Educability-and-Group-Differences-1973-by-Arthur-Robert-Jensen

140 Educability and Group Differences
can be established from family pedigrees and concordance rates
for various degrees of kinship. If samples of the two populations
being compared are sufficiently large in relation to the base rate
frequency of the gene (or the alternate allelic forms of the gene)
in the two populations, then the relative frequency of the gene in
each population can be determined. This has been done with
blood groups and many other physical characteristics which show
discrete rather than continuous variation. So far as we know, there
are very few genes that are exclusive to any one population; only
a few of the 70-odd identified blood types fall into this category.
As Thoday (1969, p. 4) notes, ‘Populations of a species do not
differ absolutely, but in the relative frequency of different genotypes.’
Continuous Traits
The characteristics we are most interested in, however, are continuous
variables, like height and IQ. Their wide variability over
a continuous range of values is said to be polygenic, that is, a result
of the combined effects of many genes, each one independently
either adding, not adding, or subtracting a small increment of
the trait. Each person’s genotype for a particular trait is comprised
of a random assortment of the parental genes; thus individuals
inherit genes, not genotypes. Parents can only pass on their genes
to their progeny, not their genotypes. Since many different assortments
and combinations of the parental genes are possible, we
see considerable variations both among parents and their children
and among the children of the same parents. For polygenic traits,
like height and IQ, how can w7e prove genetic differences between
populations
Coming back to our Watusi and Pygmy example, we can perform
what plant geneticists call a ‘transplant’ and animal geneticists call
‘cross-fostering’. That is, we rear members of one population in
the habitat of the other and vice versa. Shortly after birth, Pygmy
infants would be given to Watusi for rearing and Watusi infants
would be given to Pygmies. Will their adult height come closer
to the mean of the population of their origin or of their adoptive
population If the sample sizes are large enough, this method could
establish with considerable accuracy the relative contributions of
‘nature’ and ‘nurture’ to the mean height difference between
Pygmies and Watusi. The uncontrolled factor in this case, of