Biology Lessons Part 2: Population Biology


 Lesson 2.4: Teachers' Guide: How Do Organisms Vary?

To Ponder
Introduction and Background
AAAS Benchmarks
Part I. Mendelian Genetics
Exercise 1: Self Classification
Exercise 2: Summary Questions
Part II. Characteristics that Vary Continuously and How We Measure Them
Exercise 1: Organize Your Initial Knowledge
Exercise 2: Measuring Continuous Variation
Exercise 3: Pooling Data for Precision
Exercise 4: Knowledge of Metric System

Grade Level

Prospective and Practicing K-8 Teachers; may be adapted for use in K-12 classes


Exercises 1 and 2 will take approximately 2 1/2 hours.

To Ponder


Why do we look like members of our biological family?

We inherited all of the genetic information (which directs the growth and functioning of our cells) from our parents. During fertilization, when we were composed of only a single cell, half of our father's chromosomes combined with half of our mother's chromosomes to make up our unique genome. We ended up with two complete sets of chromosomes, one from our mother and one from our father.


How can siblings with the same two parents look completely different?

Chance dictates which portions of each of our parents genome we will inherit. Because we have about 80,000 genes, simply by chance two siblings can receive strikingly different gene combinations.


Are attributes such as your temperament and your wit due to "nature or nurture"?

A little of both. When you start to consider immeasurable qualities like personality traits, genetics becomes quite fuzzy. What we do know is that for all the traits we can measure, on average, genetics and environment each contribute about equally on average.


If all dogs are the same species, how can a Chihuahua and a Great Dane look so different?

All dogs, as well as wolves, are members of the same biological species, Canus lupis. Chihuahuas and Great Danes are different breeds. By continued inbreeding of dogs with desired physical and behavioral attributes, humans have been able to attain constancy within a breed. Examining the diversity of different breeds of dogs is a good way to observe the tremendous potential for variation in a single species. Even though they look quite different, Chihuahuas and Great Danes have very, very similar genes. Yet it should be notes that if dogs of different breeds are allowed to interbreed freely, their offspring will quickly return to the "norm."


What are the chances that your children will have the same color eyes as you do?

That depends on the color of your eyes (and the color of your partner's eyes)! Genes specifying eye color can be dominant or recessive. Dark eyes are typically dominant to light eyes. But, even if you and your partner have dark eyes, if you both carry just one copy for light eyes, your children have a 1/4 chance of having light eyes.


What is the difference between Mendelian traits and quantitative traits?

A Mendelian trait is specified by one gene or one pair of alleles. Mendelian traits can be described in discrete categories such as blue and brown eyes. A quantitative trait is specified by multiple genes. Traits affected by multiple genes vary continuously and can't be classified into discrete, separate categories. Height, weight and intelligence are quantitative traits.


These are some materials you may want for the sample experiments supplied with this lesson.

  • metric ruler
  • meter stick
  • pencil
  • graph paper
  • metric tape measure


    Once you have completed this lesson you should be able to:


Define gene, allele, dominant, recessive, homozygous, heterozygous.


Describe the differences between Mendelian traits and quantitative traits.


Describe the factors responsible for variation of individuals within a population


Explain how genes are inherited and selectively expressed as physical characteristics.
    5. Identify certain of your traits coded for by dominant or recessive gene pair
    6. Describe the methods used to quantify traits coded for by many genes.

There is a great deal of variation within nearly every population of organisms. Variation arises from a combination of genetic and environmental causes, each contributing equally on average. Owls, squirrels, rabbits and bears differ from each other by their physical characteristics, size and temperament. So do whales, porpoises, dorado fish, and most other populations of organisms. But within the population, there is also dramatic variation. Not all blue whales in the Pacific Ocean look the same! Variation is important for the continued survival of a species in changing environments.

This lesson has two parts, Mendelian Genetics and Characteristics that Vary Continuously. Together, these exercises provide an opportunity to examine genetic variation within the human population, using the most interesting specimens of all...ourselves!


Part I. Mendelian Genetics




Living things naturally exist in communities composed of some similar and some different kinds of organisms. Within populations of similar organisms, we see variations of certain traits expressed. For instance, snapdragons may have white or red flower petals; whales may be born with different patterns and markings on their flukes.


    2.  Many traits are produced by a single gene. Genes are units of inheritance. Single gene traits are called Mendelian traits because they follow the simple inheritance patterns first described by Gregor Mendel. In the 1850s, Mendel made hybrids between different varieties of pea plants and kept careful track of the traits displayed by the offspring. Mendel discovered that traits of the parent plants were passed on to the progeny plants in reproducible patterns. Mendel proposed that "units of inheritance" determined the traits in the pea plants. In humans, some Mendelian traits include hair curliness and hair color. Other traits, such as skin color and weight are affected by multiple genes.
 Question   3.

