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Q1. What is Human Population Genetics ? Critically discuss any three evolutionary forces. 20

Ans. Human Population Genetics is the branch of genetics that studies the distribution of and changes in genetic variation within and among human populations. It also investigates the various factors that cause these changes. The field integrates the principles of Mendelian genetics with Darwin’s theory of natural selection and evolution. Its primary focus is on allele frequencies (the different forms of a gene) and how they change over time. Human population genetics helps us understand human evolution, migration patterns (like the out-of-Africa migration), population structure, and the prevalence of genetic diseases. It is a crucial tool for understanding why certain diseases are more common in some populations and how gene-environment interactions affect health and disease. Evolutionary forces are the mechanisms that can change allele frequencies in a population. Three major evolutionary forces are: 1. Mutation: A mutation is a permanent alteration in the DNA sequence. It is the ultimate source of all new genetic variation, providing the raw material for evolution. Mutations occur randomly and can be beneficial, harmful, or neutral with respect to an organism’s fitness. Critical Discussion: While essential for evolution, mutation by itself is a very weak force for changing allele frequencies. Mutation rates are very low (around 10⁻⁵ to 10⁻⁶ per gene per generation). Therefore, it would take thousands of generations for a significant change in a population to occur through mutation alone. Its real power lies in providing new variations for other evolutionary forces, especially natural selection, to act upon. Mutation is random; it does not arise to fulfill a need. 2. Natural Selection: Natural selection is the process by which individuals with certain inherited traits are better able to survive and reproduce in their environment than others. This leads to an increase in the frequency of those advantageous traits over time. Selection can act in different ways: directional selection (favouring one extreme trait), stabilizing selection (favouring intermediate traits), and disruptive selection (favouring both extreme traits). Critical Discussion: Natural selection is the primary mechanism of adaptation and it is not random. It ‘selects’ for alleles that are better adapted to the environment. A classic example is sickle-cell anemia. Heterozygous (AS) individuals have a resistance to malaria, which gives them a survival advantage in malaria-endemic regions. This is a form of balancing selection called heterozygote advantage, which maintains a harmful allele (S) in the population. However, the strength and direction of selection are highly dependent on the environment; what is advantageous in one environment may be detrimental in another. 3. Genetic Drift: Genetic drift is the random fluctuation of allele frequencies due to chance events, especially in small populations. It is not caused by selection. Two major effects of drift are the founder effect (when a few individuals start a new population) and the population bottleneck (when a population’s size is rapidly reduced). Critical Discussion: Genetic drift does not lead to adaptation; it is entirely random. It can lead to the loss of beneficial alleles and the fixation of harmful ones, especially in small populations. Drift reduces genetic variation within a population but increases genetic differentiation between different, isolated populations. In human populations, the founder effect has played a significant role in the high frequency of certain genetic disorders in some isolated groups, such as the Amish or Ashkenazi Jews. It is a powerful evolutionary force, especially for most of human history when populations were small and relatively isolated.

Q2. Discuss the various environmental factors affecting complex diseases. 20

Ans. Complex diseases, also known as multifactorial diseases, arise from a combination of genetic and environmental factors. These diseases include heart disease, type 2 diabetes, asthma, hypertension, and most cancers. While an individual’s genetic makeup can determine their susceptibility to these diseases, environmental factors play a crucial role in triggering their onset and progression. The major environmental factors affecting complex diseases are as follows: 1. Lifestyle Factors: These are related to an individual’s habits and behaviours and are among the leading contributors to complex diseases in modern societies.

• Diet: A diet high in saturated fats, sugars, and processed foods is linked to an increased risk of obesity, type 2 diabetes, and cardiovascular disease. Conversely, a diet rich in fruits, vegetables, and whole grains can be protective.
• Physical Activity: A sedentary lifestyle is a major risk factor for obesity, heart disease, and hypertension. Regular exercise helps reduce the risk of these conditions.
• Smoking: Tobacco use is the single most important preventable risk factor for lung cancer, heart disease, and many other cancers.
• Alcohol Consumption: Excessive alcohol intake can lead to liver disease, certain cancers, and high blood pressure.

