Created by: CK-12/Adapted by Christine Miller
Case Study Conclusion: Your Genes May Help You Save a Life
As you have learned in this chapter, humans are much more genetically similar to each other than they are different. Any two people on Earth are 99.9 per cent genetically identical to each other — but the mere 0.1 per cent that is different can be very important, as in the case of bone marrow donation to treat diseases such as leukemia. A good match must exist between a bone marrow donor and recipient in genes that encode for human leukocyte antigen (HLA) proteins. As you have learned, antigens are molecules — often on the surface of cells — that the immune system uses to identify foreign invaders. If a patient receives a bone marrow transplant from a donor that has different types of HLAs than the patient does, antibodies in their immune system will identify the antigens as nonself and will launch an attack on the transplanted cells. Also, since bone marrow produces immune cells, antibodies in the transplanted tissue can actually attack the patient’s own cells using the same mechanism.
As you have also learned, a good HLA match is often difficult to find, even between full siblings. Finding a match in the registries is particularly hard for non-Caucasian people — and even harder for people from multiethnic backgrounds, such as seven-year-old Mateo, who you read about in the beginning of this chapter. Mateo is of African, Japanese, and Caucasian descent — a relatively rare combination. Because HLA matches are more likely to occur between people of the same ethnicity, the donor registries would ideally have sufficient potential donors from every ethnicity and ethnic combination. Unfortunately, some ethnicities are not sufficiently represented in the donor registry. According to the U.S. National Marrow Donor Program, while 97 per cent of Caucasian patients find a match, the match rate drops to 83 per cent for Hispanic or Latino patients and 76 per cent for African American or black patients. Multi-ethnic patients generally have an even harder time finding a match because the relative rareness of their particular ethnic combination in the general population makes it less likely that enough people of their same ancestry are registered donors.
As you learned in this chapter, human variation has historically been classified in several different ways, some of which resulted from or have contributed to racism. Most biological traits in humans exist on a continuum, and attempting to create biological categories of race based on discrete categories using a handful of traits is generally arbitrary and inaccurate. Gene flow through migration and mating between populations, genetic drift, and natural selection results in a gradual, clinal distribution of many human traits, rather than discrete categories. Mateo, for example, cannot be neatly placed into one racial category or another. Race and ethnic identity, however, remain important social and cultural concepts.
Mateo’s ancestry does play a role in determining his specific types of HLA proteins, and he is more likely to find a bone marrow match with a donor of an ethnic background similar to his own. Although there is much more genetic variation within races than between races, HLA types tend to correlate with ethnicity more than some other traits. As you have seen throughout this chapter, some environmental factors in different geographic regions have provided strong natural selection pressures, resulting in the development of genetic differences between people whose ancestors came from different areas. For example, adaptations to differing UV levels, diseases, altitudes, and climates all likely led to the evolution of human variations in skin colour, blood cells, and body morphology. This type of association between race and ethnicity and genetic variation is similar to the link between ethnicity and HLA type.
Mateo’s family was not able to find a match for him in the bone marrow registries, unlike the little boy pictured in Figure 6.9.1, but they are not giving up hope. His parents have started working with organizations to host bone marrow drives, where potential donors can provide cheek swabs to add themselves to the donor registry. His parents have contacted the news media with Mateo’s story, and family and friends are getting the word out on social media that more donors are needed, particular those with Mateo’s specific combination of ethnicities. They hope that even if they are unable to find a match for Mateo, bringing awareness to the issue may increase the ethnic diversity of the donor registry to save other lives.
You Can Help!
According to the Canadian Blood Services, more donors are needed to join the bone marrow registry. Currently, there is a need for more young male donors: male stem cell donors are more likely to be matched with recipients because they offer better patient outcomes after transplant. There is also a need for donors with diverse ethnic backgrounds, particularly Aboriginal, Hispanic, African-Canadian, Filipino, and more. DIverse donors are needed to acheive the closest possible match for HLA between the donor and the recipient.
Leukemia is not the only disease in which treatment involves bone marrow transplant — this course of action is often taken for conditions such as:
- Aplastic anemia
- Inherited immune system disorders
- Inherited metabolic disorders
- Bone marrow diseases
Are you registered? If not, it is a relatively simple process that could save someone’s life. A cheek swab is all that is initially needed. Only about one in 430 potential donors will actually be matched with a patient, and if you are chosen, it means that you are one of the only people on Earth who can donate to this patient because of your genetic similarity! If you decide to donate, bone marrow will either be surgically removed from the back of your pelvic bone, or blood-forming cells will be removed non-surgically from your bloodstream. Most donors are able to return to their normal activities one to seven days after donation — a small price to pay for potentially saving someone’s life!
