Definition: Inbreeding depression refers to the reduced biological fitness in a population due to increased homozygosity, which results in the expression of deleterious recessive alleles.
[Image of self-pollination in plants]Continued Self-Pollination
Increased Homozygosity
(Recessive traits appear)
Reduced Vigor, Lower Yield, Sterility
When a heterozygous plant (Aa) undergoes self-pollination, Heterozygosity ($H_n$) decreases by half every generation.
Where n = Number of generations
| Generation (n) | Heterozygosity (Aa) | Total Homozygosity (AA + aa) |
|---|---|---|
| 0 | 100% (1.00) | 0% |
| 1 | 50% (0.50) | 50% |
| 2 | 25% (0.25) | 75% |
| 3 | 12.5% (0.125) | 87.5% |
| 4 | 6.25% (0.0625) | 93.75% |
| 5 | 3.1% (0.03125) | 96.88% |
The "Bad" alleles (aa) accumulate alongside the "Good" alleles (AA). By the 5th generation, nearly half the crop is genetically compromised.
| Generation (n) | Total Homozygosity | Harmful Recessive (aa) Only |
|---|---|---|
| 0 | 0% | 0% |
| 1 | 50% | 25% |
| 2 | 75% | 37.5% |
| 3 | 87.5% | 43.75% |
| 4 | 93.75% | 46.87% |
| 5 | 96.88% | 48.44% |
Let's verify the "Generation 2" data (from the table above) using the specific calculation from the whiteboard image.
The whiteboard tracks offspring from the F1 parents (1 AA : 2 Aa : 1 aa) assuming a total pool of 16.
| Parent Type | Offspring (AA) | Offspring (Aa) | Offspring (aa) |
|---|---|---|---|
| AA Parents (x4) | 4 | 0 | 0 |
| Aa Parents (x4) | 1 *2 = 2 | 2 *2 = 4 | 1 *2 = 2 |
| aa Parents (x4) | 0 | 0 | 4 |
| TOTAL (16) | 6 | 4 | 6 |
Comparing the calculated numbers to the percentage table: