Post-zygotic mechanisms prevent hybrid offspring from reproducing. Hybrid sterility, such as in mules (a horse-donkey hybrid), occurs when hybrids are viable but cannot reproduce due to chromosomal differences.
| List I (Genetic Disorders) | List II (Genetic Alteration) | ||
| A | Down syndrome | I | Gain of X-chromosome |
| B | Klinefelter's syndrome | II | Loss of X-chromosome |
| C | Turner's syndrome | III | Trisomy of chromosome 21 |
| D | Cri du chat syndrome | IV | Deletion of small terminal portion of chromosome 5 |
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