Aging and Sex, DNA Repair in
91
Inbreeding
ordinarily
is
avoided
in
humans. In humans, body odor prefer-
ences (determined by men and women
scoring the odors of T-shirts previously
worn by men or women) would serve
to increase heterozygosity in the progeny.
Prohibitions against marriage and sexual
relations between close relatives are a near
universal feature of human social behav-
ior in many different cultures. This general
prohibition is known as the incest taboo.
The basis for the incest taboo appears
to be the avoidance of inbreeding because
of the effect of the expression of delete-
rious mutations. There is a signiFcantly
higher incidence of major congenital mal-
formations and postnatal mortality among
the progeny of consanguineous marriages
than among the progeny of nonconsan-
guinous marriages.
We have reviewed the evidence that
‘‘Level 2’’ of sexual communication, to
avoid inbreeding and promote outcross-
ing, is common in vertebrates. Since
sexual interaction in vertebrates is usually
essential for reproduction, sexual commu-
nication at ‘‘Level 1’’ for sexual interaction
is ubiquitous.
5.4
‘‘Level 3’’ of Sexual Communication Is
Used to Select for Fitness among Potential
Mates
In addition to ‘‘Level 1’’ and ‘‘Level 2’’
functions of sexual communication, the
Frst to promote homologous recombina-
tional repair and the second to promote
outcrossing, a third aspect of sexual com-
munication involves courtship behavior.
The adaptive function of courtship prob-
ably is selection of a mate with the
characteristics that optimize reproductive
success (selection for Ftness). This leads
to competition among individuals of the
same gender. Such competition is the ba-
sis for sexual selection, regarded as having
molded many features of animal behavior.
As an example, females of the guppy
Fsh,
Poecilia
reticulata
,r
e
s
p
o
n
dm
o
r
e
strongly to the mating displays of male
guppies which have brighter orange spots.
The brightness of the spots correlates pos-
itively with health and vigor and negatively
with previous parasitic infection. Thus fe-
male guppies tend to select healthier mates
that are more resistant to parasites.
Substantial evidence has been presented
on human courtship behavior suggest that
the basic biologic function of human
courtship is to optimize reproductive
success by the selection of a mate of the
highest possible Ftness.
6
Overview
The occurrence of high levels of endoge-
nous DNA damage in mammals is now
well established. DNA of mammalian so-
matic cells is the master informational
molecule, and accumulated damages in
this molecule probably cause the pro-
gressive irreversible deterioration of cell,
tissue, and organ function that deFnes
mammalian aging. When an organism
forms progeny via germ cells, it is im-
po
r
t
an
tth
a
tth
e
s
ec
e
l
l
sb
ef
r
e
eo
fDNA
damages, since such damages cause invia-
bility. To facilitate efFcient DNA repair,
the redundant information available in
the diploid cell can be used to replace
damaged information through recombina-
tional repair. Sexual reproduction appears
to be an adaptation to promote pairing and
exchange between homologous chromo-
somes for the purpose of efFcient repair of
the DNA, which is passed on to germ cells.
In eukaryotes this occurs during meiosis,
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