Stem cell donation: Step by step, hemaquebec1998, 2015.
Marrow donors talk about donating and the donation process, Be The Match, 2012.
In this chapter, you learned about human variation and its origins. Specifically, you learned that:
- No two human individuals are genetically identical (except for monozygotic twins), but the human species as a whole exhibits relatively little genetic diversity relative to other mammalian species. Genetically, two people chosen at random are likely to be 99.9 per cent identical.
- Of the total genetic variation in humans, about 90 per cent occurs between people within continental populations, and only about 10 per cent occurs between people from different continents. Older, larger populations tend to have greater genetic variation, because there’s been more time and there are more people in which to accumulate mutations.
- account for most human genetic differences. Allele frequencies for polymorphic genes generally have a clinal, rather than discrete, distribution. A minority of alleles seem to cluster in particular geographic areas, such as the allele for no antigen in the Duffy blood group. Such alleles may be useful as genetic markers to establish the ancestry of individuals.
- Knowledge of genetic variation can help us understand our similarities and differences. It can also help us reconstruct our evolutionary origins and history as a species. For example, the distribution of modern human genetic variation is consistent with the out-of-Africa hypothesis for the origin of modern humans.
- An important benefit of studying human genetic variation is learning more about the genetic basis of human diseases. This should help us find more effective treatments and cures.
- Humans seem to have a need to classify and label people based on their similarities and differences. Three approaches to classifying human variation are typological, populational, and clinal approaches.
- The typological approach involves creating a system of discrete categories, or races (no longer used). This approach was widely used by scientists until the early 20th century. Racial categories are based on observable phenotypic traits (such as skin colour), but other traits and behaviors are often mistakenly assumed to apply to racial groups, as well. The use of racial classifications often leads to racism.
- By the mid-20th century, scientists started advocating a population approach (no longer used). This assumes that the breeding population, which is the unit of evolution, is the only biologically meaningful group. While this approach makes sense in theory, in reality, it can rarely be applied to actual human populations. With few exceptions, most human populations are not closed breeding populations.
- By the 1960s, scientists began to use a clinal approach to classify human variation. This approach maps variation in the frequency of traits or alleles over geographic regions or worldwide. Clinal maps for many genetic traits show variation that changes gradually from one geographic area to another. and/or can cause this type of distribution.
- Humans may respond to environmental stress in four different ways: adaptation, developmental adjustment, acclimatization, and cultural responses.
- An is a genetically based trait that has evolved because it helps living things survive and reproduce in a given environment. Adaptations evolve by natural selection in populations over a relatively long period to time. Examples of adaptations include sickle cell trait as an adaptation to endemic malaria and the ability to taste bitter compounds as an adaptation to bitter-tasting toxins in plants.
- A developmental adjustment is a nongenetic response to stress that occurs during infancy or childhood. It may persist into adulthood and may be irreversible. Developmental adjustment is possible because humans have a high degree of phenotypic plasticity. It may be the result of environmental stresses, such as inadequate food — which may stunt growth — or cultural practices, such as orthodontic treatments, which re-align the teeth and jaws.
- is the development of reversible changes to environmental stress that develop over a relatively short period of time. The changes revert to the normal baseline state after the stress is removed. Examples of acclimatization include tanning of the skin and physiological changes (such as increased sweating) that occur with heat acclimatization.
- Cultural responses consist of learned behaviors and technology that allow us to change our environment to control stress, rather than changing our bodies genetically or physiologically to cope with stress. Examples include using shelter, fire, and clothing to cope with a cold climate.
- Blood type is a genetic characteristic associated with the presence or absence of on the surface of red blood cells. A blood group system refers to all of the (s), alleles, and possible and that exist for a particular set of blood type antigens.
- Antigens are molecules that the immune system identifies as either self or nonself. If antigens are identified as nonself, the immune system responds by forming that are specific to the nonself antigens, leading to the destruction of cells bearing the antigens.
- The ABO blood group system is a system of red blood cell antigens controlled by a single gene with three common alleles on chromosome 9. There are four possible ABO blood types: A, B, AB, and O. The ABO system is the most important blood group system in blood transfusions. People with type O blood are universal donors, and people with type AB blood are universal recipients.
- The frequencies of ABO blood type alleles and blood groups vary around the world. The allele for the B antigen is least common, and blood type O is the most common. Evolutionary forces of founder effect, genetic drift, and natural selection are responsible for the geographic distribution of ABO alleles and blood types. For example, people with type O blood may be somewhat resistant to malaria, possibly giving them a selective advantage where malaria is endemic.
