Blood XX, Skin XY, Middle 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

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?

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

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?

Human chromosomes, with the XX and XY pairs shown

Most XX boys carry a small piece of Y on one X, and that piece almost always comes from their dad.

Which blood result shows that a hidden Y piece didn’t cause the boy’s XX blood?

With a hidden Y piece, all of the boy’s cells would test XX.

Which skin result shows that a hidden Y piece didn’t cause the boy’s XX blood?

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?

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 parents?

Lesson complete

Today’s word

parthenogenetic chimaera