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Modern Genetic Analysis Chapter 18. Quantitative Genetics
Solved Problem

1. | In some species of songbirds, populations living in different geographical regions sing different "local dialects" of the species song. Some people believe that this difference in dialect is the result of genetic differences between populations, whereas others believe that these differences arose from purely individual idiosyncracies in the founders of these populations and have been passed on from generation to generation by learning. Outline an experimental program that would determine the importance of genetic and nongenetic factors and their interaction in this dialect variation. If there is evidence of genetic difference, what experiments could be done to provide a detailed description of the genetic system, including the number of segregating genes, their linkage relations, and their additive and nonadditive phenotypic effects? See answer

Solution

This example has been chosen because it illustrates the very considerable experimental difficulties that arise when we try to examine claims that observed differences in quantitative characters in some species have a genetic basis. To be able to say anything at all about the roles of genes and developmental environment requires, at minimum, that the organisms can be raised from fertilized eggs in a controlled laboratory environment. To be able to make more detailed statements about the genotypes underlying variation in the character requires, further, that the results of crosses between parents of known phenotype and known ancestry be observable and that the offspring of some of those crosses be, in turn, crossed with other individuals of known phenotype and ancestry. Very few animal species can satisfy this requirement, although it is much easier to carry out controlled crosses in plants. We will assume that the songbird species in question can indeed be raised and crossed in captivity, but that is a big assumption.

a.To determine whether there is any genetic difference underlying the observed phenotypic difference in dialect between the populations, we need to raise birds of each population, from the egg, in the absence of auditory input from their own ancestors and in various combinations of auditory environments of other populations. This is done by raising birds from the egg that have been grouped as follows:

1. In isolation

2. Surrounded by hatchlings consisting only of birds derived from the same population

3. Surrounded by hatchlings consisting of birds derived from other populations

4. In the presence of singing adults from other populations

5. In the presence of singing adults from their own population (as a control on the rearing conditions)

If there are no genotypic differences and all dialect differences are learned, then birds from group 5 will sing their population dialect and those from group 4 will sing the foreign dialect. Groups 1, 2, and 3 may not sing at all; they may sing a generalized song not corresponding to any of the dialects; or they may all sing the same song dialectthis dialect would then represent the "intrinsic" developmental program unmodified by learning.

If dialect differences are totally determined by genetic differences, birds from groups 4 and 5 will sing the same dialect, that of their parents. Birds from groups 1, 2, and 3, if they sing at all, will each sing the song dialect of their parent population, irrespective of the other birds in their group. There are then the possibilities of less clear-cut results, indicating that both genetic and learned differences influence the trait. For example, birds in group 4 might sing a song with both population elements. Note that, if the birds in the control group 5 do not sing their normal dialect, the rest of the results are uninterpretable, because the conditions of artificial rearing are interfering with the normal developmental program.

b.If the results of the first experiments show some heritability in the broad sense, then a further analysis is possible. This analysis requires a genetically segregating population, made from a cross between two dialect populationssay, A and B. A cross between males from population A and females from population B and the reciprocal cross will give an estimate of the average degree of dominance of genes influencing the trait and whether there is any sex linkage. (Remember that in birds the female is the heterogametic sex.) The offspring of this cross and all subsequent crosses must be raised in conditions that do not confuse the learned and the genetic components of the differences, as revealed in the experiments in part a. If learned effects cannot be separated out, this further genetic analysis is impossible.

c.To localize genes influencing dialect differences would require a large number of segregating genetic markers. These markers could be morphological mutants or molecular variants such as restriction-site polymorphisms. Families segregating for the quantitative trait differences would be examined to see if there were cosegregation of any of the marker loci with the quantitative trait. These cosegregated loci would then be candidates for loci linked to the quantitative trait loci. Further crosses between individuals with and without mutant markers and measurement of the quantitative trait values in F2 individuals would establish whether there was actual linkage between the marker and quantitative trait loci. In practice, it is very unlikely that such experiments could be carried out on a songbird species, because of the immense time and effort required to establish lines carrying the large number of different marker genes and molecular polymorphisms.

See question

© 1999 by W. H. Freeman and Company. All rights reserved.