- The Rhesus blood group system is a system of red blood cell antigens controlled by two genes with many alleles on chromosome 1. There are five common Rhesus antigens, of which antigen D is the most significant. Individuals who have antigen D are called Rh+, and individuals who lack antigen D are called Rh-. Rh- mothers of Rh+ fetuses may produce antibodies against the D antigen in the fetal blood, causing hemolytic disease of the newborn (HDN).
- The majority of people worldwide are Rh+, but there is regional variation in this blood group system. This variation may be explained by natural selection that favors for the D antigen, because this genotype seems to be protected against some of the neurological consequences of the common parasitic infection toxoplasmosis.
- At high altitudes, humans face the stress of , or a lack of oxygen. Hypoxia occurs at high altitude because there is less oxygen in each breath of air and lower air pressure, which prevents adequate absorption of oxygen from the lungs.
- Initial responses to hypoxia include hyperventilation and elevated heart rate, but these responses are stressful to the body. Continued exposure to high altitude may cause high altitude sickness, with symptoms such as fatigue, shortness of breath, and loss of appetite. At higher altitudes, there is greater risk of serious illness.
- After several days at high altitude, starts to occur in someone from a lowland population. More red blood cells and capillaries form, along with other changes. Full acclimatization may take several weeks. Returning to low altitude causes a reversal of the changes to the pre-high altitude state in a matter of weeks.
- Well over 100 million people live at altitudes higher than 2,500 metres above sea level. Some indigenous populations of Tibet, Peru, and Ethiopia have been living above 2,500 metres for thousands of years, and have evolved genetic adaptations to high altitude hypoxia. Different high altitude populations have evolved different adaptations to the same hypoxic stress. Tibetan highlanders, for example, have a faster rate of breathing and wider arteries, whereas Peruvian highlanders have larger red blood cells and a greater concentration of the oxygen-carrying protein hemoglobin.
- Both hot and cold temperatures are serious environmental stresses on the human body. In the cold, there is risk of hypothermia, which is a dangerous decrease in core body temperature. In the heat, there is risk of hyperthermia, which is a dangerous increase in core body temperature.
- According to , body size tends to be negatively correlated with temperature, because larger body size increases heat production and decreases heat loss. The opposite holds true for small body size. Bergmann’s rule applies to many human populations that are hot or cold adapted.
- According , the length of body extremities is positively correlated with temperature, because longer extremities are better at dissipating excess body heat. The opposite applies to shorter extremities. Allen’s rule applies to relative limb lengths in many human populations that have adapted to heat or cold.
- Sweating is the primary way humans lose body heat. The evaporation of sweat from the skin cools the body. This only works well when the relative humidity is fairly low. At high relative humidity, sweat does not readily evaporate to cool us down. The heat index (HI) indicates how hot it feels due to the humidity.
- Gradually working longer and harder in the heat can bring about heat acclimatization, in which the body has improved responses to heat stress. Sweating starts earlier, sweat contains less salt, and vasodilation brings heat to the surface to help cool the body. Full acclimatization takes up to 14 days and reverses just as quickly when the heat stress is removed.
- The human body can respond to cold by producing more heat (by shivering or increasing the basal metabolic rate) or by conserving heat (by vasoconstriction at the body surface or a layer of fat-insulating internal organs).
- At temperatures below freezing, the hunting response occurs to prevent cold injury (such as frostbite). This is a process of alternating vasoconstriction and vasodilation in extremities that are exposed to dangerous cold. Where temperatures are repeatedly cold but rarely below freezing, the hunting response may not occur, and the skin may remain cold due to vasoconstriction alone.
- Milk contains the sugar lactose, a disaccharide. Lactose must be broken down into its two component sugars to be absorbed by the small intestine, and the enzyme lactase is needed for this process.
- In about 60 per cent of people worldwide, the ability to synthesize lactase and digest lactose declines after the first two years of life. These people become lactose intolerant, and cannot consume much milk without suffering symptoms such as bloating, cramps, and diarrhea.
- In populations that herded milking animals for thousands of years, lactase persistence evolved. People who were able to synthesize lactase and digest lactose throughout life were strongly favored by natural selection. People who descended from these early herders generally still have lactase persistence. That includes many Europeans and European-Americans.
- Human populations may vary in how efficiently they use calories in food. Some people (especially South Pacific Islanders, Native Americans, and sub-Saharan Africans) may be able to get by on fewer calories than would be adequate for others, so they tend to easily gain weight, become obese, and develop diseases such as diabetes.
- The proposes that “thrifty genes” were selected for because they allowed people to use calories efficiently and store body fat when food was plentiful so they had a reserve to use when food was scarce. According to the hypothesis, thrifty genes become detrimental and lead to obesity and diabetes when food is plentiful all of the time.
