Nettie Stevens: The Genetics Pioneer Who Discovered Sex Chromosomes

At a time when women mostly married and stayed home, or were teachers or nurses if they wanted to work, Nettie Stevens became a research scientist and her discoveries changed genetics forever.

NettieStevensGeneticsPioneerOnce she graduated with her PhD in 1903, she and a colleague (Thomas Morgan) began a collaboration on the controversial and unresolved question of how sex is determined in the developing egg. Did external factors, like food and temperature, set the sex of an egg? Or was it something inherent to the egg itself? Or was sex inherited as a Mendelian trait?

She examined the yellow mealworm, Tenebrio melitor, and made a striking observation. She had observed that this species produced two classes of sperm: a type that carried ten large chromosomes, and a type that carried nine large and one small chromosome. Body cells in the females contained 20 large chromosomes while males carried 19 large and one small chromosome.

Stevens reasoned that when an egg is fertilized by a sperm that carries the small chromosome, the result is a male offspring. The presence of the small chromosome might be what decided the individual’s “maleness.”

She published her research in 1905 and it eventually evolved into the XY sex-determination system we know today: The father’s sperm, which can carry either X or Y chromosomes, determines the sex of the offspring. Before Stevens’ work, scientists thought that the mother or the environment determined if a child was born male or female.

Biography

Nettie StevensNettie Maria Stevens was one of the first American women to be recognized for her contribution to science. Yet she didn’t begin her career in genetics until later in life.

Stevens was born on July 7, 1861, in Cavendish, Vermont, to Ephraim and Julia Stevens. After the death of her mother, her father remarried and the family moved to Westford, Massachusetts.

Initially, Stevens taught high school and was a librarian for more than a decade. Her teaching duties included courses in physiology and zoology, as well as mathematics, Latin, and English. Her first career allowed her to save up for college; at the age of 35, she resigned from a high school teaching job in Massachusetts and traveled across the country to enroll at Stanford University in California.

At Stanford, she received her B.A. in 1899 and her M.A. in 1900. She also completed one year of graduate work in physiology under Professor Jenkins and histology and cytology under Professor McFarland.

Stevens continued her studies in cytology at Bryn Mawr College, where she obtained her Ph.D. Here, she was influenced by the work of Edmund Beecher Wilson and by that of his successor, Thomas Hunt Morgan. Her work documented processes that were not researched by Wilson and she used subjects that he later would adopt along with the results of her work.

At age 50 years, only 9 years after completing her Ph.D., Nettie Stevens died of breast cancer on May 4, 1912 in Baltimore, Maryland.

Bring it Home

▶︎ Dive into Genetics with a fun unit study

▶︎ Enjoy a slide show presentation on genetics

▶︎ Try out this jigsaw format activity to explore the sex determination mechanisms of seven organisms, Xs and Os

▶︎ Learn about the Father of Genetics: Gregor Mendel

▶︎ Try this Gummy Bear Genetics lab from The Science Teacher (a NSTA publication)

▶︎ Use pipecleaners and beads to show how genes and chromosomes are inherited in this Pipecleaner Babies lab.

▶︎ Use pennies to do this How Well Does a Punnet Square Predict the Actual Ratios? lab.

Science Milestones

Visit my Science Milestones page to learn more about scientists whose discoveries and advancements have made a significant difference in our lives or who have advanced our understanding of the world around us.

The bloggers of the iHomeschool Network have teamed up to create fun and original unit studies on fascinating people who were born in July.

 

Science with Harry Potter: Care of Magical Creatures (Zoology)

Care of Magical Creatures is an elective at Hogwarts, available to upper classmen. Throughout the course, students learn about a wide range of magical creatures and are taught about the care and husbandry.

Similar to herbology, the further into a student’s education the more difficult and dangerous the creatures become. The witches and wizards who succeed in the subject later become Magizoologists, like Newt Scamander.

Magical CreaturesFor this class students are required to become familiar with the many magical creatures you may encounter both at Hogwarts and in the outside world. Students should begin with the following:

  • Owl
  • Hippogriff
  • Phoenix
  • Unicorn
  • Werewolf
  • Centaur
  • Basilisk
  • Elf

C’mon, now, get a move on! Got a real treat for yeh today! Great lesson comon’ up! Everyone here? Right, follow me!” ~ Rubeus Hagrid at his first Care of Magical Creatures lesson

Students are required to keep a field notebook in which a two-page spread is created for each magical creature studied. For each magical creature you study:

  1. Make a sketch of the creature, labeling important features
  2. List any historical or literary references to the creature
  3. Describe its natural habitat
  4. Discuss its habits, temperament, and relationship to humans
  5. List its magical properties
  6. Explain the care and feeding of the creature

Advanced students may choose additional magical creatures to study. Take care to choose wisely, as your knowledge of magical creatures could one day prevent a terrible injury or death.

