A freemartin is a genetically female
calf that is born with
genitalia resembling those of a male. The freemartin is, naturally,
sterile, and by extension she will not
lactate.
This
birth defect occurs only in the case of male/female
fraternal twins. In a small percent of such pregnancies, the blood vessels of the two
embryos will overlap, so that
androgens (male
sex hormones) produced by the male calf to trigger the development of his
reproductive tract,
diffuse into the blood supply of his sister, and influence the development of hers. Because the male reproductive organs differentiate before the female reproductive organs do, his androgens get to her before her
estrogens might get to him.
...More than you thought you wanted to know from your friendly neighborhood Animal Science major.