Comprehensive Notes on Skin Color, Melanin, and Genetics

Desert Foxes and Trait Likelihood

• Desert foxes have long ears, an advantageous trait controlled by genes A and B.

• Long ears are dominant, aiding in heat release.

• The question poses: What is the likelihood that two foxes with the genotypes AaBB and AaBb will have offspring with any recessive form of either trait?

Skin Color Exploration Project

• This project (4.4 - Skin Color) connects units 1, 2, and 3.

• It ties together cells (cell parts involved in melanin production), biochemistry (how melanin is produced), and evolution (how skin color has changed over time).

Cells, Tissues, and Biochemical Pathways in Skin Color

• Before proceeding, it's important to understand the cells, tissues, and biochemical pathways involved in melanin production.

Cell Parts

• Key components include:

  • Epidermis: The outermost layer of the skin.

  • Dermis: The layer beneath the epidermis.

  • Squamous cells

  • Blood vessels (veins and arteries)

  • Melanin: The pigment responsible for skin color.

  • Basal cells: Located in the deepest layer of the epidermis.

  • Melanocytes: Cells that produce melanin.

  • Hair shaft, oil gland, hair follicle, fatty tissue, nerve, lymph vessel, sweat gland

Melanocytes and Melanosomes

• Melanocytes are cells within the basal layer of the epidermis.

• Melanosomes are organelles within melanocytes that produce melanin.

• Melanin is the pigment providing varying shades of skin color.

• Once melanosomes are filled with a genetically predetermined amount of melanin, they move into keratinocytes.

Melanin's Purpose

• Skin color is a byproduct of melanin's primary function: protecting DNA by surrounding the nucleus.

• Melanin absorbs intense UV radiation, preventing damage to DNA molecules.

• Melanin also protects folate, which is crucial for egg production and embryo implantation in females.

Melanin Biochemical Pathway

• Eumelanin: A brown-to-black pigment responsible for darker shades of hair, skin, and eyes. It is more effective in absorbing and dissipating ultraviolet radiation.

• Pheomelanin: A yellow-to-red pigment contributing to lighter shades of hair, skin, and eyes. It is not very effective in absorbing and dissipating ultraviolet radiation.

Melanocyte Activity

• Melanocytes produce melanin in melanosomes.

• Tyrosine, an amino acid, is broken down into dihydroxyphenylalanine (DOPA) by the enzyme tyrosinase.

• MSH (melanocyte-stimulating hormone) activates MC1R (melanocortin 1 receptor).

• ASIP can turn off MC1R.

• Activated MC1R stimulates the production of cyclic adenosine monophosphate (cAMP), which triggers eumelanin production (black/brown pigment).

• In the presence of cysteine, pheomelanin (red/yellow pigment) is produced.

Genes Involved

• TYR (Tyrosinase):

  • Located at locus 11q14-11q21 on chromosome 11.

• MC1R (Melanocortin 1 Receptor):

  • Located at locus 16q24.3 on chromosome 16.

MC1R and UV Light

• MC1R is activated by UV light.

• Activation stimulates cyclic adenosine monophosphate (cAMP) production, leading to eumelanin production (dark pigment).

• Mutations in the MC1R gene can prevent its activation or binding, resulting in pheomelanin production (red/yellow pigment) and lighter skin pigmentation.

UV Radiation and Melanin Levels

• Explore the relationship between areas where UV radiation is high versus areas where it is low.

• Melanin levels range from lightest to darkest skin, correlating with UV exposure.

Rickets Risk and Melanin

• The question is posed: Who is most at risk for developing rickets?

  • The correct answer is: Children born to mothers with dark skin, living far from the equator.

Vitamin D Biochemical Pathway

• Vitamin D enters the body through food.

• UV-B radiation activates 7-dehydrocholesterol, breaking it down into cholecalciferol. Both are initially inactive.

• Vitamin D from food and sunlight convert these inactive forms to biologically active vitamin D.

Vitamin D Functions

• Growth and bone mineralization

• Regulation of muscle calcium transport

• Regulation of immune function

• Induction of apoptosis

• Regulation of phosphorus and calcium homeostasis

• Regulation of insulin secretion

• Stimulation of cell differentiation

• Control of cell proliferation

Melanoma Mutations

• Consider why mutations that cause melanoma might not be passed down to the next generation.

Albinism Mutation Comparison

• Comparison of Oculocutaneous Albinism Type 2 and Rufous Oculocutaneous Albinism.

• Focus: Differences in terms of cells/tissues/organs, biochemical pathways, and genetics.

Codominance vs. Incomplete Dominance; Multiple Alleles vs. Polygenic Traits

• Describe the difference between codominant traits and incomplete dominant traits.

• Describe the difference between traits being controlled by multiple alleles and traits being polygenic.

Feedback on Topic Flow

• Feedback is requested on whether the ideas flowed in a way that makes sense and how the topic could be improved.

Warm Up Genetics Problems

• Warm Up: Desert foxes have long ears controlled by two genes-A and B. Having long ears is dominant and more advantageous than short ears allowing them to more easily release heat. What is the likelihood that two foxes with the genotypes AaBB and AaBb will have offspring with any recessive form of either trait?

• Warm Up: Two parents (Mom- blood type IAIB, Dad- blood type IAIO) have a baby. What are the chances the baby will have a blood type different from mom and dad? $I^A I^B$ $I^A I^O$ $I^AI^A$ $I^AI^B$ $I^AI^O$ $I^BI^O$

• Warm Up: Two parents (Mom- XAXa, Dad- blood type XAY) have a baby. Muscular Dystrophy is a sex-linked recessive trait. What are the chances they will have a child with muscular dystrophy? $X^A X^a$ $X^A Y$ $X^AX^A$ $X^AX^a$ $X^AY$ $X^aY$