926
UNIT SIX
and dark brown/black. If each dominant allele contributes a
certain amount of pigment, then the greater the number of
such alleles, the darker the eye color. The bell curve arises
because there are more ways to inherit light brown eyes,
with any two dominant alleles, than there are ways to inherit
the other colors.
Other genes modify the expression of the two melanin-
controlling genes. Two genes add greenish colors, called
lipochromes. Overlying the colors and tones are specks and
ecks, streaks and rings, and regions of dark versus light that
arise from the way pigment is laid down onto the distinctive
peaks and valleys at the back of the iris.
Height and skin color are multifactorial as well as poly-
genic, because environmental factors infl
uence them: good
nutrition enables a person to reach the height dictated by
genes, and sun exposure affects skin color. Most of the more
common illnesses, including heart disease, diabetes mellitus,
hypertension, and cancers, are multifactorial.
plot the frequency for each phenotype class, a bell-shaped
curve results. This curve indicating continuous variation of
a polygenic trait is strikingly similar for different characteris-
tics, such as F ngerprint patterns, height, eye color, and skin
color. Even when different numbers of genes contribute to
the phenotype, the curve is the same shape.
Eye color illustrates how interacting genes can mold a
single trait. The colored part of the eye, the iris, darkens as
melanocytes produce the pigment melanin. Unlike melanin
in skin melanocytes, the pigment in the eye tends to stay in
the cell that produces it. Blue eyes have just enough melanin
to make the color opaque, and dark blue or green, brown or
black eyes have increasingly more melanin in the iris.
Two genes (
OCA2
and
HERC2
) control melanin synthe-
sis and deposition. The alleles of the two genes interact addi-
tively, producing distinct eye colors. ±igure 24.9 depicts how
this might happen to account for F ve distinct eye colors—
light blue, deep blue or green, light brown, medium brown,
FIGURE 24.9
Variations in eye color. A model of two genes, with two alleles each, can explain F
ve human eye colors. If eye color is controlled by
two genes
A
and
B,
each of which comes in two allelic forms
A
and
a
and
B
and
b,
then the lightest color would be genotype
aabb;
the darkest,
AABB.
AABB
AABb
AaBB
AABb
AAbb
AaBb
AaBB
AaBb
AaBb
AaBb
aaBB
AaBb
Aabb
aaBb
AaBb
Light blue
1/16
Phenotype
frequency
Number of
dominant
alleles
Deep blue or green
Light brown
Medium brown
Dark brown/black
Aabb
aaBb
aabb
AB
AB
Ab
aB
ab
Ab
aB
ab
4/16
6/16
4/16
1/16
0
1
2
3
4
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