Genetics for the clinician / C.A. Clarke.

  • Clarke, Cyril A. (Cyril Astley)
Date:
1964
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Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Genetics for the clinician / C.A. Clarke. Source: Wellcome Collection.

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    312 GENETICS FOR THE CLINICIAN Figure 20.6 Series of possible results, for marginal totals the same as the observed shown in Fig. 20.5 (see text) abed our total probability is 0-07 + 0-07 = 0-14. Thus, contrary to what we might have thought by simply looking at the figures we have not demon¬ strated in this example that protection is afforded by giving the antibody. TYPES OF DISTRIBUTION When we observe any form of living organism it is obvious that different individuals within a population vary one from another with respect to any given character, and it may be taken as axiomatic that in biology variation is an inherent basic fact. The next part of the chapter deals with the ways in which such variation may occur and the methods by which we measure it. Before explaining this in detail it is as well to remember that there are two very important parameters [definition: 'numerical characteristics of populations' (Moroney, 1956)] which are mentioned now so that the terms become famihar. These are *the mean' and 'the variance'. The first is equivalent to 'the average', and the second is a measure of dispersion around the average and is directly related to another parameter which is again often encountered, namely 'the standard error of the mean'. We now describe four important types of frequency distributions : (i) Normal Distribution Here the majority of measurements are grouped aroimd the average and there are fewer and fewer individuals in the more extreme ranges. For instance the majority of adults will vary in height roughly between 5 ft. 0 in. and 6 ft. о in. and there are hkely to be very few who measure under 4 ft. 6 in. or over 6 ft. 6 in. Age at menarche and the length of pregnancy are other examples of approximately 'normal' distribution of probability and this type of curve is the one most frequently encountered in biological work. Fig. 20.7 shows the distribution of length of pregnancy in a group of mothers. It will be seen that there is a hump near the middle