They think the second spike is due to the 1986 Chernobyl disaster in which the reactor exploded in the Ukraine(乌克兰)．

　　The effects of Chernobyl were felt locally and no effect was seen in the U．S., probably because it was too far from the disaster to have an effect.

　　“The closer the country was to Chernobyl, the stronger the effect，” said study co­author Hagen Scherb, a biostatistician(生物统计学家) at the German Research Center for Environmental Health in Munich.

　　More males were born relative to females in Belarus—the Ukraine's neighbour—than in France.

　　The study is based largely on Cold War­era statistics, but the findings are highly relevant for how gender could be affected after future nuclear disasters.

　　And in the wake of Japan's Fukushima Daiichi nuclear power plant accident experts are predicting another baby boy boom could come, especially on the U．S. West Coast.

　　Previous radiation experiments on animals may give a clue for the increase in male births. Tests showed that radiation caused damage to the X chromosome(染色体) in sperm, Dr Scherb said.

　　A human sperm cell contains either an X or Y chromosome, while an egg only has an X chromosome. An XY combination will become a boy, while an XX combination will be a girl.

　　31．How many nuclear radiation accidents are mentioned in the passage?

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