Variability and Evolution, 1996, Vol. 5: 65–73
ALEKSANDRA MARIA MICHAŁOWSKA
Department of Anthropology and Biometrics
Academy of Physical Education, Poznan,́ Poland
QUANTITATIVE VARIABILITY – THE DETERMINATION
AND INTERPRETATION IN ONTOGENESIS
MICHAŁOWSKA A. M. 1996. Quantitative variability – the determination and interpretation in ontogenesis.
Variability and Evolution, Vol. 5: 65–73, Figs. 6, Adam Mickiewicz University, Faculty of Biology,
Poznan´
Abstract. The aim of this paper is to analyse some aspects of polygenic variation based on
skin pigmentation variability. The reflectance spectrophotometry which enables the presentation of skin colour in metrical units was used in the study. It was documented that this
method is appropriate for the presentation of the polygenic inheritance of skin pigmentation,
producing the continuous pattern of its variation.
In addition a possible model of ontogenetic changes in polygenic traits was considered.
In the analyses two features were compared: skin pigmentation and body height. The study
material consisted of subjects aged from 10 to 18 years. The outcomes revealed that there
is no time specific, developmental trend that may be assigned to changes in skin pigmentation
intensity.
Key words: mode of inheritance, skin pigmentation, reflectance spectrophotometry
Phenotypic variability and its alterations during ontogenesis are substantially related to the mode of genetic determination, as it underlies the potential range of
individual variation and developmental changes. The sharp division of phenotypic
variation into qualitative and quantitative categories reflects the monogenic and polygenic mode of inheritance. At the individual level, the diversity between those
two alternatives lies primarily in the extent to which the environment and genes interact
when the trait develops. At the population level, they are detected by the kind of variation
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distribution they represent – a discrete or continuous one. Biological development
may be defined as a complex of changes leading to functional, immune and adaptive
transformations occurring during the individuals life (Woynarowska, Kozłowski 1984).
In the most general point of view, those changes undergo common main phases of
development: the progressive, stabilised and involutional periods. Whereas, data which
enable us to interpret ontogenetic processes refer to their differentiation. This may be
regarded in the context of time specific changes displayed in the shape of growth curves.
Another aspect of this problem is connected with the individual course of ontogenetic
phases estimated, for example, by the level of maturity. These kinds of interpretations
are attributed to polygenic traits, which have the potential of changes derived from
multifactorial inheritance (multiple gene-loci and gene-environment interactions).
It is the purpose of this work to describe and interpret some aspects of polygenic
variability. The first point of the analysis is to demonstrate the relation between the
polygenic mode of genetic determination and the phenotypic variation. The second
consideration shows the possible patterns of ontogenetic changes in polygenic traits.
Those aspects are to be studied mainly on the basis of skin pigmentation variability.
The reason why this feature was chosen is that skin pigmentation is viewed in both
a qualitative and quantitative manner. The additional feature analysed in the paper
is body height. The model of ontogenetic changes characteristic of body height,
shared by many features, was compared to skin pigmentary variation with age. The
analysis was based on the cross-sectional material consisted of measurements taken
from 817 subjects (410 males and 407 females) aged from 10 to 18 years.
When introducing a skin pigmentation inquiry, it is necessary to note the main
factors which determine and influence its variation. Pigments that are present in
human skin are melanin, carotene, reduced haemoglobin and oxyhaemoglobin. Melanin is the pigment which predominantly contributes to skin colour and its population variability. It is produced by melanocytes, dendroid cells of ectodermal origin,
basic numbers of which and their location in the basal layer of the epidermis is
established in the prenatal period of life. Melanin is a heteropolymer, formed in the
course of melanogenesis (the Raper-Mason pathway) from tyrosine, which is oxidised to dopa and dopaquinone (in the presence of tyrosinase) and then spontaneously and non-enzymatically polymerised. The sequence is as follows (Robins
1991):
tyrosine
tyrosinase
oxygen
dopachrome
dopa
tyrosinase
oxygen
dopaquinone
5,6-dihydroxyindole
lecodopachrome
indole-5,6-quinone
melanin
The population diversity of the epidermal pigmentary system is ultrastructurally
expressed in the size and arrangement of melanosomes within keratinocytes (Rosdahl, Szabo 1976; Herzberg et al. 1989) – a direct consequence of the melanin
quantity they contain. White skin melanosomes are less mature, smaller, and are
usually bound in groups, whereas, in black skin they are large and predominantly single.
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Quantitative variability
Studies in the mode of inheritance of skin colour reveal its polygenic model.
Most often, it is suggested that four two-allelic loci are involved and that their
effects are equal and additive (Stern 1953; Harrison, Owen 1969; Livingstone 1969;
Byard 1981). Hence, pigmentary variation is quantitative in nature. It may be demonstrated by a hypothetical frequency distribution based on four pairs of cumulative alleles. Assuming that, in each pair of alleles, one of them increases the intensity
of pigmentation (let the increment be 10 metrical units) and the other does not, one
gets the following distribution (Fig. 1).
Fig. 1. Hypothetical skin pigmentation distribution
There are various anthropological methods used for the estimation of human
pigmentary traits. The simplest way involves a verbal description of skin colour.