2. Environmental Exposures: This includes physical and chemical agents present in an individual’s surroundings.

• Pollutants: Air pollution (e.g., particulate matter) can exacerbate asthma and other respiratory diseases. Pollutants in water and soil, such as heavy metals (lead, mercury) and pesticides, have been linked to neurological problems and certain cancers.
• Radiation: Excessive exposure to ultraviolet (UV) radiation from the sun increases the risk of skin cancer.
• Infectious Agents: Certain viruses and bacteria can contribute to the development of complex diseases. For example, the Human Papillomavirus (HPV) is a major cause of cervical cancer, and the bacterium Helicobacter pylori is associated with an increased risk of stomach ulcers and stomach cancer.

3. Socioeconomic and Psychological Factors:

• Socioeconomic Status (SES): Low SES is often associated with poor nutrition, limited access to healthcare, stressful living conditions, and greater exposure to environmental hazards, all of which increase the risk for complex diseases.
• Psychological Stress: Chronic stress can impact the immune system and hormonal balance, contributing to conditions like hypertension, heart disease, and mental health disorders.

In conclusion, it is vital to understand that these environmental factors do not act in isolation. They interact with an individual’s genetic susceptibility through gene-environment interactions (GxE) . For instance, a person with a genetic predisposition to hypertension may be more likely to develop the condition if they consume a high-salt diet. Therefore, preventing and managing complex diseases requires a holistic approach that considers both genetic and environmental factors.

Q3. Write an account on Thalassemia and its subtypes. 20

Ans. Thalassemia is a group of inherited blood disorders characterized by the reduced or absent synthesis of hemoglobin. Hemoglobin is the protein in red blood cells that carries oxygen throughout the body. Inadequate production of hemoglobin leads to anemia and other serious health problems. It is a quantitative hemoglobinopathy, meaning the structure of the hemoglobin molecule is normal, but it is produced in reduced amounts. Hemoglobin is made of four protein chains: two alpha (α) globin chains and two beta (β) globin chains. Mutations in the genes controlling these chains cause thalassemia. The main subtypes of thalassemia are based on which globin chain is affected. 1. Alpha (α) Thalassemia: This is caused by a reduced production of alpha-globin chains. Each person has four alpha-globin genes, located on chromosome 16. The severity of the disease depends on how many genes are deleted or mutated.

• Silent Carrier (1 gene affected): The individual has no symptoms and is essentially healthy.
• Alpha Thalassemia Trait (2 genes affected): The individual may have mild microcytic anemia but usually no serious health problems.
• Hemoglobin H (HbH) Disease (3 genes affected): This causes moderate to severe anemia, an enlarged spleen (splenomegaly), and bone problems. Patients may sometimes require blood transfusions.
• Hydrops Fetalis (4 genes affected): This is the most severe form and is almost always fatal. The affected fetus dies in the womb or shortly after birth.

2. Beta (β) Thalassemia: This is caused by the reduced production of beta-globin chains, which are controlled by a single gene on chromosome 11. Over 200 different mutations can cause beta thalassemia.

• Beta Thalassemia Minor (Trait): This occurs when a person inherits one mutated beta-globin gene (heterozygous). These individuals are usually asymptomatic or have mild anemia and are considered carriers.
• Beta Thalassemia Intermedia: This is a clinical term describing a severity between minor and major. These patients have moderate anemia and may not require regular blood transfusions.
• Beta Thalassemia Major (Cooley’s Anemia): This is the most severe form and occurs when a person inherits two mutated beta-globin genes (homozygous). Symptoms begin in infancy and include severe, life-threatening anemia. Patients require lifelong regular blood transfusions to survive. The frequent transfusions lead to iron overload in the body, which must be managed by chelation therapy to prevent damage to organs like the heart and liver.

Thalassemia is most common in the Mediterranean, the Middle East, and Southeast Asia, a region known as the “thalassemia belt.” This high prevalence is thought to be due to providing a selective advantage against malaria, as individuals with thalassemia trait have some resistance to the malaria parasite.

