Re: Why don't humans have spotted or striped skin color?

Good question.  First, let's take a quick look at skin color in humans.  
There are two major pigments in humans: eumelanin (black) and phaeomelanin 
(yellow).  For that matter, the same two pigments are present in virtually 
every mammal, from mice to tigers, and can be combined to produce any 
color from white to red to yellow to brown to black.  I'm not sure how 
Baboons get blue muzzles, but I'm hard put to think of any other examples 
of mammalian coloration outside the red-yellow-black-white spectrum.   

The melanin pigments are produced from a precursor molecule through a 
series of intermediate molecules.  A set of enzymes exist, where each 
enzyme catalyzes the transformation of one intermediate to the next.  Each 
enzyme has specific genetic control elements regulating when, where, and 
how much of the enzyme is produced.  Variants in these control elements 
exist within all mammalian populations, and as a consequence there are 
coat color variants in virtually every species: in California there exists 
a population of grey squirrels which are almost black, presumably because 
they produce more black melanin than their grey relatives.  I'm pretty 
sure that squirrels don't get tans, so the darker coloration is likely due 
to a genetic variation in one or more of the enzymes producing black 
melanin, which increases the level of pigment produced.

In humans the same phenomenon is seen, but the fun part is that in America 
and Brazil and other multi-ethnic regions we get to see how inherently 
complicated skin color is in humans.  When a dark skinned person has kids 
with a light skinned person, the kids do indeed tend to be intermediate in 
skin tone.  Because skin color does not segregate as a single gene 
mendelian trait ( unlike green and yellow peas in Mendel's experiments) it 
is likely that the trait in humans involves the interaction of many 
variations in multiple enzymes from the melanin pathways.  Moreover, even 
within human populations there exists significant variation in skin tone: 
not all British are pale, and not every African is dark.  Thus, the 
average skin color of an ethnic group reflects the average frequency of 
the melanic variations within the group, not fixation of dark or light 
variants.

Now to spots and stripes.  Stripes are easier to explain, so I'll start 
there.  The skin cells which produce melanin are known as melanocytes.  
During development of a mammalian embryo, melanocytes are derived from the 
neural crest, a set of cells which will give rise to the spinal cord and 
brain as well.  The melanocytes differentiate near what will become the 
spinal cord, and migrate from there toward the belly of the animal.  
Sometimes not enough melanocytes form to cover the whole organism, so the 
areas closer to the neural crest (the back and sides) are pigmented, while 
more distal areas (belly and legs) are not.  One way stripes occur is when 
the paths of melanocyte migration away from the neural crest are not 
uniform: the neural crest has segments (somites) which will ultimately 
form the vertebrae.  If migration of melanocytes runs in streaks from each 
somite toward the belly of the animal, you get vertical stripes (as in 
Zebra and Tiger).  Spots are more complicated, but I believe they involve 
differential survival of the migrating melanocytes, such that random 
patches of melanocytes survive.

So why don't humans have stripes?  Well, the patterns of pigmentation in 
various species are genetically determined.  That is, there are specific 
genetic regulatory elements controlling the migration patterns of 
melanocytes in striped animals, presumably because there was an 
evolutionary advantage to stripes (either as camouflage for a predator or 
as a confusion pattern for a herd animal to escape predators).  The human 
environment evidently didn't provide such an environment, so we didn't 
evolve to have stripes.  For that matter, neither did horses or dogs. 

As for spots, a condition known as piebaldism exists in many mammals, from 
horses to dogs to mice to humans.  It's not particularly common in humans 
because it is associated with some gastrointestinal abnormalities which 
can be life threatening without treatment, but it does exist.  If you 
could control who mates with who, you could presumably create a race of 
spotted humans.  However, why you'd want to do so eludes me, especially in 
the era of modern makeup.

To sum up, dark+light=intermediate, even in humans.  As for stripes, if 
you start with an organism where stripes exist, it is pretty easy to breed 
an unstriped form, but the other way around is much more difficult.  As 
with many things, it is always easier to start with what nature/God has 
provided than to invent something completely new.

Chris