- Several assumptions underlying the thrifty gene hypothesis have been called into question, and genetic research has been unable to actually identify thrifty genes. Alternate hypotheses to the thrifty gene hypothesis have been proposed, including the drifty gene hypothesis. The latter hypothesis explains variation in the tendency to become obese by genetic drift on neutral genes.
In this chapter, you learned about how genetic variation can lead to differences in human characteristics. Genes encode for proteins, which carry out our bodies’ life processes. In the next chapter, you will learn about how proteins and other molecules make up the cells, tissues, and organs of the human body, and how these units work together in interacting systems to allow us to function.
- Explain why an evolutionarily older population is likely to have more genetic variation than a similarly-sized younger population.
- The genetic difference between any two people on Earth is only about 0.1 percent. Based on our evolutionary history, describe one reason why humans are relatively homogeneous genetically.
- What aspect(s) of human skin colour are due to adaptation? Be sure to define adaptation in your answer. What aspect(s) of human skin colour are due to acclimatization? Be sure to define acclimatization in your answer.
- For each of the following human responses to the environment, list whether it can be best described as an example of adaptation, acclimatization, or developmental adjustment:
- Reduction in height due to lack of food in childhood
- Resistance to malaria
- Shivering in the cold
- Changes in body size and dimensions to better tolerate heat or cold
- Give an example of a human response to environmental stress that involves changes in behavior, instead of changes in physiology.
- What are two types of environmental stresses that caused genetic changes related to hemoglobin in some populations of humans?
- The ability of an organism to change their phenotype in response to the environment is called phenotypic __________ .
- List three natural selection pressures that differ geographically across the world and contributed to the evolution of human genetic variation in different regions.
- You may have noticed that when a sudden hot day occurs during a cool period, it can feel even more uncomfortable than higher temperatures during a hot period — even with the same humidity levels. Using what you learned in this chapter, explain why you think that happens.
- Out of all mammals, why are humans the only ones that evolved lactase persistence?
- If the Inuit people who live in the Arctic were not able to get enough vitamin D from their diet, what do you think might happen to their skin colour over a long period of time? Explain your answer.
- Explain why malaria has been such a strong force of natural selection on human populations.
- Give one example of “heterozygote advantage” (i.e. when the heterozygous genotype has higher relative fitness than the dominant or recessive homozygous genotype) in humans.
- What is one way in which humans have evolved genetic adaptations in response to their food sources?
- Do you think adaptation to high altitude evolved once or several times? Explain your reasoning.
Be The Match. (2012, December 19). Marrow donors talk about donating and the donation process. YouTube. https://www.youtube.com/watch?v=rLO0Usg8vcY&feature=youtu.be
Canadian Blood Services. (n.d.). There is an immediate need for blood as demand is rising. https://www.blood.ca/en
hemaquebec1998. (2015, August 27).Stem cell donation: Step by step. YouTube. https://www.youtube.com/watch?v=FyriQibRhLA&feature=youtu.be
A substitution of a single nucleotide that occurs at a specific position in the genome, where each variation is present at a level of 0.5% from person to person in the population.
The transfer of genetic variation from one population to another. If the rate of gene flow is high enough, then two populations are considered to have equivalent allele frequencies and therefore effectively be a single population.
The differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations.
A genetically-based trait that has evolved because it helps living things survive and reproduce in a given environment.
The process in which an individual organism adjusts to a change in its environment, allowing it to maintain performance across a range of environmental conditions. Acclimatization occurs in a short period of time, and within the organism's lifetime.
Molecules on the surface of cells or viruses that the immune system identifies as either self (produced by your own body) or non-self (not produced by your own body).
A sequence of nucleotides in DNA or RNA that codes for a molecule that has a function.
The part of the genetic makeup of a cell, and therefore of any individual, which determines one of its characteristics (phenotype).
The set of observable characteristics of an individual resulting from the interaction of its genotype with the environment.
An antibody, also known as an immunoglobulin, is a large, Y-shaped protein produced mainly by plasma cells that is used by the immune system to neutralize pathogens such as pathogenic bacteria and viruses.
An individual who has two different forms of a particular gene, one inherited from each parent.
A condition in which the body or a region of the body is deprived of adequate oxygen supply at the tissue level.
An ecogeographical rule that states that within a broadly distributed taxonomic clade, populations and species of larger size are found in colder environments, and species of smaller size are found in warmer regions.
The principle holding that in a warm-blooded animal species having distinct geographic populations, the limbs, ears, and other appendages of the animals living in cold climates tend to be shorter than in animals of the same species living in warm climates.
A hypothesis which suggests that the carriers of the 'thrifty genes' survive because they deposit fat between famines. The implication of this is that the primary factor causing famine mortality is running out of energy reserves—that is, starvation, and that fatter people run out of reserves more slowly.