1Describe what melanocytes do
Make melanin in clusters called melanosomes
2Outline keratinocytes
Barrier cells at top of skin layer
Responsible for skin colour
3Describe what melanin does
Transported to keratinocytes
4How is hair colour produced?
At base of hair follicles, melanosomes bond with keratin cells that produce hair
5State the 2 types of human melanin
Eumelanin
Pheomelanin
6Describe eumelanin
Ranges from brown to black
7Describe pheomelanin
Ranges from red to pink
8What does the MC1R gene code for?
Melanocortin 1 receptor protein
Produced receptor regulates how much eumelanin is produced
9Consequence of loss of function of MC1 receptor
More pheomelanin produced
Red hair, lighter skin
10What did Smith find and when?
1998
3 variants of MC1R gene significantly associated with red hair
Also over represented in ppl with light skin
11What did Morgan find and when?
2018
GWAS study
n = 350,000, European descent
MC1R SNPs account for 73% heritability of red hair
Combined with other genes, accounts for 90% total heritability
So red hair is a polygenic trait
12What did Harding find and when?
2000
MC1R consensus gene different in humans comapred to chimps
16 mutation differences between humans and chimps - more nonsysonymous than synonymous
Suggests possible selection
No nonsysnonymous mutations in African samples
Suggests African MC1R gene has been under functional constraint - selection against changes in this gene
10 nonsysnonymous mutations in non-African populations - not significant from human:chimp ratio so red hair alleles not under selection?
Most likely: relaxed selection for dark hair allowing red hair alleles to drift in small populations
135 potential advantages of darker skin
+ Protection against folate photodegradation
+ Protection from sunburn
+ Protection against skin cancer
+ Shielding of sweat glands
+ Immunological function of pigment, eumelanin
141 proposed advantage of lighter skin
+ Avoiding vitamin D deficiency
15What did Mallick et al find and when?
2013
SLC24A5 allele = light skin allele shared by South Asians and Europeans by descent
16Which human population has the most variabel skin pigmentation?
African
17What is the estimated genetic heritability of pigmentation?
55 - 95%
BUT considerable missing heritability remains
18What did McEvoy et al find and when?
2006
Sets of genes determing light skin in East Asia and Europe are different
19What does DNA fingerprinting measure?
Length of short tandem repeats
202 factors that might affect the random match probability when using a DNA database
Short tandem repeat allele frequencies (need >5 in database)
Different population structures
21Limitations of using short tandem repeats to differentiate between populations
- Highly allelic
- High mutation rate
- High heterozygosity
22Limitations of assigning ppl to groups
- Group definitions change over time
- Depends on socio-political groupings in population
23How might the UK national DNA database contribut to racial stereotypes?
Profiles of >3/4 black men between 18-35y are recorded
More likely to get a hit
Fingerprinting and predicting externally visible characteristics target groups rather than individuals
Inadequate reference groups
24Homozygous for MC1R consensus sequence =
- Blonde hair
- Brunette hair
- Dark hair
- Red hair
25Homo/heterozygous for red hair variants = _____ % chance of red hair
> 90
26Problems with genetic fingerprinting
- Protection of genetic data, genome cloaking
- Balancing privacy with forensic and personal medicine use
27What can be measured to predict externally visible characteristics such as hair, skin and eye colour?
- STR length
- Specific SNP ranges