Skin Color

• Three pigments:– Melanin– Carotene– Hemoglobin

Why do veins look blue?

• Blood lower than oxygen = brownish in color• Veins are closer to the surface than arteries• Walls of veins are thinner• Light-scattering effect of epidermis: blue light

is the only light that makes it to veins

Blue people of Kentucky

• Methemoglobinemia– Variation in hemoglobin (instead of Fe2+, it has

Fe3+)– Decreased ability to bind oxygen– Arterial blood = brown instead of red– Skin appears blue-ish

Acquired vs Inherited

BACK TO NORMALCY

Melanocytes

• Produce melanin (cluster = melanosome)• Located in the stratum germinativum

How did differences in skin color among races evolve?

– Protection from UV light vs vitamin D?

Melanocytes• All humans have the same relative amount

Fraternal Twins

Quantitative variation

• A trait varies continuously along a range of values

So how do we get different skin colors? How is skin color inherited?

• Usually as the result of the interaction with the environment and several different genes

One gene? Or several?

• Recall mendelian genetics…

Multiple Gene Inheritance

• Polygenic inheritance– Each gene follows dominant/recessive pattern– Combined effects add together

How do you set up the Punnett square?

modeling

• Under this model, are the statements true or falsE?– When one parent is white (zero dominant alleles),

the progeny can be no darker than the other parent.

– When one parent is light (one dominant allele), the progeny can be no darker than the other parent.

• There are more than 7 skin colors though…– Explanation?

Freckles and Moles

• Local accumulations of melanin

vitiligo

• Loss of melanocytes• Uneven dispersal of melanin

albinism

• Melanocytes do not produce melanin• Skin = pink, hair = pale/white, irises =

unpigmented or poorly