Lactase-Persistence-Lesson-Student PDF

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The Making of the Fittest: Got Lactase? The Co-evolution of Genes and Culture

Lesson Student Handout !

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LACTASE PERSISTENCE: EVIDENCE FOR SELECTION INTRODUCTION The ability of some human adults to digest lactose—the sugar in milk—is evidence of recent human evolution. All mammalian babies can digest lactose, using an enzyme called lactase. By adulthood, however, most mammals stop producing this enzyme. One exception is a minority of human adults who retain an active lactase enzyme. These “lactase-persistent” individuals have a mutation in their DNA that keeps the lactase gene turned on into adulthood, and they are therefore able to digest lactose. Evidence from different fields suggests that these lactase-persistence mutations arose in the last 10,000 years in different populations and increased in frequency by natural selection. In this activity, you will explore some of this evidence by watching three short video segments and answering questions about the scientific data presented in three figures.

PROCEDURE PART 1: What Is Lactase Persistence? One of the mutations giving rise to lactase persistence first arose about 7,500 years ago in a population in what is now Hungary. This mutation consisted of a single nucleotide change from a cytosine to a thymine in a genomic region that controls the expression of the lactase gene. The mutation increased in frequency throughout some parts of Europe. Other “hot spots” of lactase persistence include parts of Asia and east and west Africa. Here, the responsible mutations arose more recently than the European mutation did. In all these populations, the ability to digest milk as adults likely provided a survival advantage to individuals with the lactase-persistence allele.

A mutation is a change in a sequence of DNA. When a measurable proportion of individuals has that particular change, it is referred to as a genetic variant or genetic polymorphism. Functional variants often change the section of DNA that determines the amino acid sequence of the protein produced or affect how the gene is turned on or off. One version of a DNA sequence is referred to as an allele. So individuals who are lactase persistent have one particular allele or variant and individuals who are not lactase persistent have a different allele or variant.

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Published August 2015 Page 1 of 7

The Making of the Fittest: Got Lactase? The Co-evolution of Genes and Culture

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! Figure 1. Global Distribution of Lactase Persistence. Worldwide, only about 35 percent of adults can digest lactose, and most are concentrated in particular geographic regions or “hot spots”: northern Europe, parts of east and west Africa, the Middle East, and South Asia. (Source: Adapted from Curry, Andrew. “The Milk Revolution.” Nature 500 (2013): 20–22. doi:10.1038/500020a. For an animated version of the map, see the HHMI film Got Lactase? The Co-evolution of Genes and Culture, http://www.hhmi.org/biointeractive/making-fittest-got-lactase-co-evolution-genes-and-culture, from time stamp 4:12 mintues to 4:26 minutes.)

Interpreting the Figure 1. Why would a mutation leading to lactase persistence become common in a population?

This mutation would become more common in a population if being lactase persistence given an advantage over the other. Since being able to consume milk was a matter of survival the people who couldn't would die off and reduce in population leading to more people having this mutation to be able to consume and process lactase 2. Use the map in Figure 1 to predict where you might find early evidence of dairying (i.e., using animal milk).

Places like Western Africa and Northern Europe would have early evidence of dairying because of the fact that lactase persistence was an integral part to surviving before modern civilization.

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The Making of the Fittest: Got Lactase? The Co-evolution of Genes and Culture

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! 3. Provide a scenario to explain how different global hot spots of lactase persistence could be caused by the same mutation.

The mutation could possible be cause due to migration from people with the lactase persistence allele. But there is possibility that if the surviving pressure for drinking and consuming dairy was there, the mutation could've just possibly arise independently.

4. Create a scenario to explain how different global hot spots of lactase persistence could be caused by different mutations.

Different mutations can cause the same things, if the pressure to survive from drinking and consuming diary was there, it is very possible that there can be multiple different variants that arise geographically.

5. Design an experiment to determine whether lactase persistence in the different global hot spots was caused by one or several mutations.

Collect DNA from the different locations such as Western Africa and Northern Europe and find the genome sequence and compare the differences between the two.

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The Making of the Fittest: Got Lactase? The Co-evolution of Genes and Culture

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! PART 2: Evidence of Selection Chromosome%pair%

Region%of%DNA%near%lactase%gene%

Individuals%

Lactase%persistence%muta)on%

Many% genera)ons%

Many%more% genera)ons%

Neutral%markers%

1. An%adap)ve% muta)on%for%lactase% persistence%arises%in% a%popula)on.%%

2. Through%posi)ve% selec)on,%aCer%many% genera)ons,%muta)on% becomes%common%in% the%popula)on.%

3. ACer%many%more% genera)ons,% recombina)on% breaks%up% homozygous%regions.%

Figure 2. Evidence of a Selective Sweep for Lactase Persistence. Each individual has two copies of each chromosome, including the region around the lactase-persistence gene, shown as the white rectangles next to each individual. Lactasepersistent individuals have red silhouettes; lactase-nonpersistent individuals are blue. Comparing DNA sequences around the lactase gene in different individuals has revealed several neutral variations, or markers, which are denoted by different colored bands. In the first column, the red band indicates a genetic variant or mutation associated with lactase persistence—the lactase-persistence allele. When natural selection favors a beneficial allele, such as the lactasepersistence allele (red band), that allele is more likely to be passed on from one generation to the next. Over many generations, the beneficial allele increases in frequency. The frequencies of the nearby neutral markers (orange and yellow bands) also increase. These markers do not provide any selective advantage but are “swept” along with the beneficial allele. Column 2 shows evidence of this process as a reduction in genetic diversity in this particular region of chromosome 2. Over time, the association between the beneficial allele and neutral markers breaks down as a result of recombination, and the evidence for a selective sweep eventually disappears. (Source: Adapted from Tishkoff, Sarah A., Lecture 2 – Genetics of Human Origins and Adaptation. 2011 Holiday Lectures on Science series Bones, Stones, and Genes: The Origin of Modern Humans. http://media.hhmi.org/hl/11Lect2.html?start=50:42&end=51:47.)

