Transcript Mating systems

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 Wright
1889-1988
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 depression
What is inbreeding depression?
What causes inbreeding
depression?
How do organisms avoid inbreeding
depression?
Inbreeding
Scientists find no biological reason to
stop 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
Sarah
Josiah
Wedgwood I Wedgwood
Robert
Darwin
Susannah
Wedgwood
Charles
Darwin
Josiah
Wedgwood II
Mrs.
JWII
Emma
Wedgwood
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