Magical Properties of Dragons

You’ve likely already discovered the magical property of dragon scales while researching and preparing your field notes above. Now you will learn about the properties of dragon skin and dragon down (the fluffy feathers from underneath the wing).

Young wizards and witches should have adult supervision as all parts of a dragon are highly flammable. A fire-proof cauldron is advised.

Dragon Skin: take thin slices of dragon skin and hold them next to an open flame. Bend the skin, squeezing until it bursts. You should see tiny sparks fly as the fire-breathing properties are released. This should be done very close to the flame.

Dragon Down: Put a small quantity of dragon down into a cauldron. Touch the end of a 9 volt battery lightly to the down to release the fire-breathing properties.

(Note to professors: muggles will know these items as orange peel and steel wool.)

Genetics

Students watch a video clip from Harry Potter and the Goblet of Fire to learn about genetic traits. Specifically, they realize that the ability to speak parseltongue (being able to speak to snakes) is a genetic trait possessed by some characters and their parents. Students explore the use of Punnett squares to predict trait inheritance, learning about genotypes and phenotypes.

This post is part of a five-day hopscotch. Join me each day this week as we dive into each course.

Herbology (Botany)

Care of Magical Creatures (Zoology) – this post

Potions (Chemistry)

Alchemy & Divination (Geology)

Magical Motion (Physics)

Diving into Genetics with Amy Brown Science

As a science educator, I have always relied on my own experience and the wealth of material that I have collected over the years to develop our homeschool science curriculum. I have also allowed the interests of my children to drive our studies, ensuring that we also cover a variety of topics.

Months ago, when we were in the midst of our Human Anatomy unit with our STEM Club, I realized that I needed to go more in-depth, specifically for my daughter. I hadn’t taught high school level science, however, so I was a little apprehensive. Would I be able to find labs that would be both challenging and adaptable for the homeschool setting?

As I began my research, I was delighted to discover a great resource for teaching middle school and high school biology, Amy Brown Science. As I browsed her website and the products available at her Teachers Pay Teachers store, I was immediately drawn to her Genetics Complete Bundle Unit and am delighted to share with you how we’ve begun to use it in our homeschool.

Diving into Genetics with Amy Brown @EvaVarga.netGenetics Unit Bundle

Though my son is a few years younger than my daughter, I generally teach them both simultaneously. I thereby began our study by reading aloud, Gregor Mendel: The Friar Who Grew Peas. While this is a picture book, I have found that everyone enjoys listening to quality literature and it was a great way to spark interest in the new unit. I then followed up with Amy’s Genetics Warm-ups to review the material that was covered in the book.

Note: The warm-ups are a separate item in Amy’s TpT store and are not part of the Genetics Complete Bundle. However, they are the perfect tool for review and reinforcement.

Together, we viewed the PowerPoint and took notes along the way. My daughter and I have now begun to progress slowly through the material in the bundle. There is really so very much – it will take us a while to complete everything. I am impressed with the quality of the material – the lessons are laid out very clearly and each warm-up includes a teacher’s answer key.

My favorite component are the labs: Simple DNA Extraction, Effect of Environment on Gene Expression, and The Hardy Weinberg Equation. While each is available individually, the fact that they are bundled not only saves me time but puts everything into context.

Amy does an excellent job of describing the lab procedures in a way that students can easily follow along. She also provides tips for the teacher in regards to set-up and preparation.

The genetics unit focuses almost entirely on Mendelian genetics and on genetics topics that arose after Mendel’s death. The materials do include some discussion of Darwin and natural selection.

Diving into Genetics with Amy Brown @EvaVarga.netThe Genetics Complete Unit Plan Bundle contains at least 3 weeks of materials depending on how many lab activities you want to do. This unit has everything that a teacher needs to teach a unit on genetics to Biology or life science students in grades 8-12.

The bundle includes:

  • A 61 Slide PowerPoint
  • Teacher Notes
  • Student Notes
  • 3 Labs
  • A Jeopardy-style game to review concepts
  • 5 Quizzes
  • 7 Genetics Practice Problem Worksheets
  • A Crossword Puzzle
  • A Unit Test

Wow! All that for a bundled price of just $25.99! This is a savings of 39% off the price if you were to purchase the items individually.

