© 1999 ENSI (Evolution & the Nature of Science Institutes) www.indiana.edu/~ensiweb
This material may be copied only for noncommercial classroom teaching purposes, and only if this source is clearly cited.

Return to Evolution Lessons





Therese Passerini
ENSI 1990
Columbus Ohio

Presented at NABT Convention in Reno, Nov. 1998

This lesson is now available from Science Kit & Boreal Labs (#46249-00 "Lactose Tolerance" for $45.95)


Natural Selection


Therese Passerini
presented for NABT Nov. 4, 1998

Info for Teacher (5 pages), Key
(1 page) and
Student Worksheets (5 pages)
are available for downloading in PDF format
Just click on the blue "codes" at bottom of this page



The milk produced by human mammary glands contains about the same percentage of fat and less protein than cow's milk, but it has more carbohydrate than that of other mammals. This carbohydrate is in the form of lactose, a disaccharide. Mammalian infants depend on their mother's milk for nourishrnent and immunological protection. As a fresh, temperature-controlled, uncontaminated food source, the milk from mother's breast is the best at meeting the needs of the infant.

Except for certain populations of Western humans, milk consumption stops ( or is greatly reduced) with weaning. It also happens that in animals and most humans there is a decline in the level of production of lactase with aging. Lactase is the enzyme that digests milk sugar. (Curtis).

Since the sugar lactose is known to occur only in milk, the advantages offered during infancy by this nutrient have been studied. A component of the disaccharide, called galactose, is not found free in nature, and in the synthesis of lactose by the body a molecule of glucose is converted to galactose then joined with another glucose molecule. A special advantage of galactose in infant nutrition may be as a building material in brain formation (glycolipids usually contain galactose). During digestion lactose is digested and absorbed more slowly than other sugars and may improve calcium retention and absorption for increasing the mineral content of bone. In addition, growth factors, for Lactobacillus bifudus present in human milk may establish and maintain intestinal flora that are favorable. (Kleiner and Orten)

SYNTHESIS OF LACTOSE ( Keeton and Gould)


Galactose and glucose are combined in b-linkage to yield the sugar lactose. The enzyme lactase, secreted by the brush border cells of the intestinal epithelium, is capable of splitting the molecule by hydrolysis. Without lactase the lactose would pass into the colon undigested. (Keeton and Gould)


Since in most persons the body does not continue to secrete large amounts of lactase enzyme after weaning, milk is indigestible by a large majority of the world's population. The term "primary adult lactase deficiency" refers to the NORMAL condition for most people in whom lactase production has practically ceased. The decline begins in youth and continues to decline, but the onset of symptoms varies as to age. There is also a condition known as "secondary lactase deficiency" which is temporary and the result of a nutritional deficiency or digestive tract disease.

Primary adult lactase deficiency leads to lactose malabsorption. Lactose intolerance is a condition that results from the malabsorption and produces symptoms that may cause the person to avoid milk and other dairy products (some authors would prefer to change the terrn to lactose malabsorption). (Hein) This is not an allergy to milk. Allergy is associated with hypersensitivity to the protein component of milk. (Houts)



Studies concerning the development of lactose deficiency centered on an induction hypothesis have not been supported by the evidence. Because the age at which the lactase activity declines varies (from 3 years of age to more than 20 years), it was postulated that the presence of lactose in the diet induced the intestinal cells to continue lactase production. However, long-term feeding of lactose to people who are malabsorbers did not result in an increase in lactase activity. Also, depriving persons who are absorbers of lactose for months at time, did not produce a loss of lactase activity.

Although in the entire human population the trait may actually be a genetic polymorphism, for the purpose of the study of a family, it may be considered as autosomal recessive. (Allen and Cheer)

At this time it appears that the loss or persistence of lactase is a developmental change that is part of a genetic program. The genetic difference seems to be in a regulatory gene and not a structural gene. The most favored model is that of an autosomal recessive condition. Pedigree information from families provide support for this explanation. (May)

The following three sample pedigrees of American Indian families illustrates this concept. Open symbols = lactose absorbers. Closed symbols = lactose malabsorbers (from Newcomber et al). (May)

