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Blood XX, Skin XY, High school

Red blood cells and a few white blood cells under a microscope

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?

A boy whose blood came back female and his skin came back male

Doctors tested a baby boy and found XX chromosomes in his blood cells and XY in his skin cells. His blood came from one egg’s half set of chromosomes, copied twice, while his skin had DNA from both parents. Scientists call this a parthenogenetic chimera.

A chimeric lab mouse with patchy fur next to two of its babies
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.

Your word of the day is

par-theh-no-jeh-NET-ik ky-MEER-uh

partheno-maiden, virginfrom Greek parthénos
-geneticorigin, birthfrom Greek génesis
chimaerashe-goatfrom Greek chímaira

A body made of two cell lines, where one line grew from an egg with no DNA from a father. Also spelled chimera.

Rows of blood test tubes with colored caps in a lab rack

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?

A young woman swabs the inside of her cheek for a DNA test

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?

Human chromosomes, with the XX and XY pairs shown

The boy’s body held an XX cell line and an XY cell line.

Why did the boy’s blood test miss his XY cells?

Animation of red blood cells tumbling and flowing through a blood vessel

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.

Which result from the boy’s blood rules out a hidden Y piece, and how?

Scientist in gloves using a pipette to move liquid samples inside a lab safety cabinet

A hidden Y piece on one X would leave every cell in the boy’s body XX.

Which result from the boy’s skin rules out a hidden Y piece, and how?

Scientist in a lab adjusting a microscope and looking through the eyepieces

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?

Animation of a glowing blue DNA double helix twisting on a black background

A mom-only cell line carries DNA from the mother alone.

Which test would find more people with a mom-only cell line?

Close-up of a gloved medical worker handling tubes of blood samples in a lab

Claim, evidence, reasoning

More than 98 out of 100 of the boy’s blood cells came from the mom-only line, the cells with DNA from his mom alone. Fewer than 5 out of 100 of his skin cells did.

Which claim best explains how the boy’s body grew from one egg, with mom-only cells in most of his blood but few of his skin cells?

Stained human blood smear under a microscope showing many pale pink red blood cells and a few larger purple-stained white blood cells

Claim, evidence, reasoning

Which evidence best backs the claim that one egg split early into a mom-only line and a two-parent line?

Scanning electron microscope image of human blood showing disc-shaped red blood cells, a round bumpy white blood cell and small platelets

Claim, evidence, reasoning

Which reasoning best links the boy’s marker results to an early split in one egg?

Rack of capped glass test tubes filled with dark red blood samples in a hospital lab

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.

Which question would best help explain why the boy’s two-parent cells were important?

Healthcare worker in white gloves drawing blood from a person's arm with a needle and collection tube

Which evidence would show which of the boy’s tissues need genes that switch on only in dad’s copy?

Scientist in a lab coat and gloves pipetting genetic samples into small tubes to set up a DNA copying test called PCR

Scientists think the boy’s body grew from one egg, yet his blood and skin carried different DNA.

Starting from one egg, how could mitosis and differentiation give the boy mom-only blood cells and two-parent skin cells?

Human male karyotype: 22 numbered pairs of banded chromosomes lined up in order, plus one X and one smaller Y chromosome

Lesson complete

Today’s word

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.

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Sources
  1. Strain and others, Nature Genetics, 1995 (free abstract)
  2. Yamazawa and others, Journal of Medical Genetics, 2010
  3. Masunaga and others, Clinical Epigenetics, 2021
  4. “Scientists: Boy came from egg that didn’t wait for sperm,” Tampa Bay Times, September 30, 1995
  5. MedlinePlus Genetics: 46,XX testicular difference of sex development, Klinefelter syndrome, Turner syndrome, genomic imprinting and uniparental disomy
  6. Vision and Change core concepts and competencies, as summarized in Brownell and others, CBE Life Sciences Education, 2014
  7. Word origins from Dictionary.com: chimera, chimaera, chimerical, genetic, chromosome, chromo-, chromatic, -some, ribosome
  8. Mark Cartwright, “Chimera of Arezzo,” World History Encyclopedia
  9. Spanish and French meanings from Wiktionary: quimera, chimère