本期的內(nèi)容是顯性原則。本文集的這一部分是遺傳、進(jìn)化與生態(tài)學(xué) Genetics, Evolution, and Ecology. 這門課理論上建議在閱讀完文集的第一部分的內(nèi)容之后再開(kāi)始學(xué)習(xí)，但基礎(chǔ)不足的朋友也可以嘗試閱讀喔~

這一部分的主要內(nèi)容均來(lái)自 Prof. Angela J. Roles 的 BIOL 200 課程，因此本文集的這一部分均不會(huì)標(biāo)記為原創(chuàng)。但由于文本來(lái)源不清晰，UP主還是一個(gè)字一個(gè)字碼出來(lái)的文章，本文禁止非授權(quán)的轉(zhuǎn)載，謝謝！

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.

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[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.

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[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.