VAN 211: Veterinary Histology and Embryology (2+2)

TWINS AND TWINNING

• Twins are either identical twins or fraternal twins. In uniparous animals usually only one ovum is fertilized at a time. If two individuals are derived from a single fertilized egg, each member acquires the same chromosomal heritage and hence the similarity between the twins is so striking in physical, mental and other characters. They are identical, true or duplicate twins. They are always of the same sex.
• Two or more individuals, derived due to independent ripening of two or more number of ova and fertilization, have the same, degree of family resemblance, as between brothers and sisters of different ages. They can be of the same or opposite sexes. They are fraternal twins. In animals, e.g. the cow there is identical twinning.
• Also in bovines, a condition called Freemartin is recorded, which is the female fraternal twin of a male. It is sterile, as the internal genital organs are of the male type and the external genital organs are of the female type. This is caused by placental anastomosis and the influence of the male hormone which is earlier to appear in circulation.