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Inbreeding Depression in Plants
Inbreeding depression refers to the reduced biological fitness in a population due to increased homozygosity, which results in the expression of deleterious recessive alleles. This leads to reduced vigor, fertility, and adaptability.
How Self-Pollination Leads to Inbreeding Depression
- Self-pollination increases homozygosity over generations.
- Recessive alleles, normally hidden in heterozygous conditions, become homozygous and express harmful traits.
- Results in reduced vigor, smaller plants, lower yield, weaker immunity, and sterility.
Effect of Selfing on Genetic Composition
Punnett Square (First Generation)
| Parental Gametes | A | a |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
Heterozygosity drops from 100% to 50% in the first generation, while homozygosity increases.
Inbreeding Depression
Inbreeding depression refers to the reduced biological fitness in a population due to increased homozygosity, which results in the expression of deleterious recessive alleles.
Step 1: Understanding Homozygosity Increase Over Generations
When a heterozygous plant (Aa) undergoes self-pollination, the proportion of homozygous individuals (AA and aa) increases in each generation while heterozygosity decreases.
Hn = H0 × (1/2)n
Where:
- Hn = Heterozygosity after n generations
- H0 = 1.00 (initial heterozygosity)
- n = Number of generations
Step 2: Calculate Homozygosity Increase Over 5 Generations
Homozygosity is given by:
Homozygosity = 1 – Hn
Homozygosity Increase Over Generations
| Generation (n) | Heterozygosity (Aa) Hn | Homozygosity (AA + aa) |
|---|---|---|
| 0 | 100% (1.00) | 0% (0.00) |
| 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% |
Step 3: Tracking the Recessive Homozygous (aa) Population
The formula for homozygous recessive (aa) population is:
Recessive (aa) = (1 – Hn) / 2
| Generation (n) | Total Homozygosity (AA + aa) | Homozygous Recessive (aa) |
|---|---|---|
| 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% |
By the 5th generation, nearly 48.4% of the population is homozygous recessive (aa), causing severe inbreeding depression and yield reduction.
Assertion and Reason Based Question
Assertion (A): Continuous self-pollination in plants may cause inbreeding depression.
Reason (R): Self-pollination increases homozygosity, leading to the expression of deleterious recessive alleles, reducing vigor, fertility, and adaptability.
Options:
- A) Both A and R are true, and R is the correct explanation of A.
- B) Both A and R are true, but R is not the correct explanation of A.
- C) A is true, but R is false.
- D) A is false, but R is true.
Correct Answer:
A) Both A and R are true, and R is the correct explanation of A.