Evolution in pastoral ancestors that kept dairy animals, in populations of Europeans and some Africans, is thought to have produced the modern people who are lactose absorbers throughout adult life. There may have been a selective advantage in that group during times when milk and dairy products were relied on almost exclusively for nutrition during shortages of other foods (Houts). About 10,000 years have passed since the milking of domestic animals was begun. Nigeria is a case that demonstrates the significance of this lifestyle on human evolution. In the southern region, where conditions are not favorable for cattle, milk is not part of the diet. People there develop lactose intolerance. In contrast, a nomadic tribe that has been raising milk cattle for thousands of years remain lactose tolerant. Most African Americans are descendants of the nonpastoral tribes from Western Africa and do not tolerate lactose well. (Keeton and Gould)

In Caucasians only about 15% develop lactose intolerance while 80-90% of the African American and Asian populations are affected. This has strong implications in relief efforts to aid those suffering from starvation in various parts of the world. Powdered milk has been a favorite food for shipping to needed areas but it frequently causes illness in those people the aid is meant to help. The sickness can lead to further malnutrition due to the symptoms and resulting malabsorption (Martini).

Calcium deficiency can be a dietary concern for seven out of ten African Americans who are lactose intolerant and who should avoid eating large quantities of dairy products. Foods rich in calcium that could be substituted in the diet include canned fish with bones, dark green vegetables and beans. (Meister)


The presence of lactose in the colon is a boon for organisms inhabiting that part of the digestive tract. However, the lactose intolerant patient experiences acute gastrointestinal distress as a result. The lactose acts as an osmotic laxative producing diarrhea. (Greifru) This is usually accompanied by gas pains, cramping, and bloating, Even small amounts of dairy products or a single glass of milk can cause extreme discomfort . The onset of symptoms normally occurs within 15 minutes to 3 hours after consuming the offending food. (AAP)

Studies have been done to learn whether the malabsorption of lactose and accompanying symptoms may be associated with reduced absorption of other nutrients as well. Results at this time are not conclusive. (Houts)

One can submit to clinical tests to determine his/her tolerance for lactose. The tests are of two types: one measures hydrogen breath and the other is a lactose-loading test following a period of fasting. Blood glucose levels depend on the absorption of the lactose break down products. (May) When the lactase enzyme is present in sufficient quantity, lactose in the digestive system is broken down into glucose and galactose. In the liver the galactose is changed to glucose and the level of blood glucose is elevated. If the lactose is not digested this rise is not detected. (NIH)

Another option is to test yourself in a simple experiment. For two days do not eat any foods containing lactose. Drink two glasses of skim milk on an empty stomach. If the symptoms develop soon afterward, this indicates lactose intolerance. (McCall's)


Knowledge of the causes and carefully monitoring one's intake of lactose can reduce discomfort. Some people can tolerate small amounts of lactose without too much of a problem by eating it with a meal. Whole milk is tolerated better than skim milk. Plain unflavored yogurt with active bacterial cultures may be a good substitute, however, frozen yogurt usually has been pasteurized and the bacteria eliminated. (Hunter and McCarthy) Lactose is an additive in some medications, vitamins, and processed foods. Read labels and be aware of its presence.

Several dairy products have been developed to address the problem of lactose intolerance. It is possible to buy lactose-reduced products that have had lactase enzyme added. Most of these remove up to 70% of the lactose in the milk. Persons who still have symptoms after using such products can buy a tablet or liquid to add to milk and rernove more of the lactose. The source of the lactase enzyme is a cultured bacteria. (AAP, McCarthy, and Gorman)

In summary, persons who are lactose intolerant have three options regarding dairy product consumption. The first is to consume less than their threshold amount of lactose; a second is to limit usage to fermented forms such as yogurt and hard cheeses; and the third is to add lactase to fluid milk prior to use. (Houts) If an adequate amount of calcium in the diet is a concern, broccoli, soybean products, and canned salmon with the bones are optional natural sources of this mineral.



Allen, John S. and Cheer, Susan M.(1996) The Non-Thrifty Genotype.Current Anthropology. Vol 37, No. 5, p. 831-842. Dec. 1996.

American Family Physician. (1984). Lactose Intolerance. Vol. 30 p. 272.

Cerrato, Paul L. (1992). The Patient's Eating --Why is He Losing Weight? RN Vol.55 p. 77-80 April 1992.

