SEX-LINKAGE In those forms in which the male is digametic, being XO or XY in sex-chromosome constitution, a son receives his single X from his mother, whereas a daughter gets one X from each parent.
If on the X-chromosome of the father is borne the hereditary factor for a recessive character (see HEREDITY and MENDELism) the track of the chromosome in inheritance can be followed, pro vided that in the Y-chromosome there are no factors which affect the action of those borne upon the X. Moreover if the sex-chromo somes are elements of the sex-determining mechanism and if factors for hereditary characters are borne upon them it will be seen that the mechanism that is determining whether the individual shall be male or female is also determining whether or not the individuals shall also exhibit some particular character—the char acter in its transmission from generation to generation will exhibit a sex-linked mode of inheritance.
For example, consider the inheritance of haemophilia (tendency to excessive bleeding). This appears to be a sex-linked recessive character; the interpretation which best fits the facts is as follows.
Haemophilia is an hereditary character, the factor for which is X-borne. The male therefore, possessing but one X-chromosome, is either haemophilic or else is normal: the female on the other hand can be normal, a carrier (i.e., with the factor on one of her X's only), or haemophilic (i.e., with the factor on each of her X's). Let an italicised X indicate that on this particular X the factor for this recessive sex-linked character is borne. The follow ing matings and results are possible : The marriage of a haemophilic man (XY) with a haemophilic woman (XX) can yield only haemophilic sons (XY) and haemo philic daughters (XX).
The marriage of a haemophilic man (XY) with a carrier woman (XX) can yield normal sons (XY) : haemophilic sons (XY) : carrier daughters (XX) : haemophilic daughters (XX).
The marriage of a haemophilic man (XY) with a normal woman (XX) can yield normal sons (XY) and carrier daughters (XX).
The marriage of a normal man (XY) with a haemophilic woman (XX) can yield haemophilic sons (XY) and carrier daughters (XX).
The marriage of a normal man (XY) with a carrier woman (XX) can yield normal sons (XY) : haemophilic sons (XY) : normal daughters (XX) : carrier daughters (XX).
On the other hand the facts concerning the inheritance of certain characters in butterflies, moths and birds require that here the female shall be digametic. These facts can be illustrated by reference to the fowl. A black (non-barred) cock mated with barred hens will throw barred sons and black (non-barred) daughters. A barred cock mated with black hens will throw barred sons and daughters and if these are interbred they will in their turn produce barred and black offspring, but every black indi vidual in this generation will be a male. These facts can only be explained on the assumptions that sex in this form is decided by the simplex or duplex condition of some element which when present in duplicate leads to the establishment of maleness and that the factors for the sex-linked characters are borne upon the X-chromosomes.
Non-disjunction.—So far the evidence derived from breed ing experiments and also from microscopical examination of the cell is in entire agreement : it is exceedingly strong but it is indirect. But direct proof of the chromosomal determination of sex and of the location of the hereditary factors in the chromo somes is available, for it has been shown by Bridges (1916) that certain exceptions to the normal course of sex-linked inheritance in Drosophila depend upon abnormality in the distribution of the X-chromosomes.
In Drosophila mWanogaster white eye-colour is a sex-linked recessive character. Using the symbols w for the factor for this and W for the alternative dominant normal red eye and writing these as affixes to the X-chromosomes, the result of a cross be tween a red-eyed male and a white-eyed female can be shown.