Blood XX, Skin XY

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.

• 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.

Bonus word from the video. Chromosome (KROH-muh-sohm). Chromo- comes from Greek chrōma, “color,” and -some from Greek sôma, “body.” Chromosomes are the threadlike bodies that carry a cell’s genes, named for the way they soak up dye.

Questions

1. The boy’s blood cells carried one half-set of chromosomes from his mom, copied twice. His skin cells carried one half-set from each parent. Which of the boy’s two cell lines has more genetic variety, and why?

   Hint. Think about where each copy of a chromosome came from in each cell line.

2. A blood test read the boy as XX. A skin test read him as XY. How can the boy’s blood test read XX while his skin test reads XY? And why did the blood test miss his XY cells?

   Hint. Could different tissues in one body come from different groups of cells?

3. Most boys with XX cells carry a small piece of the Y chromosome on one X. That piece almost always comes from the father’s sperm. Lisa Strain and her colleagues reported two results in 1995 that rule out a hidden Y piece in the boy. What are the two results, and how does each one rule out that cause?

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

4. As of 2021, scientists had reported six people with a mom-only cell line, carrying DNA from the mother alone. Genetic tests usually start with blood, yet in one of the six cases, none of the white blood cells tested came from the mom-only line. How could a mom-only cell line go unnoticed? And what test would find more people with mom-only cell lines?

   Hint. Think about which tissue doctors test first and which other tissues could hold a mom-only line.

Claim, evidence, reasoning

Going further: genes from dad

1. Mouse embryos made from the mother’s DNA alone don’t survive past early development. Some genes switch on only in the copy that came from the father. Yet some mice with a mix of mom-only cells and two-parent cells survive. What two questions would help explain why the boy’s two-parent cells were important? Pick one of your questions and name the evidence a scientist would need to answer that question.

   Hint. Think about what genes from dad add. Which parts of the body might need those genes most?

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