In astrology mars, venus, mercury, Jupiter and Saturn are the planets ruling over dual signs. Out of these venus, mercury and Jupiter are natural benefics.
According to rishi parashar, Planets haveing dual signs and are natural benificial , ruling over Kendra houses, looses its energy or the benificial effects in there Mahadashas and anterdashas.
1st, 4th, 7th and 10th houses are Kendra houses and the lord of these houses are termed as kendradhipati, whereas 1st house is known as Trikona house . So the lordship of the 1st house is exempt from kendradhipati dosha as its kind of Rajyoga too.
Jupiter and Mercury are the natural benific planets and they loose there good energy or benific effect in ascendant of Gemini ,Virgo , Sagittarius , Pisces .
In Gemini ascendant - Jupiter rules over Sagittarius in 7th house and Pisces in 10th house.
Virgo ascendant - Jupiter rules over Sagittarius in 4th house and Pisces in 7th house .
Sagittarius ascendant - Mercury rules over 7th house of Gemini and 10th house of Virgo .
Pisces ascendant -Mercury rules over 4th house of Gemini and 7th house of Virgo .
So for Gemini and Virgo ascendant Jupiter is not benific if it is in Kendra house .
and for Sagittarius and Pisces ascendant Mercury is not benific if it is in Kendra house.
This Dosha get activated in Mahadashas or antardasha.