All ponies are born with a natural seasonal alignment that doesn't have to be the same as their parents. It's completely random what a child will end up having. The second they are born their seasonal alignment is assigned. Below are the alignments:
Winter: When within a territory where winter is present, they will feel stronger, have an easier time with magic, and healthier. They grow naturally uncomfortable in places ruled by Summer. Winter ponies have a harder time feeling cold, however they get warm quicker than others. Those born under the Winter alignment often give off a colder temperature and smell naturally like pine. Summer: When within a territory where summer is present, they will feel stronger, have an easier time with magic, and healthier. Summer ponies grow naturally uncomfortable when placed in locations ruled by Winter. Those born under the Summer alignment often naturally smell like grass and earthy scents. Those born under the Summer alignment often give off a warmer temperature. Spring: When within a territory where Spring is present, they will feel stronger, have an easier time with magic, and healthier. Those born under the Spring alignment often naturally smell like flowers. Fall: When within a territory where fall is present, they will feel stronger, have an easier time with magic, and healthier. Those born with the Fall alignment often smell like crisp leaves.