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Variety of mating systems. Panmixia Assortative mating Disassortative mating Outcrossing Inbreeding Mixed mating. Disassortative mating. What is the equilibrium frequency of alleles S1, S2, S3, and S4? What is the fate of a new mutation that produces allele S5?. Inbreeding. - PowerPoint PPT Presentation
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Variety of mating systems
• Panmixia
• Assortative mating
• Disassortative mating
• Outcrossing
• Inbreeding
• Mixed mating
Disassortative mating
What is the equilibrium frequency of alleles S1, S2, S3, and S4?
What is the fate of a new mutation that produces allele S5?
Inbreeding
What is inbreeding?
Forms of inbreeding
Genetic consequences of inbreeding
Loss of heterozygosity
Do allele frequencies change (i.e., does inbreeding result in evolution)?
Genetic consequences of inbreeding
Heterozygosity:
Homozygosity:
Genetic variance within lines:
Genetic variance among lines:
Response to selection among lines:
Fitness of inbred lines:
The inbreeding coefficient, Wright’s F
Sewall Wright1889-1988
The inbreeding coefficient, Wright’s F
F ranges from 0-1• 0 = completely outbred (H-W)• 1 = completely inbred (homozygous)
F can be thought of as:• Proportional loss of heterozygosity• Probability of homozygosity• Half the coefficient of relatedness of
the parents
F is measured relative to some starting population, which is usually assumed to have F = 0
The inbreeding coefficient, Wright’s F
From Hartl and Clark Fig. 4.15
Genotype frequencies as a function of F
AA: p2 + pqF
Aa: 2pq - 2pqF
aa: q2 + pqF
How can F be estimated if we don’t have a
pedigree for the whole population?
HI = observed heterozygosity at neutral loci (usually with
molecular markers)
HT = heterozygosity expected under H-W (2pq)
Aa: 2pq - 2pqF = HI
2pq(1-F) = HI
HT(1-F) = HI
F = 1-(HI/HT)
F = (HT-HI)/HT
Inbreeding depressionWhat is inbreeding depression?
What causes inbreeding depression?
How do organisms avoid inbreeding depression?
InbreedingScientists find no biological reason to Scientists find no biological reason to
stop first cousins from marryingstop first cousins from marrying• Risks to children born to cousins
not as high as previously thought
By Denise Grady
The New York Times
Thursday, April 4, 2002
RobertDarwin
EmmaWedgwood
CharlesDarwin
SusannahWedgwood
JosiahWedgwood I
SarahWedgwood
JosiahWedgwood II
Mrs.JWII
Is inbreeding always bad?
By exposing recessive alleles to selection in homozygotes:
• Leads to rapid fixation of beneficial recessive alleles
• Purges deleterious recessives; aka, lightens the genetic load