Curtis, Helena. (1993). Biology. Worth Publishers, Inc.New York, New York. p. 678.

Gorman, Christine. (1993). Milking a Fad. Time. Vol 41 p. 53. May 17,1993.

Greifzu, Sherry. (1989). An inability to digest milk sugar. RN Vol 52 p.93-94, Sept. 1989.

Hein, Pam and Weinzierl, Kathy. Taking Another Look at Lactose Intolerance. The Pantagraph, Bloomington, Il. April 22,1996.

Houts, Sandra S. (1988). Lactose Intolerance. FoodTechnology. Vol 42 p.110-113, March 1988.

Hunter, Beatrice Trum. (1986). Lactose Intolerance. Consumer's Research Magazine Vol.69 p. 8-9, March 1986.

Keeton, William T. and Gould, James L. Biological Science. W.W. Norton and Company, New York. 1994.

Kieiner, Israel S. and Orten, James M. Biochemistry. The C.V. Mosby Company, St. Louis, 1966.

Martini, Frederic. FundamentalsofAnatomyandPhysiology. Prentice Hall, New Jersey, 1992.

May, Daniel S. An Interdisciplinary Look at Lactose Malabsorption. The American Biology Teacher. Vol. 47 p. 154-9 March 1985.

McCall's, When Milk Makes You Sick. Vol. 113 p. 62-63. Jan. 1986

McCarthy, Paul. Many are Cold, Few are Frozen. American Health. Vol. p. 138-9. July/August 1988.

Meister, Kathleen. Lactose Intolerance. Essence. Vol.16 p.20 Dec.1985.

Morholt, Evelyn and Brandwein, Paul. A Sourcebook for the Biological Sciences. Saunder Publishing, New York.1986.p.177.

Tamarin, Robert. Principles of Genetics. Wm. C Brown Publishers, lowa,1991

Therrien, Lois, Making the Undrinkable Thinkable. Business Week.p. 40 May 14, 1990


Research done and lab developed by Therese Passerini July,1995.

NOTE: The chemical test for glucose may be altered depending on materials available. Use the same type of milk throughout the test: whole, or nonfat even for the lactose-reduced milk. If you prefer, use Lactaid drops and reduce the milk as part af the lab procedure.

Instead of using the Benedict's solution method, glucose test strips may be used.




1. Lactose is a disaccharide type of sugar. It is made up of two subunits of monosaccharide sugars - galactose and glucose

2. The results of the test indicated the breakdown of lactose by the enzyme lactase.

3. Hydrolysis is the chemical reaction in which a bond is broken by the addition of a water molecule. One part of the molecule takes the OH and another part takes the H atom. The result is two separate molecules.

4. the hydrolysis of lactose.

5. galactose and glucose

6. the lactase enzyme acts on the beta linkage bond and causes the disaccharide lactase to break by hydrolysis

7.From the pedigree charts it appears that lactose intolerance is an autosomal recessive trait in a family. Children who inherit two recessive genes will be lactose intolerant.

8. The data from the world map suggests that lactose tolerance may have had its beginning in certain tribes in Africa. It is also evident in Northern Europe.

9. The early American settlers were mainly from European countries.

10. The founder effect can be important in establishing the gene pool for an area being colonized. The early settlers may have been lactose tolerant and their descendants exhibit this trait. Later migrants to the U.S. were from other populations and were mostly lactose intolerant.



1. The symptoms of lactose intolerance include gas, bloating, and diarrhea. Since bacteria in the intestine receive an undigested quantity of lactose in the lactose intolerant person, the bacterial activity increases with this food supply. There may be a temporary increase in the production of gas by the bacteria.

2. The parents may have been heterozygous (Ll) in their genotype. Each may pass on a recessive gene to a child.(ll) 25% chance if they are heterozygous.

3.The process of evolution involves natural selection. A factor in the environment is responsible for the survival and reproductive succes of individuals with adaptive characteristics. Members of the population with an adapative advantage would be the more fit in terms of leaving more offspring. During periods of drought, cold weather, disease or other conditions affecting the success of crops, individuals who were lactose tolerant may have had an adaptive advantage. In those isolated popualtions they may have left more offspring with the lactose tolerant trait.

