Lesson Human Genetics, Chromosomes and Alleles:
What’s Dominant?

Learning Objectives

After this lesson, students should be able to:

• State the number of chromosomes in human body, sperm and egg cells.
• Explain why sperm and eggs cells have only half the number of chromosomes found in the body cells.
• Give a brief definition for allele.
• Explain the difference between dominant and recessive alleles, and give an example.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

NGSS: Next Generation Science Standards - Science

Common Core State Standards - Math

International Technology and Engineering Educators Association - Technology

State Standards

Pre-Req Knowledge

Students should have a basic knowledge of cells and their structures. A basic knowledge of cellular reproduction (mitosis) is very helpful, but it is not necessary for students to know the details of this process. Instead, it is helpful if they understand that prior to cell division, all the structures and genetic material within the cell are duplicated, enabling each of the two new cells to have the full, required amounts of all materials once division is complete.

Introduction/Motivation

You will certainly get the class' attention if you ask, "Where do babies come from?" You may get some interesting responses, but the one you are looking for (and may have to provide yourself) is that a sperm cell from the father and an egg cell from the mother unite to form an embryo. This is what develops in the mother's uterus for nine months until birth.

Ask if anyone knows how many chromosomes humans have in their cells. If no one knows, explain that in nearly all the cells in the body are 46 chromosomes and they come in pairs. Two of the 46 chromosomes "match," that is, they both contain the same number and types of genes. For example, both might contain the many genes needed to build the eyes during fetal development, and the genes to produce several different enzymes needed for digestion, and the genes needed make the hormone insulin. A different pair of chromosomes would contain different sets of genes, such as the genes that determine hair color, or the genes that allow hemoglobin to be made in red blood cells. Twenty-three pairs is the "correct" number of chromosomes. These chromosomes contain all the genetic information needed for the body to construct all the necessary structures and perform all the necessary functions.

Next, point out that if sperm cells had 23 pairs of chromosomes, and egg cells did, too, when they joined the embryos would have 46 pairs of chromosomes. This is more than they need. When these embryos developed into babies, children and then adults, a pair of these adults would then produce embryos that had 46 times 2, or 92 chromosomes. This would make the nuclei of the cells of these embryos awfully crowded! In fact, cells are very sensitive about having the correct 23 pairs of chromosomes. For example, Down's syndrome results when just one extra chromosome exists in a human's cells.

If they weren't already interested, students should be by now. Students this age are very curious about their bodies and how they work—or how they do not work when things go wrong. Conclude the introductory part of the lesson by asking if anyone has ever told them they look like their mothers or fathers. Expect this to generate lots of comments, and possibly moans and groans. Ask them, "If we get our genes from our parents, why don't girls end up looking exactly like their mothers did at the same age, and why don't boys look just like their fathers did at the same age?" Give them a few minutes to explore this question. If they already know about mitosis and meiosis they might be able to figure out the answer. If they can't figure out the answer, don't explain it. Instead, tell students they will soon get a chance to see for themselves why they don't look exactly like their same-sex parent. Instead, they will see why they inherit some of their physical features from their mothers, and some from their fathers.

Lesson Background and Concepts for Teachers

Nearly all body cells contain 23 pairs of chromosomes—with two types of exceptions. The first is cells that do not contain nuclei and thus do not divide, such as red blood cells. Instead of dividing to create new cells, they are destroyed, mainly in the liver and spleen, when their membranes wear out. Since they have no nuclei, red blood cells have no chromosomes at all.

The second exception is the sex cells: the sperm cells of males and the egg cells of females. These are produced through the process of meiosis. Meiosis starts with a cell with the normal number of chromosomes, that is, 23 pairs. The first stage of meiosis is no different from mitosis. In the cell containing 23 pairs of chromosomes, just before division occurs the organelles are all duplicated, and so is all of the DNA within the nucleus. During the division, the DNA is organized into chromosomes, which now number 46 pairs because to the duplication process. As the process of division continues, the chromosomes and organelles are allocated to the two new cells being produced. The result so far is two new cells that each contain one set of organelles and 23 pairs of chromosomes. Duplication of a cell resulting in two cells identical to the original cell, is a form of asexual reproduction.