Can you think of an analogy that would help clarify the versatility of genes? For example, one analogy might be: "A gene is like a single note in a song. You might play variations on the theme by playing the song in a different key or changing the harmony, but the overall tune is still the same." Can you think of other analogies?

Answers may vary.


Background   4. The statistical analysis of Gregor Mendel's work suggested to him that two units of inheritance existed, which he called "factors," for each trait. We now know that normal humans have two copies of each of their 50,000 - 100,000 genes.
 Questions   5.

Where do these two gene copies come from?

During fertilization, a sperm cell injects one gene copy into an egg cell, which contains another copy of the same gene. So, one gene copy comes from each parent. These gene copies make up the genotype of the individual.


These gene copies are called alleles. Do you think both genes that we inherit are always identical? Explain.

 No, the gene for each trait can exist in two or more alternative forms, called alleles.


What does it mean for one allele to be dominant to the other? Recessive?

The allele whose trait shows is said to be dominant. The allele that is hidden each time it is paired with a dominant allele is recessive.


If your mother had curly brown hair and your father had straight blond hair, would your hair be a mixture of the two types? Do genes "blend?" Explain.

No. The gene which specifies hair curliness is a Mendelian gene. Blending would mean that each hereditary factor is permanently "diluted"each time heterozygous alleles are paired. Since we see obvious recessive traits being expressed in offspring of two heterozygous dominant parents, clearly Mendelian genes do not blend.


Traits are visible expressions of the information contained in your genes. The physical appearance of a gene is called its phenotype. What is the genotype?

The actual genetic makeup of an individual is called the genotype.


Exercise 1

Self-Classification According to Inherited Traits

 Background   1.

In this lab, we will examine our own bodies for the presence of Mendelian traits (determine our phenotype); then we will approximate our genotype by comparing our observations to a chart of physical characteristics.


To Do    2. 

Classify yourself with respect to the traits listed in Table 1, by indicating in the column marked X, for each trait, the first two pairs of symbols (homozygous dominant or heterozygous) or the third symbol (homozygous recessive). See Figure 1 for depictions of these traits.

Table 1. Trait, Genotypes, and Variations
 Trait  Phenotype X Genotype
1. Hair color  brown, black, or red hair    LL or Ll
   blond hair    ll
2. Hair type  naturally curly    TT or Tt
   naturally straight    tt
3. Tongue curling  can curl tongue    CC or Cc
   cannot curl tongue    cc
4. Mid-digital hair  hair present, middle digit of finger    MM or Mm
   hair absent, middle digit of finger    mm
5. Pigmented iris  eyes not blue    EE or Ee
   blue eyes    ee
6. Widow's peak  peak in center of hairline    WW or Ww
   no peak in center of hairline    ww
7. Bent finger  little finger curves toward others    BB or Bb
   little finger straight    bb

 Background     Here are some things to know about these traits.
     3. A dominant allele will be expressed if it is homozygous (that is, occurs with another dominant allele of the same type, such as LL) or if it is heterozygous (that is, it occurs in combination with a recessive allele, such as Ll).
     4. A recessive allele is expressed when it is paired with another recessive allele of the same type (such as ll), but it is masked when combined with a dominant allele (such as in Ll).

Why can't you differentiate between a homozygous dominant or a heterozygous dominant pair of alleles (e.g. LL / Ll)? How might you be more certain of the genotype?

A dominant allele will "mask" the presence of a recessive allele. So, if we observe a dominant trait in the phenotype, we have no way of knowing if the underlying genotype is heterozygous or homozygous.

Figure 1. Phenotypes and Genotypes
 Background   6.

If we consider the homozygous dominant (LL) and the heterozygote (Ll) to be indistinguishable, these seven pairs of alleles can produce two alternative characteristics or 14 traits altogether (that is, LL and Ll produce the dominant trait, while ll produces the recessive trait). These two alternative traits for each gene can be combined in 128 different ways. That is,

27 = 2 *2 * 2 * 2 * 2 * 2 * 2 = 128.

 To Do   7. Determine which combination (1-128) of these seven alleles represents you by mapping your type onto the genetic wheel (see next page). Begin in the center and color in L_ (for LL or Ll) or ll. Then move out into the half of the wheel you have selected and fill in T_ or tt. Then move out into the quarter wheel you have selected and fill in C_ or cc....and so on. When you have completed the wheel, write your genotype and the number of your type below.
 Question   8.

Does this help you understand why every person (except for identical twins) is so different? Explain.

