It may also be linked to the increase of growth hormones in food. Keith; Barbara Luke eds. Related Articles. Follow Us.
Monoamniotic twins occur when the split takes place after the ninth day after fertilization. Monozygotic twins can develop differently, due to their genes being differently activated. Second example: There could be a glitch in copying sex chromosomes during the formation of sperm or egg cells, resulting in a male zygote that starts out with an extra X chromosome XXY.
Office on Women's Health, U. To create a fetus you need one egg and one sperm. More in Ages and Stages. Occasionally they look quite similar.
It's unclear how many other semi-identical, or "sesquizygotic," twins are out there, but it's likely "extraordinarily rare," said lead author Dr. Healthy Living. The embryo that is formed from the cells missing the Y chromosome would develop into a female XO.
Sometimes the parasitic twin becomes an almost indistinguishable part of the other, and sometimes this needs to be treated medically. Genetically Dissimilar. Twins are two offspring produced by the same pregnancy.