The second stage of meiosis differs from the first in one important way. Although any necessary organelles are duplicated, this time there is no duplication of the DNA prior to the division. Thus, each of the two cells produced in the first stage now divide into two new cells, called "daughter" cells. However, since the DNA—therefore the chromosomes—were not duplicated, each daughter cell receives only 23 chromosomes instead of 23 pairs. So, the final result of meiosis is four new cells, each containing one set of 23 chromosomes. This type of reproduction is called sexual reproduction. The process is summarized in the diagram below:

When sexual reproduction occurs and an egg cell and a sperm cell fuse together during fertilization, the resulting embryo contains 23 pairs of chromosomes. It is thus equipped with exactly the right amount of genetic information required to develop into a human baby. Only if a problem occurs during meiosis in one of the parents, causing one of the sex cells to have too many or too few chromosomes, does the embryo end up with the wrong number of chromosomes.

Body of Lesson

Ask students if they can roll their tongues, that is, stick their tongues out and curl up the long edges so the tongue almost forms a cylinder. Many will be able to do this, but some will not.

For those that cannot, tell them not to feel bad. Explain that many genes come in two forms. Somewhere in all the genes that give the instructions for making the muscles of the tongue, there is one that either does or does not enable the tongue to be rolled. Point out that everyone has two copies of this gene. One copy originally came from the father's sperm cell and the other came from the mother's egg cell. Refer to the associated activity Heredity Mix 'n Match to help students illustrate this idea by using jelly beans (or other candy) to represent genes for several human traits such as tongue-rolling ability and eye color. 

Then explain that everyone who got two copies of the tongue rolling gene—one from mom and one from dad—can roll their tongues. Those who got two copies of the non-rolling gene, however, cannot roll their tongues. Because this gene comes in two forms, we call the two forms alleles. In this case, there is a rolling allele and a non-rolling allele of the tongue rolling gene.

Ask students what they think would happen if they got a rolling allele from mom and a non-rolling allele from dad? They might suggest that they would only be able to partially roll their tongues. If so, ask if anyone in the class can only partially roll his or her tongue, which would lend support to the idea.

Since no supporting evidence will be found, explain that if they possessed one of each allele, they would, in fact, be able to roll their tongues just as well as someone who had two copies of the rolling allele. This is because the rolling allele is dominant over the non-rolling allele. The non-rolling allele is called recessive because its effects can be hidden by the presence of just one rolling allele.

Next ask students why it is that some of them cannot roll their tongues. They may be able to reason out that those students must have two copies of the non-rolling alleles. This is correct: if two recessive alleles are present, then no dominant allele exists to hide them. Thus anyone with two non-rolling alleles will not be able to roll his or her tongue.

Using a Punet square, model this concept to show how alleles can be passed to offspring during sexual reproduction. Give students the alleles for two hypothetical parents and have them determine the four possible outcomes from the combinations of alleles.

Associated Activities

• Heredity Mix 'n Match - Students use jelly beans (or other candy) to represent genes for several human traits such as tongue-rolling ability and eye color.

  Watch this activity on YouTube

Lesson Closure

Inform students that they will have the opportunity to see what happens when they pair up and make babies(!). Well, actually, they will consider several human traits that have dominant and recessive alleles, and see what happens when these form random combinations just as would occur when the chromosomes of mothers and fathers pair up during fertilization. Will the baby look more like its mother or its father?

Vocabulary/Definitions

allele: One form of a gene that can occur in two or more forms; for example, three different alleles code for a protein found on the surface of red blood cells, giving rise to the A, B and O blood types.

dominant: A visible or otherwise observable gene for a trait that can mask a recessive form of the same gene.

hemoglobin: The iron-containing protein found in red blood cells that carry oxygen.

meiosis: A type of cell division in which one cell undergoes two divisions, resulting in four new cells, each containing half the amount of genetic material that was in the original cell. Meiosis is a form of sexual reproduction.

mitosis: A type of cell division in which one cell divides into two new cells, each genetically identical to the original cell. Mitosis is a form of asexual reproduction.

recessive: A gene for a trait that can be masked or hidden by a dominant form of the same gene.

Assessment

At this point, assess student learning by askng them to:

• State the number of chromosomes in the human body, sperm and egg cells.
• Explain why it is necessary for sperm and eggs cells to have only half the number of chromosomes found in the body cells.
• Define an allele.
• Explain the difference between dominant and recessive alleles, and give an example.
• Explain the outcomes of sexual and asexual reproduction (that is, are the cells produced during each type of reproduction genetically identical or not?).

Copyright

Contributors

Supporting Program

Acknowledgements

This content was developed by the MUSIC (Math Understanding through Science Integrated with Curriculum) Program in the Pratt School of Engineering at Duke University under National Science Foundation GK-12 grant no. DGE 0338262. However, these contents do not necessarily represent the policies of the NSF, and you should not assume endorsement by the federal government.