The function "par" is used in order to set the parameters for high level plotting. We save the current setting of plotting in the object "op" and set the plotting region to contain six plots in two rows and three columns. After plotting, the default setting is restored.

Examine the six plots in Fig. 2.2. The first two plots describe the distribution of the phenotype in the two pure inbred strains. Note that the distribution follows the bell shape of the normal distribution. The two distributions look the same, but they have different means. The distribution of the phenotype among the Fi mice is identical to the distribution among the inbred strain with the low expression, since the genetic effect is recessive. The picture is the same for the BC1 = aa x Aa backcross. All animals in this cross have at most a single A allele. Genetic variability emerges in the last two plots. Observe that the standard deviation of the phenotype increases from 0.5 to 1 in the case of the intercross and to 1.12 in the case of the backcross. Note that the distribution is no longer normal, but rather a mixture of normals.

3 4 5 6 7 N = 100000 , Bandwidth = 0.0452

Fig. 2.2. Segregation of the phenotype in crosses

Fig. 2.2. Segregation of the phenotype in crosses

The mixture frequencies are (1/2,1/2) for the backcross and are (3/4,1/4) for the intercross.

The investigation of the backcross and the intercross will proceed throughout most of the part of the book that deals with experimental genetics. Other crossing designs will be considered occasionally. In particular, we will deal with recombinant inbred strains, which are important resources in genetic mapping. These inbred strains are created by the formation in parallel of several F2 mating pairs, followed by several generations of inbreeding within each pair. The result is a set of inbred strains, all originating from the initial cross of the two inbred strains. In Chap. 3 we will explain in more detail the genetic population dynamics of the formation of inbred strains and their genetic properties.

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