Question

What are monozygotic embryos? What are dizygotic embryos? How many babies can a mother in a single pregnancy have? Explain your answer.

Answer 1

Monozygotic twins are twins that are identical. They are derived from splitting of a single fertilized zygote to form two separate embryos. Such embryos that are obtained from a single fertilized egg are known as monozygotic embryos. They will have identical genetic material as a single sperm has fertilized a single egg. However, there may be epigenetic changes accounting for difference which are influenced by the maternal environment.

Dizygotic twins are twins obtained from two different embryos that implant at the same time in the uterus. The dizygotic embryos are embryos that are obtained by fertilization of two separate egg by different eggs at the same time. These embryos will implant in the uterus at same time and will give rise to live births in a multiple pregnancy. The genetic material differs in both embryos as different eggs are fertilized. Further, environmental changes will also account for epigenetic differences in these twins as well. Hence, such twins are non-identical.

A single pregnancy is a pregnancy involving on embryo/fetus. Hence, there is only one fertilized embryo/baby that is carried to term by the woman in a single pregnancy. Single indicates one and thus single pregnancy occurs when a single fertilized egg will implant in the uterus of the women and develop to term. Most human pregnancies are single pregnancies as one ovary usually produces one egg in single menstrual cycle. It is quite uncommon for both ovaries to release one egg each in a menstrual cycle during ovulation.

Conversely, multiple pregnancies will result in multiple births as more than one embryo will be developed in the uterus at the same time. Twins are examples of multiple pregnancies. The maximum number of multiple births known in humans is 8 (octuplets). They are rare and mostly a result of implanting multiple embryos in the uterus during in vitro fertilization procedures.