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What does it mean, in practice?
100%
100%100%
Members of our community are only slightly less different from us than members of distant populations
85%
85%100%
Variances among continents are small, but not zero. Do genotypes naturally cluster in continental or subcontinental groups?
Assigning a genotype to its continent by discriminant analysis
training dataset
query genotype
?
Genes, as well as morphology, suggest inconsistent clusterings of genotypes
Africa
Asia, Europe, Australia, Americas
Americas
Africa, Asia, Americas,Oceania
Asia Europe
Africa, Asia,EuropeOceania
Y chromosome: Romualdi et al. 2002
Alu insertions: Romualdi et al. 2002
X chromosome: Wilson et al. 2001
Europe,Ethiopia
S. Africa N. Guinea
Asia
Genes, as well as morphology, suggest inconsistent clusterings of genotypes
377 STR loci: Rosenberg et al. 2005
Melanesia Eurasia N Africa N America
Maya
S. Africa
377 STR loci: Barbujani and Belle 2006
E Africa
C Africa
Piapoco
Suruì
Karitiana
Kalash
W. Eurasia
E. Asia
Africa
Americas
Oceania
8
6
2
45
9
10
1
7
3
Genomic boundaries inferred from diversity at 377 STR loci(Barbujani and Belle 2006)
Less than 8% of the alleles are continent-specific, and more than half of these are African
Rosenberg et al. (2002)
Avg. numbers of haplotype blocks in 51 genome regions (1.5 million base pairs): limited continental
differentiation
Africa is special: Genetic diversity in all continents is often a subset
of African genetic variation
Tishkoff et al. (1998)
Approximate inferred colonization dates
Human genetic diversity largely reflects patterns of migration
A pointillist view of human evolution and variation: 1
© 1999 Kenneth K Kidd, Yale University
A pointillist view of human evolution and variation: 2
© 1999 Kenneth K Kidd, Yale University
A pointillist view of human evolution and variation: 3
© 1999 Kenneth K Kidd, Yale University
Genetic evidence on modern humans’ origins
• Extensive allele sharing across continents• Extensive haplotype sharing across continents• Largest proportion of human genetic diversity within
populations• No obvious continental clusters of populations• Genetic diversity out of Africa a subset of African
diversity• Broad-scale clines
We are a young
and mobile species
• Il nostro genoma è molto piccolo• Il nostro genoma è molto grande• I nostri genomi sono molto simili• I nostri genomi sono molto differenti
La diversità genomica umana
Mind the numbers
Humans and chimps share >98% of their genomes
Among the 2% differences, 1.9% are fixed differences within species
The remaining fraction, 0.1%, contains all human genomic variation
85% of that 0.1% represents differences among members of the same population
The differences among the main racial or continental groups represent 10% of 0.1% of the total, that is, 0.01%
But 0.01% of 3 billion DNA sites means 300 000 variable sites
DNA-based forensic identification
Many DNA regions are characterised by a Variable Number of Tandem Repeats (VNTR): …CTAGACCCGAGAGAGAGAATTCCATGC… [5] …CTAGACCCGAGAGAGAATTCCATGC… [4] …CTAGACCCGAGAGAGAGAGAGAATTCCATGC… [7]
Each of us carries a potentially unique combination of alleles at VNTR loci
Isolation of VNTRs
DNA fingerprintingAlec Jeffreys et al.: Hypervariable minisatellite
regions in human DNA, Nature, 314:67-73, 1985.
DNA fingerprinting: an application
DNA fingerprinting: an application
DNA fingerprinting: a paternity case
A case of sexual violence
But if races do not exist, how come forensic scientists are so good at finding them?
a. UK forensic classification (before 2005):European, Afro-Caribbean, Indian Subcontinent, South East Asian, Middle Eastern
b. UK forensic classification (after 2005)White-British, White-Irish, White-other, Asian-Indian, Asian-Pakistani, Asian-Bangladeshi, Asian-other, Black-Caribbean, Black-African, Black-other, Chinese, + 4 razze miste e Other
c. USA forensic classification Caucasian, African-American, East Asian, Hispanic, Native American
d. Bribri classificationBribri, ña
Any list can be Ok for certain purposes, no list gives an all-purpose biological classification of humankind
Think about Hispanics
Consigli e sconsigli