The Father of Genetics: Gregor Mendel

When my son was diagnosed with congenital nystagmus as an infant, I became increasingly interested in heredity or the passing of phenotypic traits from parents to their offspring. Both my brothers also have nystagmus and it was now clear that I carry the gene as does my daughter.

The Father of Genetics, Gregor Mendel @EvaVarga.net

This post contains affiliate links.

Nystagmus is inherited in an X-linked pattern. A condition is considered X-linked if the mutated gene that causes the disorder is located on the X chromosome, one of the two sex chromosomes in each cell. In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two copies of the X chromosome), one altered copy of the gene in each cell can cause the condition, although affected females may experience less severe symptoms than affected males.

Genetics is the study of genes, heredity, and genetic variation in living organisms.

Single gene inheritance is also referred to as Mendelian inheritance as it follows transmission patterns Gregor Mendel, a late 19th-century scientist and Augustinian friar, observed in his research on peas. As a result of his work, he is known as the Father of Genetics.

Mendel studied ‘trait inheritance’ or how different traits are passed on from parents to their children. He observed that organisms (pea plants) inherit traits by way of discrete “units of inheritance”. This term is a somewhat ambiguous definition of what is referred to today as a gene.

The four types of Mendelian inheritance patterns he described include:

Autosomal: the gene responsible for the phenotype is located on one of the 22 pairs of autosomes (non-sex determining chromosomes).

X-linked: the gene that encodes for the trait is located on the X chromosome.

Dominant: conditions that are manifest in heterozygotes (individuals with just one copy of the mutant allele).

Recessive: conditions are only manifest in individuals who have two copies of the mutant allele (are homozygous).

Biography

The Father of Genetics, Gregor Mendel @EvaVarga.netJohann Mendel was born July 20, 1822, in Heinzendorf bei Odrau, a small village in the Czech Republic (formerly the Austrian Empire). His parents were small farmers who made financial sacrifices to pay for his education.

He entered the University of Olomouc in 1840. He took courses in physics, mathematics and philosophy. He was advised by a professor to join the Abbey of St. Thomas in Brünn as a monk when he was 21 years of age to help offset the cost of his education.

The Abbey actually had a good reputation for its teaching of sciences, and its director, Abbot Franz Cyril Napp, was particularly interested in heredity of traits in plants and animals on farms. Upon joining the Abbey, he took the name Gregor.

My scientific studies have afforded me great satisfaction; and I am convinced that it will not be long before the whole world acknowledges the results of my work.  ~ Gregor Mendel

After completing his studies, he returned to the monastery in 1854 and took a position as a physics teacher at a school at Brünn, where he taught for the next 16 years. The monastery had a 5 acre garden, and two former professors encouraged Mendel to pursue his interest in heredity by using the garden for experiments.

Mendel set himself the very ambitious task of discovering the laws of heredity. To achieve this, he embarked on a highly systematic, eight year study of edible peas, individually and carefully recording the traits shown by every plant in successive generations. His work involved growing and recording the traits in about 30,000 plants.

One of the keys to his success was breeding from closely related pea varieties which would differ in only a small number of traits. The seven traits of pea plants that Mendel chose to study: seed wrinkles; seed color; seed-coat color, which leads to flower color; pod shape; pod color; flower location; and plant height.

In 1866, he published his heredity work, though it was not immediately recognized for the wealth of information it contained.

Bring it Home

🌱 Explore genetics in more depth with a Genetics Unit Bundle from Amy Brown.

🌱 Use M&M’s to visually see the genetic probability that an offspring will carry a certain trait. Just don’t each them all before you finish!

🌱 Explore the genetics of sickle cell disease through Punnett squares, pedigrees, and an optional chi-square analysis with this classroom activity from the Howard Hughes Medical Institute.

🌱 Enjoy this 3-D animation, available in both basic and advanced detail, which depicts the molecular machinery responsible for quickly and accurately copying DNA.

🌱 Here is a cute video of an interview with Gregor Mendel from Untamed Science.

🌱 Download the free printables from Stephanie Harrington and enjoy the book Gregor Mendel: The Friar Who Grew Peas.

🌱 Create a DNA model of your own using pipe cleaners and beads or if you prefer, Lego bricks.

🌱 Erect a DNA Double Helix Table Sculpture Puzzle

🌱 If you still have those M&Ms, consider this Easter egg and M&M genetics activity from All Things Beautiful!

Science Milestones

Visit my Science Milestones page to learn more about scientists whose discoveries and advancements have made a significant difference in our lives or who have advanced our understanding of the world around us.

The bloggers of the iHomeschool Network have teamed up to create fun and original unit studies on fascinating people who were born in July.