Interpreting the Figure 1. In column 1, the red band represents the lactase-persistence mutation. Why is it more prevalent in column 2, which shows the same DNA region in the same population after many generations?

It is more prevalent because this gene allows you to consume lactase with out any negative side effects. This leads to these individuals staying alive for a longer amount of time, and through the process of natural selection they were able to pass down this gene with greater diversity. www.BioInteractive.org!

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The Making of the Fittest: Got Lactase? The Co-evolution of Genes and Culture

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! 2. In column 2, why do the orange and yellow bands always border the red band? What do the orange and yellow bands represent?

The orange and yellow bands are more prevalent because they are physically linked to the red allele that are "swept" along when the red allele gets passed down. They represent nothing and are really just markers for the re allele of lactase persistence. 3. How many individuals in column 2 are homozygous for the lactase-persistence allele? How many are homozygous for the surrounding region (i.e., the white rectangle)?

Only 3 of 5 as they are homozygous for the lactase persistence allele.

4. Why would scientists want to identify regions in the genome that are homozygous in many individuals in a population (i.e., regions of homozygosity, or reduced diversity)?

This can help us understand how lactase persistence came to be, and what genome it resides in. It may be possible in the near future to be able to replicate lactase persistence to make a larger portion of the population lactase persistent 5. How many individuals in column 3 are homozygous for the lactase-persistence allele? How many are homozygous for the whole region?

6.

How do you explain the differences between columns 2 and 3?

As more and more generations arise there are more and more variants of the lactase persistence allele because mating will happen with people from other regions making the gene more "convoluted"

ADDITIONAL DISCUSSION QUESTIONS 1. Is lactase persistence a dominant or recessive trait? Use Figure 2 to explain your answer. 2. What does the length of the region of homozygosity around an allele reveal about its evolutionary history? 1. dominant as the red silhouettes indicating lactase persistence only really have one allele for lactase

persistence meaning that only one allele for lactase persistence is need to be persistent. 2. Homozygosity was more prevalent because of the fact that populations were smaller in earlier generations

but then recombination, or mating with people in different geographic locations led to more heterozygous individuals. www.BioInteractive.org!

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The Making of the Fittest: Got Lactase? The Co-evolution of Genes and Culture

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! PART 3: Lactase Persistence in East Africa Dr. Sarah Tishkoff and colleagues investigated lactase persistence in East Africa. The mutation leading to lactase persistence in this population is different from the one found in lactase-persistent individuals from Europe. Both genetic changes occur in the regulatory region of the lactase gene.

! Figure 3. Evidence of Selection for Lactase Persistence in African and European Populations. Lactase-persistent individuals in African and European populations have large regions of homozygosity around the lactase-persistence alleles. Each blue and green bar represents a region of homozygosity in one individual; the lengths of these regions are shown at the bottom of the figure. Large areas of homozygosity indicate strong, recent selection. (Source: Adapted from Tishkoff, Sarah A., Lecture 2 – Genetics of Human Origins and Adaptation. 2011 Holiday Lectures on Science series Bones, Stones, and Genes: The Origin of Modern Humans. http://media.hhmi.org/hl/11Lect2.html?start=50:42&end=51:47; and Figure 6 in Tishkoff, Sarah A., et al. “Convergent Adaptation of Human Lactase Persistence in Africa and Europe.” Nature Genetics 39 (2007): 31-–40. doi:10.1038/ng1946.2007.)

Interpreting the Figure 1. In the African population, why do lactase-persistent individuals have large areas of homozygosity compared to individuals who are not lactase persistent?

Because this allele was relatively new leading to recombination not being able to disrupt to homozygosity.

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The Making of the Fittest: Got Lactase? The Co-evolution of Genes and Culture

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! 2. Use evidence to explain whether either the African or European population shows evidence of strong, recent selection in the genome.

African culture shows evidence of strong recent selection because of the fact that there are a large amount of regions with a high frequency of homozygosity indicating that recombination has not happened yet, which indicates more recent selection. 3. The African and European populations have different lactase-persistence alleles. Use data from Figure 3 to formulate a hypothesis about which allele originated more recently.

African populations had the lactase persistent allele generate more recently due to the fact that the graph for homozygous individuals has a higher frequency then the other.

4. What additional evidence might support this hypothesis?

Samples from African artifacts such as pots compared to European ones, and the presence of milk in those pots

PART 4: Summary Discussion 1. What is evolution? The change of allele frequencies over time. 2. Do the data on lactase persistence support the idea that humans are evolving? Use data to support your answer. Yes as there is evidence showing recent change of allele changes in the context of lactase persistence 3. Does lactase persistence provide an example of a selective sweep? Use data to support your answer.

There is a lower frequency of different alleles as shown by the sheer amount of homozygousity in lactase persistence. Meaning that there is evidence for a selective sweep.

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