Final answer:
The two different meanings of horizontal lines in a pedigree are representing family relationships and indicating the passing on of a specific trait.
Step-by-step explanation:
In a pedigree, horizontal lines have two different meanings. The first meaning is that a horizontal line joining a male and a female symbolizes that the couple had offspring. The children are then connected to this line with vertical lines. This represents the family relationships and shows the generations within the pedigree.
The second meaning is that a horizontal line within a circle or square, connecting the parents to their children, represents the passing on of a specific trait being traced. For example, if the trait being traced is a genetic disorder, the horizontal line would indicate that the parents passed on the gene for the disorder to their children. So therefore in a pedigree, horizontal lines have two meanings: representing family relationships and indicating the passing on of a specific trait.