Studies in quantitative inheritance

Summary

• 1. A crossing method is described for creating all possible combinations of major chromosomes from pairs of inbred lines ofDrosophila melanogaster. The twenty-seven different genotypes in females, eighteen in males, provide the basis for different tests which throw light on the genetic control of body size.

• 2. Complete chromosome analyses have been carried out on two pairs of contrasted lines of different size, descended from the Nettlebed and Edinburgh wild stocks. Each such pair comprises a small line, descended from a strain selected for small body size, and an approximately normal-sized line, inbred without selection from the same stock. Three unrelated lines inbred without selection have been studied in a similar way, except that twenty-one out of the twenty-seven possible combinations for each pair have been studied in females only.

• 3. The accuracy of the method of combining chromosomes was demonstrated by the agreement between preparations of the same genotype by different means, and also by the level of the within-culture variance, which was generally of the same order as that for untreated inbred lines.

• 4. The within-culture variance is not constant for all genotypes, but tends to decline with an increase in the number of heterozygous pairs of chromosomes.

• 5. When the unselected and small lines are crossed, a highly non-additive situation is revealed by the size of theF ₁ which may be as great, or nearly as great, as the size of the unselected parent line.

• 6. In the analysis of the unselected and small Edinburgh lines the size of the different types could be accounted for by aggregate dominance of the chromosomes of the larger

• 7. In the Nettlebed combinations, aggregate dominance and additive combination of non-homologous chromosomes account for the size of the majority of the types. But there are also a number of striking interactions which increase or decrease size, leading to different effects of particular substitutions and different dominance relations in different genetic backgrounds. Most of the larger interactions occur in genotypes carrying several chromosomes from the small line. The behaviour of the X-chromosome of the small line is exceptional in being incompletely recessive in all backgrounds.

• 8. In the combination of chromosomes from the unrelated, unselected, inbred lines, interactions between non-homologous chromosomes are much more frequent and striking. The substitution of a single chromosome or of a homozygous pair may increase or decrease size, according to the genetic background.

• 9. Inter-crossing these unrelated inbred lines always leads to heterosis in theF ₁, which exceeds both parent lines in size. This heterosis cannot be accounted for merely in terms of the summation of the effects of dominance or over-dominance on different chromosomes, but must be considered in terms of gene interaction. The effects of making each pair of chromosomes heterozygous in otherwise homozygous backgrounds may be compared with the joint effects of making two or more pairs heterozygous. In several cases, the presence of a single pair of heterozygous chromosomes may lead to a body size quite as large as in the fully heterozygous type, and actually exceeding the size of types with two heterozygous pairs. But, in one series, on the other hand, increase in the number of heterozygous pairs of chromosomes increases size more than the sum of the individual effects.

• 10. The results are discussed in relation to the mechanism of heterosis, inbreeding decline and possible ways in which selection has changed the genotype to produce small size.