The document discusses the concept of inbreeding coefficient in population genetics, highlighting how inbreeding and mating patterns can influence allele and genotype frequencies over generations. It describes mechanisms such as self-fertilization, sexual selection, and disassortative mating that can affect mating success. The inbreeding coefficient is defined and its practical uses in predicting inbreeding depression and assessing genetic risks are outlined.

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Comsats Institute of information technology
ASSINGMENT TOPIC
Inbreeding coefficient
ASSINGMENT OF
POPULATION GENETICS
SUBBMITTED TO: DR. ABDUL REHMAN KHAN
SUBBMITTED BY: ZOHAIB HUSSAIN(SP13-BTY-001)
SUBBMITTED ON: 5th
October, 2015
Inbreeding coefficient

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[2].
Inbreeding and self-fertilization
Genotypes mate at random with respect to their genotype at this particular locus.
There are many ways in which this assumption might be violated:
• Some genotypes may be more successful in mating than others, sexual selection.
• Genotypesthataredifferentfromoneanothermaymatemoreoftenthanexpecteddisassortative
mating, e.g., self-incompatibility alleles in flowering plants, MHC lociinhumans (the
smelly t-shirt experiment)
• Genotypesthataresimilartooneanothermaymatemoreoftenthanexpectedassortativemat
ing.
• Some fraction of the offspring produced may be produced asexually.
• Individuals may mate with relatives inbreeding.
– self-fertilization
– sib-mating
– first-cousin mating
– parent-offspring mating
– etc.
When there is sexual selection or disassortative mating genotypes differ in their chances
of being included in the breeding population.
As a result, allele and genotype frequencies will tend to change from one generation to
the next. Infact, we’ll also ignore assortative mating, since it’s properties are fairly similar
to those of inbreeding

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Self-fertilization
Self-fertilization is the most extreme form of inbreeding possible, and it is characteristic of
many flowering plants and some hermaphroditic animals, including fresh water snails
and that darling of developmental genetics, Caenorhabditis elegans.
• All progeny of homozygotes are themselves homozygous.
• Half of the progeny of heterozygotes are heterozygous and half are homozygous.
So you might expect that the frequency of heterozygotes would be halved every generation, and
you’d be right. To see why, consider the following mating table:
Offspring genotype
A1A2 ×A1A2 x12
1 1 1
A2A2 ×A2A2 x22 0 0 1
Inbreeding coefficient
Using the same technique we used to derive the Hardy-Weinberg principle, we can calculate
the frequency of the different offspring genotypes from the above table.
Now you’re going to have to stare at this a little longer, but notice that xˆ12is the
frequency of heterozygotes that we observe and 2pq is the frequency of heterozygotes
we’d expect under Hardy-Weinberg in this population if we were able to observe the
genotype and allele frequencies without error. So
xˆ11 = p 2 + σpq/ 2(1 − σ/2
xˆ12 = 2pq – 2(σpq/2(1 − σ/2)
xˆ22 = q 2 + σpq/2(1 − σ/2)
Mating frequency A1A1 A1A2 A2A2
A1A1 ×A1A1 x11 1 0 0

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f is the inbreeding coefficient. When defined as
1-(observed heterozygosity)/(expected heterozygosity). It can be used to measure the
extent to which a particular population departs from Hardy-Weinberg expectations.
When f is defined in this way, I refer to it as the population inbreeding coefficient.
Practical Uses of F
1. Predicting the Effects of Inbreeding Depression
2. Assessing the Risk of Inheriting Genetic Defects
In a large random-mating population, where the frequency of a harmful recessive gene (a) is q, the
proportions of affected individuals and 'carriers' can be estimated from the Hardy-Weinberg Law as
follows:
Aa (carriers) aa (affected)
2q(1 − q) q2
However, if any inbreeding has occurred, Wright's Equilibrium Law enables a further prediction to be
made about the increased risk of inheriting any harmful conditions caused by homozygous recessive genes.
The expected frequency following inbreeding rises to: aa (affected)
q2 + Fq(1 - q)
References
1. http://www.genetic-genealogy.co.uk/Toc115570144.html
2. http://www.uvm.edu/~biology/Classes/271/inbreeding.pdf
3. http://www.ihh.kvl.dk/htm/kc/popgen/genetics/4/2.htm
4. [K E Holsinger. The population genetics of mating system evolution in homosporous
plants. American Fern Journal, pages 153–160, 1990.
5. C Wedekind, T Seebeck, F Bettens, and A J Paepke. MHC-dependent mate preferences
in humans. Proceedings of the Royal Society of London, Series B, 260:245–249, 1995.
6. http://www.uvm.edu/~biology/Classes/271/inbreeding.pdf

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