First generation are the only ones that are actually "sex-linked", i.e. the sex is linked to the gender. It's how you guarantee you're getting all females.
When you breed sex links together, for a second generation, you get a mix of red and white birds and you lose the connection between color and gender. Birds of both genders will be various shades of red and white. They still lay quite well, but you lose the ability to sex them at hatch by sight.