4. The earliest settlers in the United States were mostly descended from European countries, where cattle and dairy products were used a a main source of nutrition. Natural selection may have produced populations of lactose tolerant individuals there and when they migrated the founder effect established a gene pool of lactose tolerance.

[Student Handout]


BACKGROUND INFORMATION: What is the topic of this ad? Who would use such a product? Why would they need it?

According to statistics approximately one-third of all Americans feel ill after consuming milk and other dairy products. We are taught that we should "drink our milk" by our parents. "It is good for you, will help you have strong bones and teeth, and grow healthy and tall." The nutritional value of milk is a fact. However, most animals stop drinking milk after they are weaned and their body chemistry changes so that they can no longer digest the sugar in milk. Worldwide this is also true for the human population. That is what is normal. It is actually unusual for adults to be able to digest milk easily.

In this lab activity you will learn more about lactose intolerance You will understand the chemical structure of lactose, why a person may have difficulty digesting milk, how lactose tolerance may have started, why so many Americans can digest it, and how lactose-reduced products can help people.


MATERIALS: 3 test tubes and rack Benedict's solution

distilled water boiling water bath

milk 3 10-ml pipettes

100% lactose-reduced milk


PROCEDURE: Part A. Test for lactose

1. Place 10 ml of distilled water in test tube A

Place 10 ml of milk in test tube B

Place 10 ml of 100% lactose-reduced milk in test tube C


2. Add 5 ml of Benedict's solution to each test tube

Heat the tubes for 3 minutes in the boiling water bath

3. Compare the tubes

4. Record the results in the table:

Test Tube: A B C


PROCEDURE: Part B. Lactose Intolerance in Families

1. Fill in the pedigree charts for three families. Parents Joe and Lucy Anderson are both lactose intolerant. The four children: Alicia, Eric, Ben, and Rodney are all lactose intolerant.
* Remember that we shade in persons with the phenotype being studied.

2. Parent Mary Wallace is lactose intolerant and her husband, John is lactose tolerant. They had five children. Ann, David, and Dan are lactose intolerant. Nancy and Scott are lactose tolerant.

3. Parents Mike and Donna Miller are both lactose tolerant. Their children Fred, Niles, and Linda are lactose tolerant. The other child, Jane i s lactose intolerant.

PROCEDURE: Part C. Study the map of the world. The numbers indicate the percentage of the population in each area that is lactose intolerant.


USE THE MAP DATA TO MAKE A GRAPH (draw vertical and horizontal axes below, using the labels for the axes as given)








of origin



Percentage of lactose intolerance




1. Which type of sugar is lactose?


2. What did the results of the Benedict's test for sugar mean?



3. Define hydrolysis.



4. What reaction is catalyzed by the enzyme lactase?



5. What are the products of lactose digestion?



6. What is the effect of adding lactase enzyme to milk?



7. Does it appear that lactose intolerance (or tolerance) is an inherited characteristic? Explain your answer.



8. Where did lactose tolerance probably first originate?



9. Which parts of the world were the original home of the early American settlers?



10. How can migration and gene flow affect a population?




1. When the body does not secrete lactase enzyme in the intestine, the lactose sugar is not digested. Bacteria that are a normal part of the colon use the lactose for food and produce gas. How does this contribute to the symptoms of lactose intolerance?



2. How can two parents who both tolerate lactose (digest it) produce children who do not?


What is the most probable mode of this inheritance?


Show your reasoning with a Punnett square or pedigree chart.









3. During the past 10,000 years agriculture has been important to human populations. In sorne isolated areas crops did not perform as well or the climate did not permit relying on them year round. In these places animals and their milk were the main food supply. Use your knowledge of evolution and natural selection to explain how some populations may have become lactose tolerant.






4. What would explain the statistics that approximately 30% of all Americans are lactose intolerant compared to other parts of the worid where that number is more than 80%?




PDF Version of this material is available for downloading.


 Milk Intolerance Lesson, Teacher Background (5 pages,)
(447 K, lots of figures, long download)

Milk Intolerance Lesson,Student Info. and Worksheets (5 pages)
(495 K, lots of figures, long download)

Key to Worksheet Questions (1 page)