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which is completely dominant to the other. Genetics, however, is not
always that simple.
Sometimes, two alleles combine to give an entirely new phenotype, as
with flower color in snapdragons. If one crosses a true-breeding red
snapdragon with a true-breeding white snapdragon, the offspring will
all have pink flowers. This phenomenon is called incomplete dominance.
In the human ABO blood type system, we find another example. If a
person who is homozygous for the type A allele (I A I A ) marries a person
who is homozygous for the type B allele (I B I B ), their children will
all be type AB, with a genotype I A I B . This situation is called
codominance—the phenotypes (in this case, the particular sugars on the
red blood cell membranes) of both parents are found in the child.
Some genetic loci have multiple alleles. We return to the human ABO
blood type system for an example. In addition to the A, B, and AB blood
types mentioned above, humans may also have blood type O, which has
the genotype ii. This means that there are three alleles which may be
found at the blood type locus—I A , I B ,ori. The i allele is recessive to
either of the I A or I B alleles. Thus, a person with type A blood may have
genotype I A I A or I A i, and a person with type B blood may be I B I B or I B i.
Some traits are sex linked, in that they occur on one of the sex
chromosomes of a species (such as the human X chromosome). Such
traits are therefore present in a single copy in the sex with the unmatched
''pair'' of sex chromosomes (such as the human male, in which there are
22 pairs of chromosomes and an unmatched set, XY). When calculating
the frequency of X-linked alleles in the human population, one needs to
take into account the sexes of the individuals comprising the population.
E XERCISE 3-3
If p is the frequency of the I A allele, q is the frequency of the I B allele, and r is
the frequency of the i allele, how would you state the frequencies of the
blood types A, B, AB, and O, assuming Hardy-Weinberg conditions?
E XERCISE 3-4
Assume a gene C is found on the human X chromosome and that it
occurs in only two alleles, C and c. Using p to represent the frequency
of C and q to represent the frequency of c, how would you express the
genotype frequencies in males and females?
The Hardy-Weinberg theorem and the selection observed with
maladaptive genes might prompt us to believe the genetic make-up of
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