More advanced procedures are based on “the best matching” of skin colour to model
categories such as Gate’s tinted papers, Luschan’s skin colour tablets, the Munsell
system or the Bradley colour top (a quasi-metrical colour-matching technique). It
should be pointed out that any description of skin pigmentation in terms of a category
of colour, irrespectively of subjectivity inherent to this kind of evaluation, views
skin pigmentation in a discrete way – inadequate to the continuous character of its
variability. The use of optical properties of skin gives the possibility of numerical
assessment of skin pigmentation. Precisely, the measured value is visible light reflected from the skin surface. For the reflectance is mostly determined by the quantity of pigment present in the skin; the lighter the skin the higher the reflective
properties (Edwards 1953; Weiner 1951). The reflectance spectrophotometer [FO-02]
used in the study is capable of giving automatic measurement of spectral components
of the light reflected from the examined surface. It is designed in the form of three
main units: the microprocessor control (based on the IBM PC computer), a built-in
minimonitor and the measurement head. The measurement head contains the follo-
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wing basic functional parts: a light source, an optical system based on a diffraction
grating, built-in black and white standards, and photosensitive elements reading the
reflected light. The eventual value – the percentage of light reflected by the skin –
is calculated according to the following formula:
R=
Rn – Rblack
× 100%
Rwhite – Rblack
where, R [Reflectance] – numerical outcome of transformation: light intensity – current
frequency; Rn – reflectance value of given visible light wave length; Rblack (white) – reflectance value of black (white) standards.
The results obtained for particular monochromatic light beams produce a spectral
percentage reflectance curve which represents the relative intensity of skin pigmentation – indicated by its location in the reference system of 0% to 100% reflection
of light. The characteristic shape of the reflectance curve (Fig. 2) arises from the
overlapped absorption bands of skin pigments, especially of their strong absorption
peaks*. It has been shown (Harrison, Owen 1956) that in vitro, melanin concentration is linearly proportional to the reciprocal of the reflectance values; however, this
relation is much more evident for long wavelengths. Therefore, the analyses of
melanin pigmentation variability are based on the reflectance values at 684 nm.
Fig. 2. Mean reflectance curve in the male sample
The data analysed here consist of skin reflectances obtained at 684 nm from the
medial aspect of the right upper arm. In order to avoid pigmentary variation coming
from exposition to the summer sunlight the examinations were made during maximum loss of the secondary pigmentation. The obtained results are presented in the
* The maximal absorption bands for the pigments are the following: carotene 482 nm,
oxyhaemoglobin: 576 nm and 542 nm, reduced haemoglobin: 556 nm, melanin: absorption
decreases gradually from 400 nm to 700 nm
Quantitative variability
69
Fig. 3. Skin reflectance (R684) distributions in the male samples
form of frequency distributions of pigmentary variation (Fig. 3). As shown, the use
of reflectance spectrophotometry enables the display of skin pigmentation in a continuous manner, identifying its gradation from low to high intensity. Such a phenotypic picture of skin pigmentation remains consistent with the polygenic mode of
its genetic determination. To present polygenic traits, complete frequency distributions of body height variation are also shown (Fig. 4).
In order to study the course of ontogenetic changes in skin pigmentation, mean
age reflectances were analysed using a one-way analysis of variance, followed by
a Newman-Keuls test of multiple comparison (Rosner 1986). The final results are
presented graphically (Fig. 5) – mean values are in ascending order and the groups
of means that are not statistically different are put within common borders. As
shown, pigmentation intensity in the female sample is maintained on the same level
in all age groups (F = 1.70, p = ns). In the males slight alterations in reflectance
values are visible. The decrease in pigmentation intensity in boys at the age of 13
years is statistically significant in relation to its level in 10 and 15-year-old groups
(F = 2.99, p<0.01). Generally, the obtained results support findings reported by other
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Fig. 4. Body height distributions in the male samples
authors investigating age variation of pigmentary traits with the use of reflectance
spectrophotometry. Most often, if any significant changes in skin pigmentation are
observable, they are temporary. It is widely suggested that fluctuations of skin pigmentation may be associated with physiological changes occurring during adolescence (Kalla 1973; Banerjee 1984; Mehrai, Sunderland 1990; Rebato et al. 1993).
Thus, it must be concluded that the course of ontogenetic changes in skin pigmentation is very different from the pattern of development characteristic of body height.
As is widely known, body height increases directionally towards the final value
during its progressive ontogenesis. The difference between age variability in skin
pigmentation and body height is clearly noticeable when the results of statistical
analysis presented in the paper are compared (Fig. 5) – in the male sample F = 209.24,
p<0.001 and in the female sample F = 89.78, p<0.001.
The dependence between age and a trait may be revealed by means of the correlation coefficient and regression. This aspect will be discussed here only partially
– a simple regression is to be considered. A statistically insignificant correlation
Quantitative variability
71
Fig. 5. Mean age curves and homongenous groups
(r-Pearson’s correlation coefficient), the regression line course and a weak goodness-of-fit (R squared) of the regression line indicate no association between age
and pigmentary changes in both sexes (Fig. 6). Apparently, a different relationship between age and body height is present, because highly significant correlation coefficients
are indicated and a regression line of a high goodness-of-fit can be determined.
Conclusions
1. In anthropology many traits studied at the individual level may be regarded as
physical body properties (like its size, mass or colour), which are very distant from
the direct effect of gene action attributed to the molecular level. The way the trait should
be defined is indicated by the kind of variation it produces at the population level,
derived from the mode of inheritance. Since skin pigmentary traits arise from polygene
action, their proper phenotypic manifestation should be quantitative in nature. Pigmentation intensity exhibited by the reflectance measurement proved this to be so, as was
demonstrated by the continuous frequency distribution of the reflectance values.
2. Polygenic variability is subject to possible changes. When there are many
traits, the considered period of progressive ontogenesis is the time of characteristic
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A. M. Michałowska
Fig. 6. Characteristics of simple regression
alterations leading to their final form. The inquiry into a pattern of ontogenetic
changes in the skin pigmentation period suggests that there is no time specific,
developmental trend that may be assigned to them.
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