ANTH103 Response Paper:
Mendelian Inheritance 

Part 1 Analysis:

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Darwin’s Theory of Natural Selection and Sickle Cell Anemia

Darwin’s theory of natural selection shows that traits that help an organism survive or reproduce in its environment become more common over time. In the case of sickle cell anemia, the disease is caused by a mutation in the hemoglobin gene, which makes red blood cells take on a sickle shape. These sickle-shaped cells can cause health problems, especially when oxygen levels are low. However, in areas where malaria is common, having one sickle cell gene (being a carrier) actually helps protect against malaria, giving carriers a survival advantage.

Heritable Variation and Environmental Context:

The hemoglobin gene can be passed down from parents to their children. In regions where malaria is common, people who carry one sickle cell gene (HbAS) have an advantage because their red blood cells are less likely to be infected by the malaria parasite. This increases their chances of surviving and having children. As a result, the sickle cell gene remains common in these areas because carrying just one copy of it helps protect against malaria, even though having two copies (HbSS) causes the serious condition of sickle cell anemia.

Differential Reproductive Success:

People with one sickle cell gene (HbAS) are more likely to survive and have children in areas where malaria is common. This means they can pass the sickle cell gene to the next generation. However, people who inherit the sickle cell gene from both parents (HbSS) often suffer from sickle cell anemia, which can shorten their lives and make it harder for them to have children. Even though sickle cell anemia is serious, the sickle cell gene remains in the population because having one gene (HbAS) helps protect against malaria.

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Why the Disease Persists:

Sickle cell anemia hasn't been eliminated by natural selection because the gene that causes it also gives a survival advantage in areas where malaria is common. The sickle cell gene stays in these populations because having one copy of the gene (being a carrier) is helpful in fighting malaria. This situation is called a balanced polymorphism, where natural selection favors carriers, keeping the gene in the population.

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Biological and Cultural Interactions:

The presence of malaria and cultural practices like migration, intermarriage, and public health efforts can affect how common the sickle cell gene is in a population. In areas without malaria, the sickle cell gene might become less common because it doesn't provide an advantage. Improvements in medical care and public health could also lower the death rate from sickle cell anemia, which would impact how common the gene is. During your lifetime, the frequency of the sickle cell gene could change if malaria is eliminated in some regions or if people move to new areas, changing the conditions that influence how the gene is passed on.

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Part 2 Questions:

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1. Mendel's Contributions to Genetics:

Gregor Mendel was a 19th-century scientist and monk who helped start modern genetics with his experiments on pea plants. He discovered that traits are passed down in specific units (now called genes) instead of blending together from parents. Mendel's work showed how dominant and recessive traits interact to create different characteristics in offspring, leading to the concepts of genotype (genetic makeup) and phenotype (physical traits). His ideas, especially about how traits are separated and inherited independently, are key to understanding how traits are passed from one generation to the next and how differences appear in populations.

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2. Gene Flow vs. Genetic Drift:

Gene flow and genetic drift are two ways that genetic variation changes within and between populations. Gene flow happens when people move from one population to another and have children, bringing new genes into the population and increasing diversity. Genetic drift is different because it’s a random process that changes how common certain genes are, especially in small populations. This can lead to some genes disappearing and less genetic diversity. An example of genetic drift is the bottleneck effect, where a natural disaster greatly reduces a population’s size, causing some genes to become more or less common just by chance.

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3. Founder’s Effect:

The founder’s effect happens when a small group of people starts a new population, and their genes do not fully represent the original population. This can lead to less genetic diversity and a higher chance of certain genes, including harmful ones, becoming common. For example, Tay-Sachs disease is more common in the Ashkenazi Jewish population because the group started from a small number of founders. Another example is the higher rate of polydactyly (extra fingers or toes) in the Amish population, which was founded by a small group of people who carried the gene for this trait.

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4. Sexual Selection:

Sexual selection is a form of natural selection where certain traits help an individual attract mates and reproduce. These traits might not help with survival but make the individual more successful at mating. For example, in many bird and fish species, males are more colorful and showy than females because these traits attract mates. Bright colors and flashy displays signal good health and strong genetics, making the male more likely to reproduce. This difference between males and females happens because females usually invest more in their offspring and are pickier about their mates, so males evolve traits that appeal to them.

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5. Genetic Basis of Evolutionary Change:

Evolutionary change happens through changes in genes. Mutations change the DNA sequence, leading to new traits. If these traits help an organism survive or reproduce better, natural selection will make them more common. Gene flow brings new genes into a population from others, while genetic drift can randomly change gene frequencies, especially in small populations. Over time, these processes alter the genetic makeup of populations, driving evolution and helping species adapt to their environments.

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