本期的內(nèi)容是繼承與隔離。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(mén)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 12: Inheritance and Segregation

[1] Overview

Mendel’s pea plant phenotypes

????Mendel observed patterns in PHENOTYPE and inferred basic patterns of?inheritance (with NO knowledge of underlying genetics).

????He simplified -?only examined traits with DISCRETE inheritance (i.e.,?yellow or green, nothing in between)

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Mendel’s Laws: Phenotypic Inheritance?Patterns

-?Assumed a single inherited ‘factor’ stored the information for each?phenotypic trait.

-?Only studied traits with exactly 2 discrete phenotypic categories.

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Mendel’s “Laws”

?Segregation

? ? -?The 2 factors for a trait remain distinct across generations.

????- Each gamete inherits only a single copy of each factor.

?Dominance

????- “Dominant” = one copy of the factor sufficient to produce phenotype.

????- “Recessive” = two copies of the factor required to see phenotype.

?Independent Assortment

????-?Factors for different traits are not inherited together but sort into gametesindependently of each other.

?????Our current understanding encompasses these principles AND explains?more complex patterns ---?traits with variation fully explained by the?simple rules above are RARE in nature

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[2] Segregation

Principle of Segregation

?Remember from meiosis: segregation ensures that each gamete?receives only a single allele for each gene (the pairs of a homologous?chromosome separate during meiosis I).

????- Segregation enables sexual reproduction of a diploid via the fusion of two?haploid cells?--- thus maintaining ploidy across generations.

Segregation during meiosis

?Consider that zygote with 3 chromosomes. There are 4 possible?outcomes of meiosis.

????- Note that chromosomes descended from the same parental chromosome are?inherited independently of each other.

????PAUSE and try to draw all 4 outcomes before moving to the next slide.?(Ignore crossing-over for the purposes of this exercise.)

?

?Consider that zygote with 3 chromosomes. There are 4 possible?outcomes of meiosis.

????- Note that chromosomes descended from the same parental chromosome are?inherited independently of each other.

[3] Nondisjunction

????Violations of segregation: Segregation is known to be violated by two mechanisms:

- Nondisjunction during meiosis I

?????Homologous chromosomes fail to separate so one “haploid” daughter?cell ends up with 2 copies instead of 1 (remember aneuploidy?);

?????Many (but not all) such mutations will be lethal to the gamete.

- Meiotic drive / segregation distortion (quite rare)

?????Heterozygotes, instead of producing 50% of gametes for each allele,?primarily produce gametes with the “driver” or “distorter” allele.

?????Mechanisms remain unknown but a selfish genetic element is suspected.

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PAUSE to practice your chromosome skills

?????Take the parental cell below and draw a diagram of meiosis for this cell.?For each step,?label the ploidy of the nucleus and use appropriate?descriptors for the chromosomes (e.g., homologous pairs, replicated?chromosomes, single chromatids).

?????Next, draw the outcome of meiosis if nondisjunction happens for the?large chromosome.

[4] Meiotic drive

Meiotic drive in the stalk-eyed fly

? ? In some populations of stalk-eyed flies, there is a female-biased sex ratio?and most?broods of offspring are almost completely female. (Hurst and?Pomiankowski 1998, Nature)

? ? ?A “driving” X(d)?chromosome destroys?Y-bearing gametic cells, thus most?offspring are female for males with X(d).

????? Some males have a Y -linked allele that suppresses the?“driving” X(d), reversing direction to produce?male-biased broods.

?????513 genes on the X-chromosomes are expressed?differently in males with X+?versus X(d)?but the genes?causing the drive are not known. (Reinhardt et al. 2014,?PLOS Genetics).