Anonymous
Anonymous asked in 社會與文化語言 · 1 decade ago

barr body and color-blindness

(i don't need a translation...)i put up this question in the human biology section.. but no one answered... i mean.. there are a lot of univeristy-leveled students around here.. so i thought i give it a try...so here's my question:normally, when we're talking about color-blindness (protan and deutan), we seem to ignore the presence of the barr body and assume that, in a female carrier, the dominant allele is expressed and not the recessive one. barr body is a condensed X chromosome whose genes are not expressedif that is true, then say a female carrier XXC whose barr body is the dominant X chromosome, leaving the only expressible genes (on the X) to be the recessive color-blindness allelewhy do we not see more female carriers who are color-blinded?orwhy do we ignore the presence of barr body when it comes to color-blindness..?

Update:

that's not my question....

if i didn't know the probabilities to begin with i wouldn't have asked this question...

2 Answers

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  • C C
    Lv 6
    1 decade ago
    Favorite Answer

    okie, so i've answered this before.... here was my answer:

    X chromosome inactivation is random and not imprinted for somatic cells (but they seem to be imprinted for extraembryonic tissue during development, but that's an aside). Therefore every female is essentially a "mosaic" such that some cells will randomly have the wild type allele inactivated, while other cells will randomly have the diseased (color-blind) allele inactivated. Therefore for the most part carrier females are still expressing enough gene products from that particular locus to see color like normal people. But it must be a dosage effect, so I think compare to completely normal male or female, the carrier females will have a decreased sensitivity to color.

    (and just to explain a bit more, barr bodies are just the result of x-chromosome inactivation. After the x-chromosome is inactivated it is condensed into the structure called barr body, where there's also evidence of special types of histone associated with it to cause and maintain its inactive state)

    oh and lastly, just an aside... it's also known that the inactivated X is not completely inactivated... there are still some genes that escape inactivation, despite the chromosome being condensed into a Barr body. So how this is done is an active area of research.

    Source(s): I am a graduate student in genetics
  • Anonymous
    1 decade ago

    這是基因遺傳的問題,因為色盲是屬於隱性基因,而其基因位在X染色體中

    女生的基因是XX,男生則是XY,女生需要兩個色盲基因才能成為色盲

    而男生只要一個就會有色盲。

    舉例來說:(具有色盲基因的染色體為x)

    男孩A有色盲,則母親的基因可能是xx或Xx,而父親可能是XY或xY

    男孩A無色盲,則母親的基因可能是XX或Xx,而父親可能是XY或xY

    女孩A有色盲,則母親的基因可能是xx或Xx,而父親必定是xY

    女孩A無色盲,則母親的基因可能是XX或Xx,而父親可能是XY或xY

    就是這樣,男生色盲機率為6/24,女生是3/24

    **另一種說法

    男生的染色體是XY

    女生的染色體是XX

    如果是X染色體上的缺陷的話~只有一條X的男生~就會表現出來~

    而女生則可以由另一條X來補~所以還可以正常運作~

    假設:

    某個遺傳性疾並在X染色體上的發生機率是10000分之1

    (也就是說此遺傳性疾病在男生的發生率是10000分之1)

    而女生的情況則是...10000 x 10000 = 1億分之1 (兩條X都出問題)

    (也就是說此病在女生發病的機率為1億分之1)

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