Suffolks produce hybrids in which the males all have horns, but none of the females. "A ram will develop horns with only one dose of the horned character which is present in the heterozygous condition of that character: but the ewe needs the two doses which are only present in the homozygous condition of that character."

7 CASTLE, W. E. Heredity, pp. 172-174.

It appears then that in many cases a male will show a given character if it has one determiner, that is, is simplex, while the female must be duplex. Why this is true we have no idea. It applies to human beings as well as animals as will be shown.

Color-blindness is much commoner among men than among women. The children of a color-blind man married to a normal woman are themselves normal, but when the girls of this generation marry half of their sons will be color-blind. A color-blind woman is found only when a color-blind man marries a woman whose father was colorblind. If it is true that man is color-blind if he is simplex while the woman only when duplex we should expect all the sons of color-blind women to have the trait and no exception to this is known. Bateson was able to list seventeen cases all of whom were color-blind.

Among other sex-linked traits in man Guyer names: "hemophilia, a serious condition in which the blood will not clot properly, thus rendering the affected individual liable to severe or fatal hemorrhage; near-sightedness (myopia) in some cases; a degenerative disease of the spinal cord known as multiple sclerosis; progressive atrophy of the optic nerve (neuritis optica); Gower's muscular atrophy; some forms of night-blindness; in some cases ichthyosis, a peculiar scaly condition of the skin.” 8 All of these characters must be considered as recessives. How many of the human traits follow the Mendelian principles is not known, but there is reason to think the number is large.

"Already a long list of characteristics, which are inherited in man in accordance with one form or another of the Mendelian principles, is known. This list in8 GUYER, M. F. o. c., pp. 64-65.