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inbreeding genetics university exam answer inbreeding coefficient effects

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I now have enough information from the textbooks to write a thorough university-level answer with a diagram. Here it is:

Inbreeding (Consanguinity) - University Answer

Definition

Inbreeding (also called consanguinity) is the mating between two individuals who are related by common ancestry - i.e., they share one or more common ancestors. In genetics, inbreeding increases the probability that an individual inherits two identical copies of an allele that are identical by descent (IBD) from a common ancestor.

Inbreeding Coefficient (F)

The coefficient of inbreeding (F) is a key measure defined as:

F = the probability that two alleles at any given locus in an individual are identical by descent (IBD)

F ranges from 0 (no inbreeding) to 1 (completely inbred)
For first cousins: F = 1/16 (6.25%)
For sibling mating: F = 1/4 (25%)
For uncle-niece / aunt-nephew: F = 1/8 (12.5%)

Simple Diagram: First-Cousin Mating

        [I-1] ─────── [I-2]       (Common Grandparents)
              │
     ┌────────┴────────┐
   [II-1]           [II-2]        (Siblings: Uncle & Aunt)
     │                 │
   [III-1]         [III-2]        (First Cousins)
     └────────┬────────┘
           [IV-1]                 (Offspring of first cousins)
              F = 1/16

I-1 and I-2 are the shared common ancestors
II-1 and II-2 are their children (siblings)
III-1 and III-2 are first cousins who mate
IV-1 is the inbred offspring with F = 1/16

How Inbreeding Increases Homozygosity

Inbreeding does not create new harmful alleles - it unmasks existing recessive alleles that are present in the common ancestor.

Example:

Common ancestor carries a rare recessive allele "a" (carrier: Aa)
This allele is passed down through both lines of the pedigree
When two cousins who are both carriers (Aa) mate, their offspring has a 1/4 chance of being aa (homozygous affected)

  Carrier ancestor (Aa)
         │
    ┌────┴────┐
  Aa (carrier)  Aa (carrier)
    │              │
 Aa (cousin)   Aa (cousin)
    └──────┬──────┘
     1/4 chance → aa (AFFECTED)

Genetic Consequences of Inbreeding

Effect	Explanation
Increased homozygosity	More loci become homozygous (both alleles identical)
Inbreeding depression	Reduced biological fitness - lower survival, fertility, and health
Unmasking of recessive diseases	Rare autosomal recessive conditions appear more frequently
Reduced heterozygosity	Less genetic diversity in the population
Increased disease risk	Risk for any abnormality is ~doubled in offspring of first cousins vs. unrelated parents

Effects on Autosomal Recessive Disease

This is the most important clinical consequence. Inbreeding dramatically increases the frequency of autosomal recessive conditions such as:

Cystic fibrosis
Phenylketonuria (PKU)
Tay-Sachs disease
Congenital deafness
Metabolic enzyme deficiencies

In first-cousin offspring, the risk for any congenital abnormality is approximately double the baseline risk (baseline is ~15-20 per 1000 live births for any child).

Inbreeding and Hardy-Weinberg Equilibrium

Inbreeding violates the Hardy-Weinberg principle because:

H-W requires random mating (panmixia)
Inbreeding = non-random mating (preferential mating between relatives)
Result: Observed heterozygote frequency is lower than expected under H-W

Key Formulas

Relationship	Coefficient of Kinship (Φ)	Inbreeding Coefficient of Offspring (F)
First cousins	1/16	1/16
Double first cousins	1/8	1/8
Uncle-niece	1/8	1/8
Siblings	1/4	1/4
Parent-offspring	1/4	1/4

Summary Points (for exam)

Inbreeding = mating between relatives sharing a common ancestor
Measured by the inbreeding coefficient F (probability of alleles being identical by descent)
First-cousin mating gives F = 1/16
Main danger: increases frequency of autosomal recessive disorders
Does NOT create new mutations - just increases homozygosity for existing alleles
Violates the Hardy-Weinberg assumption of random mating
Leads to inbreeding depression - reduced fitness of the population

Sources: Thompson & Thompson Genetics and Genomics in Medicine (9th ed.); Emery's Elements of Medical Genetics and Genomics

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