Q4. Write short notes on any two of the following : 10+10 (a) Law of Independent Assortment (b) Hardy-Weinberg’s law (c) Hypertension

Ans. (a) Law of Independent Assortment The Law of Independent Assortment is the second law proposed by Gregor Mendel. This law states that the alleles of genes responsible for different traits separate independently of one another during the formation of gametes. In other words, the inheritance of an allele for one particular trait does not influence the inheritance of an allele for a different trait. This principle applies specifically to genes that are located on different chromosomes or are very far apart on the same chromosome. For example, in a dihybrid cross where a plant is heterozygous for both seed shape (Round R, wrinkled r) and seed colour (Yellow Y, green y) (RrYy), the R and r alleles will segregate independently from the Y and y alleles during gamete formation. This results in four types of gametes (RY, Ry, rY, ry) being produced in equal proportions. When two such plants are crossed, the offspring exhibit a phenotypic ratio of 9:3:3:1. This law is fundamental to genetic recombination and the production of new combinations of traits in offspring, contributing to genetic diversity. However, genes that are located closely together on the same chromosome (linked genes) do not assort independently. (b) Hardy-Weinberg’s law The Hardy-Weinberg law is a fundamental principle of population genetics. It describes an ideal population in which allele and genotype frequencies remain constant from generation to generation, as long as no other evolutionary influences are acting upon them. There are five key conditions for this equilibrium to be maintained:

1. No mutation
2. Random mating
3. No gene flow
4. No natural selection
5. An infinitely large population size (so no genetic drift).

The law is expressed mathematically by two equations:

• p + q = 1 (where p and q are the frequencies of two alleles)
• p² + 2pq + q² = 1 (where p², 2pq, and q² are the frequencies of the three genotypes: AA, Aa, and aa)

In practice, no real population meets all these conditions. Therefore, the main utility of the Hardy-Weinberg law is as a null hypothesis. If the observed genotype frequencies in a population differ from the frequencies predicted by the law, it indicates that one or more evolutionary forces—such as mutation, non-random mating, gene flow, natural selection, or genetic drift—are at work.

Q5. What is Genetic Polymorphism ? Describe the models explaining the maintenance of genetic polymorphism. 20

Ans. Genetic Polymorphism is the occurrence of two or more distinct alleles at a gene locus within a population, where the frequency of the rarest allele is at least 1%. This threshold distinguishes a polymorphism from a rare mutation that may exist in only a few individuals. Polymorphism is the basis of genetic variation within a species, providing the raw material for evolution and adaptation. Classic examples of polymorphism in human populations include the ABO blood group system, PTC tasting ability, and the HLA (Human Leukocyte Antigen) system. Given that forces like natural selection and genetic drift often act to reduce variation, the question arises as to how polymorphism is maintained in populations. Several models explain this: 1. Balancing Selection: This is a type of natural selection that actively maintains multiple alleles in a population. It takes several forms:

• Heterozygote Advantage (Overdominance): This occurs when the heterozygous genotype (Aa) has a higher fitness than both homozygous genotypes (AA and aa). The classic example is the sickle-cell trait in malaria-endemic regions. Heterozygotes (AS) are resistant to malaria and do not suffer the severe complications of sickle-cell disease. Homozygotes (AA) are susceptible to malaria, and homozygotes (SS) have sickle-cell disease. The survival advantage of the heterozygote therefore maintains both the A and S alleles in the population.
• Frequency-Dependent Selection: In this model, the fitness of a phenotype depends on its frequency in the population. Negative frequency-dependent selection, where rare phenotypes are favoured, can maintain polymorphism. For example, if a predator learns to target the most common prey type, rare variants will have a survival advantage, causing their frequency to rise, until they become common and are targeted in turn.
• Varying Selection Pressures in Time and Space: If selection pressures vary over time (e.g., seasonal changes) or in different geographical locations, different alleles may be advantageous at different times or places. This can maintain multiple alleles in the overall population.

2. Mutation-Selection Balance: Although deleterious alleles are constantly removed by selection, they are also constantly reintroduced into the population by recurrent mutation. An equilibrium is reached when the rate at which new alleles are created equals the rate at which they are removed by selection. This can maintain a low-level polymorphism of harmful alleles for many genes. 3. Neutral Theory of Molecular Evolution: Proposed by Motoo Kimura, this theory suggests that much of the genetic variation observed at the molecular level is functionally neutral (has no effect on fitness). The frequency of these neutral alleles is governed primarily by a balance between mutation (which creates them) and genetic drift (which randomly alters their frequency). This theory has been very influential in explaining the extensive polymorphism observed at the protein and DNA levels.

