Determination of Sex and Identical vs. Fraternal Twins

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  • 0:07 Boy, Girl or Twins
  • 1:00 Sex Determination
  • 2:26 Twins
  • 3:39 Lesson Summary
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Lesson Transcript
Instructor: Lisa Roundy

Lisa has taught at all levels from kindergarten to college and has a master's degree in human relations.

Have you ever heard an old wives' tale about what determines whether a child will be a boy or a girl or what causes twins to be born? In this lesson, we will explore the biology behind these myths on how sex is determined and how twins are created.

A Boy, a Girl or Twins

Bob and Tracey are excited because they're planning to start a family. While they know the basics about how to make a baby, they have heard lots of rumors about what they can do to increase the chances of having a baby boy or having a baby girl. For instance, Tracey was told she would have a greater chance of having a boy if she eats only spicy foods while she's trying to get pregnant. They have also heard rumors about how to conceive twins. They are curious about what really determines the sex of their child. What about twins?

In order to answer these questions, we have to look at what happens when a child is conceived. Conception occurs when the father's sperm fertilizes the mother's egg. Whether or not the child will be a boy or a girl, or whether twins will develop, depends on the details surrounding this event. The term zygosity refers to the characteristics of the cell union during conception.

Sex Determination

First, let's look more closely at the determination of the child's sex. A child will inherit 23 chromosomes from its mother and 23 from its father to create 46 total chromosomes that occur in pairs. So, when the egg and the sperm join, the 23 pairs of chromosomes match up to determine the genetic makeup of the child.

The last, or 23rd, pair of chromosomes are known as the sex chromosomes because they will determine the sex of the child. The other 22 pairs of chromosomes do not have any influence on the sex of the child. If the child inherits two X chromosomes (XX), she will be female. If the child inherits an X and a Y sex chromosome (XY), he will be a male.

The mother's egg will always provide the child with an X sex chromosome because the mother has no Y chromosome to pass down. The father is male, so he has both an X sex chromosome and a Y sex chromosome that can be passed down. This means that the sex of the child is determined by the sex chromosome provided from the father's sperm. About half of the sperm will contain the X sex chromosome, and the others will possess the Y sex chromosome. In other words, if Bob and Tracey conceive a child, the sex will depend on which sex chromosome was present in the sperm that fertilizes Tracey's egg, no matter how much spicy food she eats!

Identical and Fraternal Twins

Now let's look at what could cause Tracey to become pregnant with twins. There are two types of twins that Tracey could conceive: identical and fraternal. Tracey could have identical twins, or monozygotic twins, if one of her eggs is fertilized and then splits into two genetically identical parts before the embryo starts to develop.

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