遗传 进化与生态学 13 - Principle of Dominance
本期的内容是显性原则。本文集的这一部分是遗传、进化与生态学 Genetics, Evolution, and Ecology. 这门课理论上建议在阅读完文集的第一部分的内容之后再开始学习，但基础不足的朋友也可以尝试阅读喔~
这一部分的主要内容均来自 Prof. Angela J. Roles 的 BIOL 200 课程，因此本文集的这一部分均不会标记为原创。但由于文本来源不清晰，UP主还是一个字一个字码出来的文章，本文禁止非授权的转载，谢谢！
Lesson 13: Principle of Dominance
[1] Dominance & Allelic relationships
Principle of Dominance: relating genotype to phenotype
- Observed phenotypic pattern
▸Flower color = purple (AA, Aa) or white (aa).
-Homozygous dominant genotype (AA) and the heterozygote (Aa) have purple flowers
- Homozygous recessive genotype (aa) has white flowers
Note: Phenotypes are NOT dominant or recessive, homozygous or heterozygous.
- Actual genetic mechanism
▸The “dominant” allele (A) encodes and expresses functional protein. The phenotype of AA results from 2 expressed, functional copies of this allele.
▸The “recessive” allele (a) does not produce functional protein for this gene. The phenotype of aa is what happens with no protein produced for that gene.
- Amino-acid mutations may cause loss of function.
- Regulatory region mutations may prevent transcription of a functional copy of the gene.
▸The heterozygote has one A and one a allele. If Aa has the same phenotype as AA, then the A allele is haplo-sufficient: one copy produces enough protein to yield the same phenotype as 2 copies.
Allelic relationships
If not dominance, then what?
- How do we know if a phenotype DOESN’T show a dominance pattern?
(1) More than 2 phenotypes are present: Heterozygotes do not resemble either homozygote (for example, ABO blood types);
(2) Phenotypes can’t be categorized into distinct classes (i.e., they are continuous like height);
(3) Offspring phenotype depends on more than just parental phenotypes.
[2] Non-dominant allelic relationships
▸Note 1: Underlying inheritance is not different—individuals still have 2 copies of all autosomes.
▸Note 2: Dominant/recessive does NOT imply that one allele or phenotype is “better” than another. Nor that one allele “masks” the other one.
- Recessive alleles often represent loss-of-function mutations (no protein or a non-functional protein is produced).
- Dominant alleles represent cases of haplo-sufficiency (one allele is sufficient to produce enough functional protein for the full phenotype).
More than 2 possible categorical phenotypes
(How does dominance work at cellular level?)
▸Partial or incomplete dominance
- The heterozygote’s phenotype is in-between the phenotypes of the homozygotes.
- Haplo-insufficiency: 1 functional allele doesn’t produce enough protein to achieve the “full” homozygous phenotype.
▸Codominance:
- Heterozygote phenotype shows both homozygotes’ phenotypes at once;
- Both alleles encode functional proteins (differ slightly in function);
- Represents a special case of incomplete dominance.
▸Overdominance:
- Heterozygote’s phenotype is more extreme than either homozygotes’
- No general mechanism, usually involves natural selection favoring the heterozygote.
Note: Allelic relationships depend not on the gene or alleles but on how you define the phenotype.
[3] Multiple alleles & Trails
Schematic of cell surface antigens produced by individuals with each blood type:
STOP: Identify the inheritance pattern for each allele pair.
Polygenic traits: multiple genes contribute to a single phenotype
Phenotype varies continuously due to the additive effects of many genes.
Example: human skin pigmentation varies continuously. At least 100 genes are implicated in pigment production in humans.
Imagine pigment defined by 3 genes each with 2 alleles
▸For 3 genes, each with 2 alleles, you get 64 possible unique genotypes;
▸If each + allele contributes the same amount of pigment, that gives you 7 distinct color phenotypes - some genotypes produce the same shade of color;
▸With this simple scenario (and no environmental influence), you already get much closer to a continuous distribution of color.
Phenotypic plasticity (AKA polyphenism)
Phenotypic plasticity: One genotype can develop different phenotypes, depending on environment.
