Body Parts: Double-Jointedness, Hitchhiker’s Thumb, and More

Written by Buffy Silverman

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Hitchhiker’s-thumb? What on earth is that? Are people born with such a thumb?  Yes, indeed they are. Thumbs are straight or bent. A thumb that is bent is called a hitchhiker’s thumb.

“What makes people different are their traits.” Young readers learn that all humans share traits that distinguish them from animals: we all walk with two legs, talk and sing, but each person has his or her own traits.

Body Parts: Double-Jointedness, Hitchhiker’s Thumb, and More is a simple introduction to genetics. “Our bodies have directions for our traits. These directions are called genes.” Our traits are determined by the genes, so it is the genes that differentiate one person from another. Genes get passed on from the birth parent. We learn that genes come in pairs, one gene from the mom and the other from the dad.

We also learn of the allele, a different form of gene, that also comes in pairs. The examples show us that in every situation one allele of the pair takes control. The controlling allele could be inherited from the mom or the dad. The boy in the story got an allele for stretchy thumb joints from his dad and the allele for normal joints from his mom. The boy’s thumb is bent; the stretchy allele is in control. This could be a discussion point, allowing the teacher to mention (depending on the grade level) Mendel’s experiments and laws.

Did you know that there are 10 traits that influence the structure of the hand? Fascinating information. The back matter activity, ‘Track the Traits”,  tells how to make a tally chart for the ten traits. This is  sure to have second grade readers hopping around as they investigate their own, and their classmate’s hands.

The differently colored pages make the photographs stand out and more vividly illustrate the concepts.

A good read aloud book.

Additional Resources:
Author Bio:
What is a Trait:
Gregor Mendel:

  • Body PartsTitle: Body Parts: Double-Jointedness, Hitchhiker’s Thumb, and More
  • Author: Buffy Silverman
  • Publisher: Lerner Publications Company
  • Reviewer: Anjali Amit
  • Hardback: 32 pages
  • ISBN: 978-0-7613-8942-2
  • Genre: Nonfiction/Science

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