Yes! We have only looked at seven different genes and already seen the possibility for 128 different variants. Imagine how much variation exists when we consider the total number of genes contained in the human genome... the potential for about 280,000 variants!


If more genetic traits were included in the wheel analysis, would the number of pattern types in the class increase of decrease? Explain. an exponential rate. Try including just one more trait. Our total number of pattern types goes from:

27 = 2 * 2 * 2 * 2 * 2 * 2 * 2 = 128


28 = 2 * 2 * 2 * 2 * 2 * 2 * 2 * 2 = 256

Figure 2. Genetic Wheel (obtained from Lawrence Hall of Science, U.C. Berkeley)

For a higher resolution genetic wheel click below:

High-Resolution Wheel


Exercise 2

Summary Questions and Exercises

Background 1.

Here are some other traits of human beings that are produced by a single pair of alleles:

Table 2. Other Medelian Traits in Humans
Dominant Form   Recessive Form
 long ear lobes  short ear lobes
 white skin spotting  uniform skin color
 astigmatism  no astigmatism
 normal body size  midget
 6 or more fingers  5 fingers
 varicose veins  no varicose veins

To Do 2.

Design a genetic wheel to summarize these characteristics.

Wheel should resemble Figure 2 with one less trait.









Question 3.

How many different combinations of these 6 traits exist?

26 = 2 * 2 * 2 * 2 * 2 * 2 = 64.



Based on what you see in Table 2, would you say that all desirable genes are dominant? Are all dominant genes desirable? Is the dominant allele typically the most prevalent form of a gene in the population? Why or why not?

No, not all dominant genes are more appealing than the recessive forms. Certainly vericose veins, six fingers and white skin spots are not culturally "desirable," though they are the dominant forms of these three genes. Likewise, the dominant forms of genes are not always the most prevalent form in a population. If that were the case, the majority of the population would have six fingers and astigmatism!


Part II. Characteristics that Vary Continuously and How We Measure Them


1. Some characteristics cannot be separated into discrete categories like Mendelian traits. These characteristics are said to "vary continuously." Continuous variation results when many different genes affect a trait. In humans, skin color is affected by several pairs of genes. And behavioral qualities such as intelligence are affected by many, many different genes!

There are also a variety of environmental influences which contribute to the variation of living things. Can you give some examples?

Diet, climate, cultural influences...For example, if we examine clothing from 100 years ago, we see that humans, on average were much smaller than nowadays. Have large-scale mutations occured in the population to cause such a dramatic difference? No, it is not likely that these changes have resulted from mutations in the human genome. It is easy to attribute the difference in size to the improvement of diet and nutrition which has steadily taken place over the past 100 years.


This lesson will also review metric measures as a means of quantifying continuous traits.


Exercise 1


Organize Your Initial Knowledge



What is the metric system of measurement?

The metric system is a classification system which assigns a numerical quantity to a physical quality of an object. The metric system is based on the number ten and can be used to quantify such parameters as length, mass, and volume.




Arrange the following quantities from largest to smallest.

1 centimeter, 1 millimeter, 1 kilometer, 1 meter

1 kilometer, 1 meter, 1 centimeter, 1 millimeter.


Are the above units of mass or units of distance?

These are units of distance. The gram is the metric unit of mass.


What is the approximate equivalent commonly used in the US of each of these quantities in the English system of measure?

1 centimeter ~ 2.54 inches

1 millimeter ~ 1/32 inch

1 kilometer ~ 0.6 miles

1 meter ~ 3 feet


What is the approximate metric equivalent for each of the following?

1 pound ~ 450 grams

1 cup ~ 1/8 liter

1 quart ~ 1 liter


What advantages are provided by the metric system compared to the English system?

Since it is based on the decimal system, it is easier to make calculations and dimentional transformations with the metric system. It's difficult to remember how many feet are in a mile. Yet with the metric system, you can make these sorts of conversions simply by moving a decimal point! Also, the metric system typically has more sub-divisions, making it more of a precise measurement scale. Because of these obvious advantages, the metric system is almost always preferred in scientific analyses.


Exercise 2

Measuring Continuous Variation

  Metric measurements are preferred in science because they are more precise due to the increased number of regular intervals. It is a good idea to become very familiar with metric measures.
    Using an appropriate metric tool, obtain the following measurements:
    1. Your height, in centimeters: __________
    2. Your arm span, finger tips to finger tips, in centimeters: __________

Your weight, in kilograms: __________ (you will have to calculate this unless you have a metric scale available)

The conversion formula is:

(weight in pounds) / 2.2 = (weight in kilograms)

    4. Your forearm length, inside of your elbow to wrist, in centimeters: ________

The length of your foot, heel to longest toe, in centimeters: ________

The length of your forearm is approximately equal to the length of your foot!


