What is the Inheritance Pattern of Single and Double Eyelids?

If both parents have single eyelids, the child is generally born with single eyelids. If both parents have double eyelids, the child should generally have double eyelids. However, occasionally, the child may have single eyelids. This may be because although both parents have double eyelids, they both carry the genetic factor for single eyelids. When the genetic factors for single eyelids from both parents combine, the child will have single eyelids. Regarding the genes on the chromosome that determine whether humans have single or double eyelids, we can assume that the letters A and a represent the genetic factors for double eyelids and single eyelids respectively. Because double eyelids are dominant, when A and a are present together, as long as A is present, the phenotype will be double eyelids. Only when there are two a genes will the phenotype be single eyelids.

1. If both parents have double eyelids, the genes can be represented as: AA+AA, meaning both parents carry the dominant gene for double eyelids, and the child's genes will be AA, resulting in double eyelids; AA+Aa, meaning one parent carries the recessive gene for single eyelids, and the child's genes will be AA or Aa, still resulting in double eyelids; Aa+Aa, meaning both parents carry the recessive gene for single eyelids, and the child's genes may be AA, Aa, or aa, meaning the child may have double or single eyelids.

2. If one parent has double eyelids and the other has single eyelids, the genes can be represented as: AA+aa, and the child's genes will be Aa, resulting in double eyelids; Aa+aa, meaning the parent with double eyelids carries the recessive gene for single eyelids, and the child's genes may be Aa or aa, meaning the child may have double or single eyelids.

3. If both parents have single eyelids, the genes can be represented as: aa+aa, meaning both parents do not carry the dominant gene for double eyelids, and the child will only have single eyelids.