6R genetics question #192

[kimijohni]

Here is the question:
If all genotypes are equally fit and if there are no genetic modifiers of the sex trait ration, what will be the ultimate fate of a population in which 50% of the X chromosmes are Xi and 50% are Xs?


Passage information:
XsY in male is standard; he sires equal numbers of sons and daighters.
XiY male expresses the sex ratio trait: he sires only daughters.


The answer is extinction. I dont understand the question, what does it mean that there are no genetic modifiers of the sex ratio trait? Can someone please clear this question up for me.

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[NapeSpikes]

No genetic modifiers just means the genes won't change. As I recall, the Xi gene causes lizards to produce only girls. The lizards become extinct because all the Xi male lizards pass on their bad X gene to all of their offspring (all girls), while the healthy male lizards only pass their good X genes to half their offspring (they pass on the Y to the boys). The result is a greater and greater percentage of lizards will be girls after each generation, resulting in eventual extinction.

Sorry for the crappy phrasing, but I don't have the test in front of me.