case has a sufficient number of characters been studied to show whether there is any correspondence between the number of hereditary groups of characters and the number of chromosomes. In the fruit fly, Drosophila ampelophila, we have found about 125 characters that are inherited in a perfectly definite way. On the opposite page is a list of some of them.
It will be observed in this list that the characters are arranged in four groups, Groups I, II, III and IV. Three of these groups are equally large or nearly so; Group IV contains only two characters. The characters are put into these groups because in heredity the members of each group tend to be inherited together, i.e., if two or more enter the cross together they tend to remain together through subsequent generations. On the other hand, any member of one group is inherited entirely independently of any member of the other groups; in the same way as Mendel's yellow-green pair of characters is inherited independently of the round-wrinkled pair.
| Group I Abnormal Bar Bifid Bow Cherry Chrome Cleft Club Depressed Dot Eosin Facet Forked Furrowed Fused Green Jaunty Lemon Lethals, 13 Miniature Notch Reduplicated Ruby Rudimentary Sable Shifted Short Skee Spoon Spot Tan Truncate intensifier Vermilion White Yellow | Group II Antlered Apterous Arc Balloon Black Blistered Comma Confluent Cream II Curved Dachs Extra vein Fringed Jaunty Limited Little crossover Morula Olive Plexus Purple Speck Strap Streak Trefoil Truncate Vestigial | Group III Band Beaded Cream III Deformed Dwarf Ebony Giant Kidney Low crossing over Maroon Peach Pink Rough Safranin Sepia Sooty Spineless Spread Trident Truncate intensifier Whitehead White ocelli | Group IV Bent Eyeless |
If the factors for these characters are carried by the chromosomes, then we should expect that those factors that are carried by the same chromosome would be inherited together, provided the chromosomes are definite structures in the cell.
Fig. 52. Chromosomes (diploid) of D. ampelophila. The sex chromosomes are XX in the female and XY in the male. There are three other pairs of chromosomes.
In the chromosome group of Drosophila, (fig. 52) there are four pairs of chromosomes, three of nearly the same size and one much smaller. Not only is there agreement between the number of hereditary groups and the number of the chromosomes, but even the size relations are the same, for there are three great groups of characters and three pairs of large chromosomes, and one small group of characters and one pair of small chromosomes.