Page:Popular Science Monthly Volume 84.djvu/13

Rh transcend the number of the chromosomes. As a matter of fact, several strains are known in which the number of Mendelian characters is greater than the number of chromosomes; but just here a remarkable phenomenon has come to light that shows in certain cases that many of the characters do not segregate independently, but are linked to each other as though they belonged to some common system. As a result, peculiar ratios appear, that differ from the expected Mendelian ratios in so 'far as that expectation rests on independent assortment. The question here raised has, therefore, taken on a new aspect, and it has become essential to discover whether there are as many, or more, or fewer groups of linked characters than there are kinds of chromosomes.

Correns appears to have been the first to call attention to a case in which peculiar ratios appeared which he attributed to coupling. Bateson and his coworkers have described several instances of the same kind. They have found that certain characters in sweet peas do not fulfill the expectation for independent assortment of different pairs of characters, although they do show Mendelian segregation when each pair is taken separately. This phenomenon has been described as coupling or repulsion. I shall refer to it as linkage.

It was found, for instance, in sweet peas that when plants with blue flowers and long pollen grains were crossed to plants having red flowers and short pollen grains in the grandchildren, the blues had for the most part long pollen and the reds short pollen. Again when blue and red flowers with erect and hooded standards were used all the red grandchildren had erect standards.

We have met with these same phenomena at nearly every step in our studies of heredity in the fruit fly, Drosophila. Over one hundred mutants have appeared from many of which pure races have been formed. At present we have studied fifty-nine of these sufficiently to show that they fall into three great groups.

The characters in the first group show sex-linked inheritance. They follow the sex chromosomes. The second group is less extensive. Since the characters in this group are linked to each other we say that they lie in a second chromosome. The characters of the third group have not as yet been so fully studied, except to show that they are linked. We