502 - Heredity and Evolution 



IDENTICAL TWINS 

 Are products of 



A single and A single 



sperm / ^ egg 



tj 



In an early stage 

 the embryo divides 



The halves go 

 on to become 

 separate 

 individuals 



Usually — but not always — identical 

 twins share the same placenta and 

 fetal sac 



But regardless of how they develop, 

 they carry the same genes and are 

 therefore 



Always of the same sex — two boys 

 or two girls 



FRATERNAL TWINS 



Are products of TWO different eggs 

 fertilized by TWO different sperms 



They have different genes and may 

 develop in different ways, usually — 

 but not always — having separate 

 placentas and separate fetal sacs 



Also, as they are totally different in- 

 dividuals, they may be 



— or two girls 



—Or a 



mixed 

 pair 



One One 



boy girl 



Fig. 26-24. (From Scheinfeld, You and Heredity. Permission of J. B. Lippincott Co.) 



