Video Transcript
What is a characteristic of
monozygotic twins? (A) They are not genetically
identical. (B) They are always the same
sex. (C) They develop with the same
umbilical cord. (D) They are formed from one egg
and two sperm.
To answer this question, we need to
recall what the term “monozygotic” means and how it describes twins.
First, let’s break down the term
“monozygotic.” It contains the prefix mono-, which
means one, and the root word “zygotic,” which refers to a zygote. A zygote is formed when a sperm
cell fertilizes a mature egg cell. In most cases, this zygote will
divide via mitosis, increasing the number of cells until it becomes an embryo. This mass of cells will then
implant in the uterus where it will further develop into a fetus.
However, in rare cases, a single
zygote can split early in development, producing two embryos. This will cause the development of
two genetically identical offspring, since these cells have not undergone
differentiation. Since both embryos result from the
same single sperm and egg pair, each fetus will possess the same genetic
information. This means that each fetus will
contain the same sex chromosomes, either two X chromosomes, resulting in female
offspring, or one X and one Y chromosome, resulting in males.
These fetuses will also have the
same placenta in most cases. The placenta is the structure that
provides the fetus with the support and nourishment they require to develop. Even though the fetuses are in the
same placenta, they’re connected to the shared placenta by distinct umbilical
cords. The umbilical cord is the structure
that contains blood vessels that connect the developing fetus to the placenta.
After reviewing how monozygotic
twins are formed, we can eliminate the answers choices we know are incorrect. For instance, we just recalled that
the phrase “monozygotic twins” means twins that result from one zygote, which means
they are genetically identical, they develop from the fusion of one sperm and one
egg cell, and while they share one placenta, they’re attached to the placenta
through two distinct umbilical cords. Moreover, since monozygotic twins
develop from one zygote and are genetically identical, this means that they’re
always the same sex.
Now we have all the information
that will help us answer our question correctly. The characteristic that describes
monozygotic twins is that they are always the same sex.
