图11显示太平洋某部分岛屿上几种鸟类的分布及迁徙情况。图12显示其中的S鸟不同种群的等位基因频率与代数的关系，其中n代表种群的个体数。
S鸟有黑羽（AA）、杂羽（Aa）、灰羽（aa）三种表现型，当S鸟迁至Y岛后，在随机交配产生的后代中统计发现灰羽个体只占1%，Y岛S鸟种群中A基因的频率为（）。
估算Y岛S鸟密度的调查方法一般采用（）。
参考答案：0.9；标记重捕法
A gap year (also known as year abroad, year out, year off, deferred year, bridging year, time off and time out) is a year during which students take time off and do something other than schooling, such as travel or work. The gap year is most commonly taken after secondary school and before starting university. The practice of taking a gap year developed in the United Kingdom in the 1960s. During a gap year, a student might travel, engage in volunteer work overseas or undertake a working holiday abroad. In 1978, the Prince of Wales and Colonel John Blashford-Snell began what is now known as Raleigh International by launching "Operation Drake," a gap year expedition voyage around the world following Sir Francis Drake's route. The Center for Intermin Programs in the United States was originally founded in 1980 to assist students in making a more effective transition from high school to college. The gap year has grown very popular among students in the UK, Australia, New Zealand and Canada. A trend for gap years is to participate in international education programs that combine language study, homestay, cultural immersion, community service, and independent study. The high pressure increasingly leaves students of high school tired out and longing for refreshment. Also, counselors are coming to bless the gap-year option, and colleges increasingly are offering a delayed enrollment option as more and more “gappers” arrive on campus with enhanced motivation and maturity—all of which prepares well foe their undergraduate years in college. In 2010, gap year travel has increased among school, college and university leavers, as this is seen as an attractive option for future career development.
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