5.4 Non-Mendelian Genetics
Keywords
| English Term | 中文翻译 | Definition & Explanation |
|---|---|---|
| Linked Genes | 连锁基因 | Genes located on the same chromosome that tend to be inherited together. |
| Map Distance | 图距 | The distance between genes on a chromosome, measured in map units (centimorgans). |
| Incomplete Dominance | 不完全显性 | A pattern where the heterozygote phenotype is a blend of the two homozygotes. |
| Codominance | 共显性 | A pattern where both alleles are simultaneously expressed in the heterozygote. |
| Sex-linked Traits | 伴性遗传 | Traits determined by genes located on the sex chromosomes (X or Y). |
| Non-nuclear Inheritance | 非核遗传 | Inheritance of genetic material found in organelles like mitochondria and chloroplasts. |
1. Linked Genes and Genetic Mapping
When genes are located on the same chromosome, they are referred to as genetically linked. These genes do not follow the Law of Independent Assortment because they travel together during meiosis unless separated by crossing over.
- Recombination Frequency: The probability that linked genes segregate (separate) during meiosis.
- Map Units (cM): Used to represent the distance between genes.
- \(1\%\) recombination frequency = \(1\) map unit.
- Quantitative Analysis: If observed phenotypic ratios significantly differ from the predicted \(9:3:3:1\) (for a dihybrid cross), it often indicates linkage.
2. Deviations from Complete Dominance
Mendel's "Dominant vs. Recessive" model is often too simple for complex traits.
| Pattern | Phenotypic Expression | Classic Example |
|---|---|---|
| Incomplete Dominance | A blend of parental traits (e.g., Red + White = Pink). | Snapdragon flowers (Antirrhinum) |
| Codominance | Both traits appear distinctly (e.g., Red + White = Spotted). | AB Blood types in humans |
| Pleiotropy | One single gene affects multiple seemingly unrelated traits. | Sickle-cell anemia |
3. Sex-Linked Traits
Traits determined by genes on the sex chromosomes exhibit unique inheritance patterns.
- X-linked: Much more common in males (\(XY\)) because they only have one X chromosome. A single recessive allele on the X will be expressed.
- Pedigree Analysis: You can often identify sex-linked traits if they appear more frequently in one biological sex or if they skip generations in specific ways.
[Image of an X-linked recessive pedigree]
4. Non-Nuclear (Maternal) Inheritance
Not all DNA is in the nucleus! Chloroplasts and mitochondria contain their own circular DNA and are assorted randomly to daughter cells.
- Maternal Inheritance: In most animals and plants, mitochondria (and chloroplasts in plants) are transmitted via the ovule/egg, not the pollen/sperm.
- Result: Traits determined by mitochondrial or chloroplast DNA are typically inherited from the mother only.
Pro-tip: Evolution Connection
Remember from Unit 2.10 that mitochondria and chloroplasts evolved via endosymbiosis. This explains why they have their own DNA and follow non-Mendelian inheritance patterns.
Quiz
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