Many genetics books and curricula ask students to create and analyze their own family trees. Given the complexities of modern families, single parent households, step-families, adopted families, etc., please be cautious about asking children to compare their physical traits with their biological relatives. The activity outlined here enables children to model inheritance patterns in a more sensitive way.

Background and Introduction

Genetics is the study of how characteristics of living things are passed from one generation to the next.

Until 150 years ago, people believed that parents' characteristics blended together in their children. In the 1830s, Gregor Mendel, a Moravian monk, conducted experiments cross-breeding peas. He determined that certain traits, such as color, shape, and size, were passing from the parents to their offspring in a predictable way. He called these characteristics "factors." We now call them genes.

Mendel observed that the traits of the parents did not blend in the offspring. A six-foot-tall pea plant crossed with a one-foot-tall pea plant did not produce a three-foot-tall pea plant. All the crosses with the six-foot plants produced six-foot plants. Mendel hypothesized that each organism has a pair of factors that determine each trait. Offspring receive one half of each pair from each parent. In the offspring, the two factors, or genes, work together. He discovered that one form of the trait was almost always stronger than the other (dominant). Further experiments revealed that the hidden (recessive) gene can reappear in future generations, if it is combined with a recessive gene like itself.

Although Mendel recorded his observations, published them, and sent them to many leading institutions, his work was ignored. Mendel's work was rediscovered around the turn of the century. However, it was still unclear what the precise agent of inheritance was and how it functioned. In 1907 Thomas Morgan demonstrated that the controlling mechanisms of inheritance are contained in chromosomes inside each cell. The work of Mendel, Morgan, and others showed that mechanisms for inheritance existed at the level of cells. The term gene was accepted as a description of the genetic mechanism located in the chromosome.

In the 1930s, our understanding of genetics was aided by the discovery that the molecule deoxyribonucleic acid (DNA) is present in the nucleus of virtually every cell, but its function and shape were a mystery. In the 1950s, DNA was identified as the carrier of genetic information. Rosalind Franklin conducted x-ray diffraction studies of the DNA molecule in 1951, which suggested DNA had a helical structure. At the same time James Watson and Francis Crick were exploring how DNA functioned. They realized that the form of a double helix would not only explain the shape of DNA, but also explain how the molecule passes on information. After physically building a model of their proposed structure, they realized they had solved the riddle of how genetic information is transmitted. In 1962, Watson, Crick and another researcher Maurice Wilkins, received the Nobel Prize for their discovery. Unfortunately, Rosalind Franklin's death in 1958 at age 38 prevented her from sharing in their recognition.

We now know that in the center of every cell are pairs of chromosomes. Each chromosome is a tightly wound thread of DNA. These very long threads contain repetitions and variations of four different chemicals, adenine (A), thymine (T), cytosine (C) and guanine (G). Segments of these strings of DNA are called genes. Genes have from a few hundred pairs to many thousands of pairs of As, Ts, Cs, and Gs joined together in a precise code that is unique to each gene. Human DNA contains 3 billion of these pairs. Genes are recipes for making proteins. These proteins determine how an organism grows and what it is like. Some proteins make cells, other proteins tell the cells how to function. Your cells are making new cells all of the time. Different genes are active in different cells at different times.

There is a special kind of cell division that happens in sexual reproduction to form the cells that are passed on to the next generation. This division, meiosis, results in cells that have half the number of chromosomes as the original cell. Each parent produces cells that contain half of his or her genetic information. When these two special cells merge together, the two half sets of chromosomes combine to produce a fertilized egg or seed. In this way an organism inherits some traits from its mother and some from its father. The reason every organism is different is because each mixture of genes is slightly different.

A new person starts as just one tiny cell with 23 pairs of chromosomes containing a unique mixture of genes. As the person grows, the cells divide. The first cell becomes two cells, the two become four and so on. Before each cell divides, the DNA is copied and each new cell receives one of these copies.

Mendel's theory about simple dominant factors was the start of our understanding of heredity. We now know that most traits are influenced by many pairs of genes working together. Sometimes there is no dominant gene, and the offspring actually is a blend of both traits. Genes give instructions about what can be done, but genes also have to work with the environment in which the organism grows. Short parents can produce tall children. Tall parents can produce short children. Your height can also be influenced by your diet.

Today, geneticists around the world are working on mapping the entire sequence of the billions of chemicals that compose human DNA. They have already charted over 2,300 of the suspected 100,000 genes that make up the human genome.

Benchmarks for Science Literacy [1]⁰

grades 3-5	Some likenesses between children and parents, such as eye color in human beings, or fruit or flower color in plants, are inherited. Other likenesses, such as people's table manners or carpentry skills, are learned. For offspring to resemble their parents there must be a reliable way to transfer information from one generation to the next.
grades 6-8	In some kinds of organisms, all the genes come from a single parent, whereas in organisms that have sexes, typically half of the genes come from each parent.
grades 9-12	The information passed from parents to offspring is coded in DNA molecules. The genetic information in DNA molecules provides instructions for assembling protein molecules. The code is virtually the same for all life forms.

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Experiment -- Unique You

Materials
pencils
index card or small slip of paper for each person

Background for Instructors
In every population there are similarities and differences. In this activity, campers are asked to make and record careful observations. Each person will end up with a list of attributes that describes him uniquely, just like a person's combination of genes makes him unique.

