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to the amount of active gene product, we would expect
the heterozygote phenotype to be exactly intermediate
between the homozygotes (show no dominance).
For many quantitative traits, however, neither of
these simple cases is the rule. In general, heterozygotes
are not exactly intermediate between the two homozy
gotes but are closer to one or the other (show partial
dominance), even though there is an equal mixture of
the primary products of the two alleles in the heterozy
gote. Suppose that two alleles,
a
and
A,
segregate at a lo
cus in±uencing height. In the environments encountered
by the population, the mean phenotypes (heights) and
frequencies of the three genotypes might be:
of children with biological parents is
r
5
1.00, but with
adoptive parents it is
r
5
0. (Remember that a correla
tion between two sets of numbers does not mean that
the two sets are identical, but that, for each unit of in
crease in one set, there is a constant proportional in
crease in the other set—see the Statistical Appendix on
statistical analysis at the end of this chapter.) This per
fect correlation with biological parents and zero correla
tion with adoptive parents means that
H
2
5
1, given
the arguments just developed. All the variation in IQ
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.
 Spring '08
 JOHNSON
 Biology

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