Q6. Define and discuss the inbreeding and outbreeding in Indian populations. 20

Ans. Inbreeding is defined as mating between individuals who are more closely related genetically than two individuals chosen at random from the population. The most common form is consanguineous marriage, such as unions between uncle-niece, or first cousins. Outbreeding refers to mating between unrelated individuals. In the context of population structure, this often refers to mating between individuals from different sub-populations, castes, or religious groups (inter-caste marriage). Discussion in Indian Populations: Inbreeding: The Indian population is known for high rates of consanguinity, though there is significant regional, religious, and caste-based variation in its prevalence.

• Prevalence: The practice of inbreeding is particularly prevalent in South India, especially in states like Andhra Pradesh, Tamil Nadu, and Karnataka. In these regions, uncle-niece and first-cousin marriages are culturally preferred in many communities. In contrast, the practice is much less common in North India, where gotra exogamy (avoiding marriage within the same lineage) is followed. Consanguineous marriages are also common in some Muslim and other religious communities across India.
• Reasons: The reasons for inbreeding include cultural traditions, strengthening family ties, keeping property within the family, and ease of marital arrangements.
• Genetic Consequences: The main genetic consequence of inbreeding is an increase in homozygosity . This elevates the incidence of rare autosomal recessive genetic disorders. When related individuals have children, the chance of a child inheriting two copies of a harmful recessive allele from a common ancestor increases. This has resulted in a high burden of diseases like thalassemia, sickle cell anemia, and various metabolic disorders in parts of India with high rates of inbreeding.

Outbreeding: Traditionally, Indian society has been highly stratified by the caste system and religion, which strictly enforced endogamy —marriage within one’s own group (jati). This created thousands of genetically isolated, in-marrying groups. Therefore, while inbreeding might occur within a jati, outbreeding between jatis was historically limited.

• Modern Trends: In recent decades, urbanization, education, increased mobility, and social reforms have led to a rise in inter-caste and inter-religious marriages. This trend, though still relatively slow, is breaking down the traditional endogamous barriers.
• Genetic Consequences: Outbreeding, or the breakdown of endogamy, increases heterozygosity . It mixes different gene pools, a phenomenon known as admixture . Genetically, this reduces the risk of recessive disorders as harmful alleles are more likely to be in a heterozygous carrier state. Over time, this process will reduce the genetic differences between the various groups of the Indian population. It represents a significant ongoing change in India’s complex population structure and health patterns.

Q7. Give an account of comparison between Man and Apes with respect to foot, vertebral column and pelvis. 20

Ans. The differences in the skeletal structures of humans and apes (like chimpanzees, gorillas) primarily reflect their different modes of locomotion. Humans are adapted for habitual bipedalism (walking on two legs), while apes are adapted for quadrupedalism and brachiation (swinging through trees). This distinction is clearly visible in the foot, vertebral column, and pelvis. 1. Foot:

• Man: The human foot acts as a rigid, arched platform for weight-bearing and propulsion. It has a non-opposable, robust big toe (hallux) that is aligned with the other toes, which is crucial for the ‘toe-off’ phase of walking. The human foot features two arches—a longitudinal and a transverse arch. These arches act as shock-absorbing springs during walking and running.
• Apes: The ape foot is flexible and grasping. The big toe is opposable, functioning much like a thumb, which is very useful for climbing trees. The ape foot lacks the developed arches of humans; it is largely flat.

2. Vertebral Column:

• Man: The human vertebral column has four distinct curves that form an ‘S’ shape: cervical (forward), thoracic (backward), lumbar (forward), and sacral (backward). This S-shape helps to position the body’s center of gravity directly over the feet, providing balance and shock absorption during upright locomotion. The lumbar vertebrae are large and robust to support the weight of the upper body.
• Apes: The ape vertebral column has a single, gentle ‘C’-shaped curve. This is suitable for a quadrupedal or semi-erect posture, where the torso is pitched forward. They lack a distinct lumbar curve.

3. Pelvis:

• Man: The human pelvis is broad, short, and bowl-shaped. This structure provides support for the abdominal organs and a stable base for the torso. The iliac blades are short and wide, and they flare out to the sides. This positioning allows for the attachment of powerful gluteal muscles that are crucial for stabilizing the hip and propelling the body forward during bipedal locomotion. The birth canal has to accommodate a large-headed infant, which places another constraint on the shape of the pelvis.
• Apes: The ape pelvis is long, narrow, and flat. The iliac blades are tall and extend up the back. This structure is suited for quadrupedal locomotion and climbing, providing attachment points for muscles that power the hind limbs in a different manner than in humans.

