Melanin Storyline Notes

Humans have 46 chromosomes, which equals 23 homologous pairs.
- Each pair contains 1 chromosome inherited from the mother and 1 chromosome inherited from the father.
Chromosomes are made of DNA.
DNA contains genes which code for proteins.
- Proteins create physical characteristics called phenotypes.

A family tree that tracks a specific trait.
- Different symbols have different meanings in a pedigree.

Traits can be:
- Dominant (Genotype = AA or Aa)
  - Phenotype examples = Freckles and Dimples
- Recessive (Genotype = aa)
  - Phenotype examples = Hitchhiker’s Thumb and Attached Earlobes

Trait = a characteristic that is inherited
- Ex. Skin Color
Alleles = different versions of a gene
Locus = a gene’s address
- Which chromosome the gene is found on & where on the chromosome it’s found

Phenotype = how a trait is expressed (how it looks)
- Ex. Normal Skin Pigment (dominant) vs. Albino Skin (recessive)
Genotype = the TWO ALLELES an individual has for a trait (1 from each parent)
- AA and Aa code for normal skin pigmentation
- aa codes for albino skin
- Homozygous = the 2 alleles are the same (AA or aa)
- Heterozygous = the 2 alleles are different (Aa)

Example:
- Mother’s Genotype: Aa
- Father’s Genotype: Aa
- Genotype %: AA 25% Aa 50% aa 25%
- Phenotype %: Normal Skin Pigment 75% Albino Skin 25%

Autosomal traits can be DOMINANT (AA or Aa) or RECESSIVE (aa)
- Albinism, which is an autosomal recessive trait.
Autosomal Traits: the gene of the trait is carried on an autosomal chromosome (any chromosome #1-22 = does not include the sex chromosomes #23)

Trait: Skin Color
Phenotype: Normal Skin Pigment (dominant) vs. Albino Skin (recessive)
Genotype:
- Remember genotypes are made up of TWO ALLELES (1 from each parent)
- AA and Aa code for normal skin pigmentation
- Aa is considered a carrier of the trait
- aa codes for albino skin

Trait: Huntington’s Disease
Phenotype: Huntington’s (dominant) vs. No Huntington’s (recessive)
Genotype:
- Remember genotypes are made up of TWO ALLELES (1 from each parent)
- AA and Aa code for Huntington’s Disease
- aa codes for NO Huntington’s Disease
- NO carriers → you either have the disease or don’t

Sex-Linked Traits: the gene of the trait is either carried on the X or Y chromosome (sex chromosomes are pair #23)
- Males and females do not have the same sex chromosomes which causes a difference in how these sex-linked traits are expressed
- Use XX for females and XY for males
- The letters that represent the trait being studied will correspond to the name of the trait in some way
- Those letters are written as superscripts ( $X^BX^b$ vs. $X^BY$ )

Trait: Color Vision
Phenotype: Normal Vision (dominant) vs. Color Blindness (recessive)
Genotype:
- Males only inherit 1 allele vs. females inherit 2 alleles
Males:
- $X^BY$ - normal vision
- $X^bY$ - color blindness
Females:
- $X^BX^B$ - normal vision
- $X^BX^b$ - normal vision, carrier of the trait
- $X^bX^b$ - color blindness

Trait: Fragile X Syndrome
Phenotype: Fragile X Syndrome or No Disease
Genotype:
- Males only inherit 1 allele vs. females inherit 2 alleles
- NO carriers → you either have the disease or don’t
Males:
- $X^BY$ - Fragile X
- $X^bY$ - No disease
Females:
- $X^BX^B$ - Fragile X
- $X^BX^b$ - Fragile X
- $X^bX^b$ - No disease

TRY Gene = Chromosome 11
- Below the centromere (14.3)
OCA2 Gene = Chromosome 15
- Below the centromere (13.1)
TRYP-1 Gene = Chromosome 9
- Above the centromere (23)
SLC45A2 Gene= Chromosome 5
- Above the centromere (13.2)
The 4 genes are NOT linked because all the genes are found on different chromosomes

The process of using genes from the DNA to create a protein
- Information moves from DNA to mRNA to protein
Proteins control the majority of the processes in the body
- Melanin is the protein that gives skin color

Important Information
- DNA Has… RNA Has…
  - A U
  - T A
  - G C
  - C G
For Translation, look up mRNA on the codon chart in sets of 3 bases to find the amino acid it codes for
- Ex. AUG = MET, UCU = SER, CCU = PRO, etc.

Mutation = a change in the order of the bases in the DNA
- Possible causes:
  - Mistakes during DNA replication
  - Mistakes during cell division
  - Mutagens = environmental factors that can damage DNA (ex. Radiation, chemicals, etc.)
Mutations can occur at different levels
- DNA - base pairing rules are not followed
- Genes - point mutations change just one gene
- Chromosomal - changes to the overall structure of the chromosome & can affect multiple genes
Mutations themselves are not good or bad
- Natural selection acts on mutations to increase or decrease them in a population

Frameshift - the entire frame of nucleotides shifts (most amino acids after are different)
- Insertion: addition of a nucleotide
- Deletion: deletion of a nucleotide
Substitution - one nucleotide is substituted for another
- Silent: no change in amino acid sequence = same protein is made
- Nonsense: results in an early STOP codon = short, usually nonfunctional, protein
- Missense: one amino acid changes = a change in the protein’s function/shape