 Exercise 3

Pooling Data for Precision


  1. Group data is generally more precise than individual data because small errors in measurement are averaged. Whenever possible, we perform experiments individually, and then summarize the results of all groups. The group results should generally be more meaningful than any individual result.



Let's use our data to check the following hypothesis:

Arm span is approximately equal to height.

Do you think this hypothesis is plausible? Why?


To Do


To test it, first we need to summarize the data for the whole class and then obtain an average arm span and average height for all individuals (use Table 3).

Table 3. Arm Span & Height of Class Members in Centimeters
Initials  Arm Span, cm. Height, cm. 



Next, interpret the data. Do the class averages support or refute the hypothesis?



To see how continuous traits vary, draw two different lines on the graph below (with metric measures) to summarize (a) the arm spans and (b) the heights of all individuals in the class. Or, use graph paper and set up your graph like the example below (Figure 3).

Figure 3. Height and Arm Span of Individuals, cm.

 To Note


Your graph should have the following characteristics:

a. an informative title

b. a vertical axis, labeled with name of measure (Lgth, cm)

c. a horizontal axis labeled with # of individuals

d. values that are spaced equidistantly

e. a dot at the intersection of each height with number of individuals at that height.



Compare the ratio of arm span to height by graphing one against the other (Figure 4). What do you predict the graph will look like?

Given that arm span is approximately equal to height, the graph should be linear (slope = 1).

Figure 4. Arm Span versus Height for All Class Members


 Exercise 4

Your Knowledge of the Metric System


  1. 15 meters equals_1500 _centimeters.
    2. 1500 meters equals how many kilometers? _1.5 Km_


Relate metric to English measures in the following:







Two and one-half centimeters are approximately equal to

a. an inch

b. a foot

c. a yard

    4. One meter is approximately equal to a _yard_.
    5. A liter is similar to a _quart_ (but slightly bigger).
    6. A kilometer is six-tenths (slightly more than half) of a __mile__.

A kilogram is approximately equal to

a. 1 pound

b. 2.2 pounds

c. 10 pounds


Five grams is approximately equal to a

a. dime

b. nickel

c. quarter

d. half dollar


Postlethwait, J. H. & Hopson, J. L. (1995). The Nature of Life, Third Edition. San Francisco: McGraw-Hill, Inc.

Klare, Roger. Gregor Mendel: Father of Genetics. Enslow Publishers. 1997.

Landa, Norbert and Patrick Baeuerle. Ingenious Genes. Barron's Juveniles. 1998.

Marshall, Elizabeth L. The Human Genome Project: Cracking the Code Within Us. Franklin Watts, Inc. 1997.




Section B: Scientific Inquiry

Grade K-2 Benchmark 2 of 4

Tools such as thermometers, magnifiers, rulers, or balances often give more information about things than can be obtained by just observing things without their help.

Section C: The Scientific Enterprise

Grade K-2 Benchmark 2 of 3

In doing science, it is often helpful to work with a team and to share findings with others. All team members should reach their own individual conclusions, however, about what the findings mean.

Grade 3-5 Benchmark 1 of 3

Science is an adventure that people everywhere can take part in, as they have for many centuries.



Section A: Diversity of Life

Grades K-2 Benchmark 1 (of 3)

Some animals and plants are alike in the way they look and in the things they do, and others are very different from one another.

Section B: Heredity

Grades K-2 Benchmark 1 (of 2)

There is variation among individuals of one kind within a population.

Grades K-2 Benchmark 2 (of 2)

Offspring are very much, but not exactly, like their parents and like one another.

Grades 3-5 Benchmark 1 (of 2)

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.

Grades 3-5 Benchmark 2 (of 2)

For offspring to resemble their parents, there must be a reliable way to transfer information from one generation to the next.

Grades 6-8 Benchmark 1 (of 3)

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.


Section A: Human Identity

Grades K-2 Benchmark 1 (of 3)

People have different external features, such as the size, shape, and color of hair, skin, and eyes, but they are more like one another than like other animals.

Section B: Human Development

Grades K-2 Benchmark 1 (of 2)

All animals have offspring, usually with two parents involved. People may prevent some animals from producing offspring.


Chapter 12: HABITS OF MIND

Section B: Computation and Estimation

Grades K-2 Benchmark 5 (of 5)

Make quantitative estimates of familiar lengths, weights, and time intervals and check them by measurements.

Section D: Communication Skills

Grades 6-8 Benchmark 1 (of 5)

Organize information in simple tables and graphs and identify relationships they reveal.