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Blood XX, Skin XY

Middle school learnscience.org

Name Date

Predict

Chromosomes are tiny threads inside your cells that carry genes. A one-year-old boy’s blood test shows two X chromosomes (XX), a pattern doctors usually see in girls. Could a test on another part of the boy’s body give a different answer than the blood test?

Circle one. Yes / No

Why?

Watch

  1. A boy whose blood came back female and his skin came back maleyoutube.com/shorts/rYCzxUn_5s0

Word of the day

Parthenogenetic chimaera (par-theh-no-jeh-NET-ik ky-MEER-uh). A body made of two cell lines, where one line grew from an egg with no DNA from a father. Also spelled chimera.

Word partComes fromMeaning
partheno-Greek parthénosmaiden, virgin
-geneticGreek génesisorigin, birth
chimaeraGreek chímairashe-goat, and a monster in Greek myth
  • The Chimera of Arezzo, a bronze statue the Etruscans made about 2,400 years ago, shows the monster as a lion with a goat’s head on its back and a snake for a tail.
  • In Spanish, quimera can mean a pipe dream, something impossible to achieve.

Questions

  1. Most people get one copy of each chromosome from mom and one from dad. The boy’s blood cells had two matching copies from his mom, and his skin cells had one copy from each parent. Which of the boy’s cells have more genetic variety, meaning more genes with two different versions?

    Circle one. His blood cells / His skin cells

    Hint. Two matching copies carry the same version of every gene. Where could a second version of a gene come from?

  2. Doctors often check a person’s chromosomes with a blood sample. The boy’s blood cells tested XX, but his skin cells tested XY. What can doctors miss when they test only one part of the body?

    Hint. What would doctors have known if they’d tested only the boy’s blood?

  3. Some boys have cells with two X chromosomes. Most of these boys carry a small piece of the Y chromosome stuck on one X, and that piece almost always comes from their dad. Which two test results showed that a hidden Y piece didn’t cause the boy’s XX blood?

    Hint. Look for one result about his blood and one result about his skin.

  4. As of 2021, scientists had reported six people with mom-only cells, which carry DNA from a person’s mom and none from their dad. Why might doctors have trouble finding people with mom-only cells?

    Hint. Think about how often people get a chromosome test. Which part of the body do doctors usually test?

Exit ticket

The tests found different DNA in the boy’s blood and skin. How can the boy have blood cells with DNA from only his mom and skin cells with DNA from both of his parents?

Hint. His body grew from one egg. Did dad’s DNA join every early cell, or only some of them?

Sources