How does a Freemartin calf occur?

Blood cell chimerism is particularly significant when it involves heterosexual twins (i.e. male and female). The female fetus will thus have blood cells with XY chromosomes derived from the male in addition to her own blood cells with XX chromosomes. The Y-chromosomes suppress the development of the female genital system, resulting in a sterile heifer known as a freemartin. Freemartins develop in about 85 % of heterosexual twin pregnancies.