Frequências de Alleles e Genótipos em Populações Diferentes: Origens e Medida da Diferença, Notas de estudo de Genética

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Differences between populations:

Origins and quantifying

Recap: Calculating genotype/ allele

frequencies, and testing HW

• MN blood type in Navajo
  • MM: 305 0.845 p(M) = 0.845 + ½ (0.144) = 0.
  • MN: 52 0.144 q(N) = 0.011 + ½ (0.144) = 0.
  • NN: 4 0. TOTAL: 361 HW Predicted: MM p 2 = 0. Close to HW MN 2pq = 0. NN q^2 = 0.

Populations differ...

• May have different allele & genotype frequencies
• May have alleles at some genes not found in other

populations if

• Very recent new mutation, or
• Complete isolation

Difference(s) arose via mutation,

then spread

• Ancestors aa bb
• Population splits

into two groups

• In #1, new mutation to A
• In #2, new mutation to B
• New mutations spread

ANCESTOR All aa bb

Mutation to A

A allele spreads

Mutation to B

B allele spreads

AA bb aa BB

aa bb aa bb

POP 1 POP 2

How to quantify?

• Simplest to quantify - all individuals differ
  • “Fixed” difference
    • Population 1: all AA Population 2: all aa

How to quantify?

• Simplest to quantify - all individuals differ
  • “Fixed” difference
    • Population 1: all AA Population 2: all aa
• Happens, but not very common within a species, and

generally not

true among modern

human ethnic groups

Deviation from HW allows you to

quantify allele freq differences!

• Assume two populations at HW
  • If sample each by itself , see HW
  • If sample both together , see deviation from HW
    • Wahlund effect
• How big the deviation is from HW when sampling both

together quantifies difference in allele frequencies

Measure we’ll use: FST

• FST ranges from 0 to 1

0 : no allele frequency differences 0 < FST < 1 : allele frequencies differ somewhat 1 : “fixed” difference between populations

• FST = HW predicted 2pq – % observed hetz

HW predicted 2pq

aa: 100/200 = 0.5 (predicted-obs)/predicted

 - FST Example 

• • POPULATION 1 POPULATION
  • – AA 100 AA
  • – Aa 0 Aa
  • – aa 0 aa
• TOTAL- AA: 100, Aa: 0, aa: - FST Example
• • POPULATION 1 POPULATION
  • – AA 100 AA
  • – Aa 0 Aa
  • – aa 0 aa
• TOTAL- AA: 100, Aa: 0, aa: 100 N=
• AA: 100/200 = 0.5 p(A) = 0.5 ; q(a) = 0.
• Aa: 0/200 =
• aa: 100/200 = 0. - FST Example
• • POPULATION 1 POPULATION
  • – AA 100 AA
  • – Aa 0 Aa
  • – aa 0 aa
• TOTAL- AA: 100, Aa: 0, aa: 100 N=
• AA: 100/200 = 0.5 p(A) = 0.5 ; q(a) = 0.
• Aa: 0/200 = 0 HW 2pq = 0.
• aa: 100/200 = 0. - FST Example
• • POPULATION 1 POPULATION
  • – AA 100 AA
  • – Aa 0 Aa
  • – aa 0 aa
• TOTAL- AA: 100, Aa: 0, aa: 100 N=
• AA: 100/200 = 0.5 p(A) = 0.5 ; q(a) = 0.
• Aa: 0/200 = 0 HW 2pq = 0.
• aa: 100/200 = 0. - FST Example
• • POPULATION 1 POPULATION
  • – AA 100 AA
  • – Aa 0 Aa
  • – aa 0 aa
• TOTAL- AA: 100, Aa: 0, aa: 100 N=
• AA: 100/200 = 0.5 p(A) = 0.5 ; q(a) = 0.
• Aa: 0/200 = 0 HW 2pq = 0. - FST = (0.50-0)/(0.50) = 1. - FST Example
• • POPULATION 1 POPULATION
  • – AA 250 AA
  • – Aa 500 Aa
  • – aa 250 aa
• TOTAL- AA: 740, Aa: 920, aa: - FST Example
• • POPULATION 1 POPULATION
  • – AA 250 AA
  • – Aa 500 Aa
  • – aa 250 aa
• TOTAL- AA: 740, Aa: 920, aa: 340 N=
• AA: 740/2000 = 0.37 p(A) = 0.6; q(a) = 0.
• Aa: 920/2000 = 0.
• aa: 340/2000 = 0. - FST Example
• • POPULATION 1 POPULATION
  • – AA 250 AA
  • – Aa 500 Aa
  • – aa 250 aa
• TOTAL- AA: 740, Aa: 920, aa: 340 N=
• AA: 740/2000 = 0.37 p(A) = 0.6; q(a) = 0.
• Aa: 920/2000 = 0.46 HW 2pq = 0.
• aa: 340/2000 = 0. - FST Example
• • POPULATION 1 POPULATION
  • – AA 250 AA
  • – Aa 500 Aa
  • – aa 250 aa
• TOTAL- AA: 740, Aa: 920, aa: 340 N=
• AA: 740/2000 = 0.37 p(A) = 0.6; q(a) = 0.
• Aa: 920/2000 = 0.46 HW 2pq = 0.
• aa: 340/2000 = 0.