U.S. ambassadors have been historically male: less than ten percent of the 4,600 diplomats have been women. Ratios have improved over time, but a significant number of countries the United States have relations with have never employed a female ambassador, including Germany, Canada, China, and Russia.

Source: http://www.forbes.com/sites/niallmccarthy/2016/07/25/the-countries-where-u-s-women-have-served-as-ambassadors-infographic/?utm_source=FBPAGE&utm_medium=social&utm_content=526263844&utm_campaign=sprinklrWomen@Forbes%20-%20womenatforbes#481ee7107728

Image Source: By lagunalife4 (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons