Why is lymphatic drainage important in the mammary gland?

Lymphatic drainage serves as pathway for metastasis of malignant tumor cells.

Explanation:

Malignant tumor cells have the ability to travel from one body part to others and cause cancer in various body parts. The ability of tumor cells to migrate from one body part to others is known as metastasis. The lymphatic system serves as the most common pathway for metastasis of malignant tumor cells.

Therefore, the presence of cancer cells in mammary drainage serves in the diagnosis of cancer. Tumor cells are not present in the lymphatic drainage of mammary glands in case of localized breast cancer. However, the presence of cancer cells in the lymphatic drainage of mammary glands marks their distant metastasis.

the father in the first generation probably had the recessive colorblind trait in his x chromosome. if he and his wife both had the colorblind trait, even if she had 1 recessive and 1 dominant, then that trait would be passed on to their kids.

(since you didn't attach the family tree this is just an assumption)