This is a binary sorting method, similar to what is used by many computer programs. For every trait, campers either possess the trait or do not possess the trait. Each step of the sort has two possible outcomes, just like an electronic signal is either on or off. For example, don't divide into groups by shirt color. Instead divide into two groups, white shirts and not white shirts.

Objectives

• carefully observe similarities and differences in a population
• model a binary sorting method
• create a list of traits that uniquely identifies each member of the group

Procedure

• Each person needs a slip of paper and a pencil, and starts by writing her name on her paper.
• Discuss some ways that scientists describe, sort, and organize the world. Choose one attribute that describes some of the campers, "wearing black" for example. Everyone wearing black should go to one side of the room, while those not wearing black should go to the opposite side of the room.
• After they've divided into two groups, tell the campers to record the attribute that applied to them on their piece of paper (wearing black or not wearing black).
• The two groups will then separately decide on a second attribute that will divide their group into two smaller groups. If groups seem stumped you can suggest some attributes such as: wearing glasses / not wearing glasses; shoes with laces / shoes without laces; long sleeves / not long sleeves; shirt with words / shirt with no words; wearing a belt / not wearing a belt.
• Individuals should record each attribute on their papers after each division.
• The four new groups divide themselves according to another attribute.
• Repeat the process until each camper is a "group" of his own with a list unique to him.
• Test whether you have accurately and uniquely described all the members of the group. Collect all the papers and have everyone stand. Choose one paper and read off each attribute. Ask those to whom the attribute does not apply to sit down. By the end of the list only one camper should be left standing -- the one who has all these attributes and whose name is on the paper!

Materials
none

Background for Instructors
All living things are determined partially by the genes that they inherit, partially by their biology as they develop, and partially by the external environment. Some things change as we grow, such as our height . Other traits remain virtually constant, such as our eye color. Some physical features can be changed by our behavior. For example, you can increase your heart rate by running for a few minutes. Tanning changes your body chemistry and skin color. We have less control over other things, such as our foot size.

Determining inheritability is a complicated subject. Often things are said to be 80% inherited or 20% inherited. Most patterns of inheritance are not simple. Many traits are controlled by multiple genes. There are still fervent debates about many traits, such as intelligence or certain behaviors.

Objective

• stimulate thought and discussion about the different influences that determine how an organism looks and behaves, particularly humans

Procedure

• Discuss with the group a variety of traits that describe people, and consider what determines them. For example:
  Gender. Whether you are a boy or a girl is determined entirely by genetics. Your biological mother always contributes an X chromosome. Your biological father contributes either an X chromosome (making you female) or a Y chromosome (making you male). However in some other species, such as turtles or fish, environmental conditions such as temperature can affect whether the children are male or female.
  Eye color. Your genes make proteins which determine your eye color. Eye color isn't influenced by the environment (unless you wear colored contact lenses).
  Height. How tall you are is partially determined by your genes, but may also be affected by your diet and your overall health as you grow.
  Native Language. Genetics play no role in determining the language you speak. Your language is determined by exposure early in your life.
  Musical ability. No genes for musical ability have been found. Genes do make proteins that determine ear and brain structure. Exposure to music, training, and cultural reinforcement contribute to musical ability.
  
• Discuss other traits. Hypothesize whether the traits are inherited or determined by the environment? How could scientists test the hypotheses?

Materials
face templates (included)
facial features worksheet (included)
colored markers, crayons, or pencils
pennies

Background for Instructors
People carry two genes for every trait, one contributed by your biological mother and one contributed by your biological father. When you reproduce, you pass one of those genes on to the next generation (the other one is contributed by your partner). Which one of the two genes is passed on is decided randomly, like flipping a coin.[1]¹ You can't purposefully flip heads. In the same way, you can't choose to contribute red hair or a cleft chin. There are thousands of genes and thousands of combinations.

The introductions to this section and the previous activity indicate the complexity in determining what features of our appearance are genetically determined. Nevertheless, this activity introduces some features that are generally accepted to be inherited. Several types of inheritance are described:

• Chin shape, freckles, and ear shape are considered traits that exhibit complete dominance. These features are controlled by a single gene that is either dominant or recessive. When the dominant gene is paired with the recessive gene the dominant trait is exhibited.
• Hair color, hair curliness, and lip color are traits that exhibit incomplete dominance. When two different forms of the gene are combined, a third intermediate type results. For example, when black hair is crossed with blonde hair the result might be brown hair.
• Eye color is controlled by multiple genes. This model uses two genes, one that determines brown pigment and a second that determines yellow pigment, which combine to determine the final eye color.

Objective

• model how the random contribution of parents' genes helps to determine the appearance of the offspring

Procedure

• Each camper needs a face outline, facial features worksheet, two pennies, and coloring tools.
• Flip the two pennies to determine the combination of genes the this imaginary person inherits from its parents. If the coin lands on heads (H) a dominant trait is contributed. If the coin lands on tails (T), a recessive trait is contributed.
• After each set of coin flips, circle the appropriate type on the worksheet.
• When all the traits have been determined, have campers use the face outline to draw a picture of each imaginary person.
• Compare the different pictures and observe the diversity and similarities. Are some traits more common than others? Are any two pictures exactly alike? Are some traits more common than others? Are any two pictures exactly alike?

Footnotes:
[10] AAAS. pp. 106-108, 114
[11] Some genes are linked together on one chromosome, making the process not entirely random. For this activity we'll keep it simple.