Linkage Relations of Autosomal Factors in the Fowl.

OR the past 20 years data have been accumulated a t the KANSAS AGRIF CULTURAL EXPERIMENT S ATION on the linkage relations of factors in the domestic chicken with the objective of establishing chromosomal maps for this species. The possibility of obtaining sufficient data for locating gene markers on all the chromosomes seems rather remote, since each succeeding publication on the cytology of the chicken increases the proposed number of chromosomes. One of the earliest careful studies on the chromosomes of the fowl was by GUYER (1916), and he found the diploid number to be 15 to 19. Later SHIWAGO (1924) reported 32 chromosomes, and HANCE (1926) reported 36 chromosomes. WHITE (1932) found 66 chromosomes in the fowl and YAMASHINA, in 1944, placed the number of chromosomes a t 78. Numerous other workers have reported chromosome numbers between the above mentioned extremes. The lack of agreement among cytologists is due to the difficulty of obtaining satisfactory preparations for study and the rather large number of very small c romosomes. Whether there are 16 or 39 pairs of chromosomes in the chicken will not make too much difference to the geneticist since even the smallest proposed number makes the problem of complete mapping a complicated one. I t is true, however, that the chicken is the one domestic animal which offers material that is a t all promising for gene mapping. Some of the sex-linked factors in the fowl show what appears to be independent assortment among themselves. This would indicate that map distances on the sex chromosome are so great that the percentage of crossing over is large enough to obscure evidence of linkage. The earlier cytologists believed that the largest member of the chromosomal complex was the sex chromosome but more recent workers hold that several of the autosomes are larger than the sex chromosomes. If the latter interpretation is correct, we can expect additional difficulty in obtaining gene maps of the autosomes of this species because of long distances. Fortunately, the chicken has a considerable number of non-interfering characters which are satisfactory for linkage tests. This permits the testing of several characters in one cross, thus reducing the work and facilities required. Because of mortality which may be large during growth, it is desirable to have characters which express themselves early in life, preferably in the day-old chick. Many individuals are lost because of failure to hatch. The late-stage deaths in shell may be salvaged for traits identifiable a t that time, since sex may be determined then by examination of the gonads. Characters which