In summary, these three structures clearly demonstrate how natural selection has shaped the human skeleton for efficient, long-distance travel on two legs, while the ape skeleton remains adapted for a life that involves using all four limbs in trees and on the ground.

Q8. Write short notes on any two of the following : 10+10 (a) Gorilla (b) Genetic load (c) Random mating

Ans. (a) Gorilla The gorilla is the largest living primate and a member of the great ape family. They are found in the forests of central sub-Saharan Africa. Taxonomically, the genus Gorilla is divided into two species: the Western Gorilla and the Eastern Gorilla, each with two subspecies. Gorillas are characterized by their large, robust bodies and dark fur. There is marked sexual dimorphism, with adult males, known as ‘silverbacks’, being much larger and heavier than females. They are primarily terrestrial, moving around by ‘knuckle-walking’. Gorillas are mainly herbivorous, eating leaves, stems, and fruit. They live in stable family groups called troops, typically led by a dominant silverback. Despite their intimidating appearance, they are generally peaceful and shy animals. Genetically, humans and gorillas share about 98% of their DNA. Unfortunately, all subspecies of gorilla are critically endangered due to poaching, habitat destruction, and disease. (b) Genetic load Genetic load refers to the reduction in the mean fitness of a population compared to a hypothetical ideal or optimal genotype. It represents the burden of deleterious or low-fitness alleles present in a population’s gene pool. This “load” is the price a population pays for not being composed entirely of the best possible genotype. There are several causes of genetic load:

• Mutational Load: This is caused by recurrent harmful mutations. Selection removes these mutations, but they are constantly being re-generated, leading to a low-level, steady-state frequency of them in the population.
• Segregational Load: This occurs when the heterozygote has the highest fitness (overdominance). In this case, the population pays a “price” by producing less-fit homozygotes in every generation (e.g., the AA and SS individuals in the case of sickle-cell anemia).

The concept of genetic load is important for understanding why no population is perfectly adapted and it quantifies the cost of maintaining genetic variation in a population.

(c) Random mating

Random mating, also known as panmixia, is a mating pattern where individuals choose their mates without regard to their genotype or phenotype for a particular trait. In a truly randomly mating population, the probability of any two individuals mating is directly proportional to their relative frequencies in the population.

Random mating is one of the five key assumptions of the Hardy-Weinberg equilibrium. When mating is random, genotype frequencies can be predicted directly from allele frequencies (as p², 2pq, and q²). It prevents the stratification of alleles into specific subgroups.

This is in contrast to non-random mating patterns, such as:

• Assortative Mating: Where individuals choose mates who are phenotypically similar (positive) or dissimilar (negative) to themselves.
• Inbreeding: Where mating occurs between related individuals.

It is important to note that non-random mating changes genotype frequencies but does not, by itself, change the allele frequencies in the population.

Q9. (Compulsory) (a) Describe the procedure to detect colour-blindness. 10 (b) Describe the procedure for PTC tasting ability test. 10

Ans. (a) Describe the procedure to detect colour-blindness. Colour blindness, or colour vision deficiency, is the decreased ability to see colour or differences in colour. The most common form is red-green colour blindness, which is an X-linked recessive trait. The most widely used and standard procedure to detect it is the Ishihara Test . Procedure:

1. Materials: The Ishihara test consists of a set of plates. Each plate features a circular pattern of dots of varying colours and sizes.
2. Test Setup: The test should be conducted in a room with adequate, natural daylight, avoiding artificial light. The subject is seated comfortably.
3. Administration:
   • The examiner holds the plates about 75 cm from the subject, tilted so they are perpendicular to the line of sight.
   • The subject is given about three seconds to identify the number or pattern on each plate.
   • The first plate is usually a control plate (e.g., the number 12), which both normal and colour-blind individuals can see. This ensures the subject understands the task.
   • In subsequent plates, some dots form a number or shape that is visible to those with normal colour vision but is invisible or difficult to see for those with a red-green deficiency.
   • Some other plates are designed so that only those with a colour deficiency can see a number.
4. Interpretation: The examiner records the subject’s responses for each plate. The number of misread plates and the specific nature of the errors allow the examiner to determine the presence and type (protanopia – red deficiency; deuteranopia – green deficiency) of red-green colour vision deficiency.

It is a quick, simple, and reliable screening tool used widely in schools, industries, and clinics.

(b) Describe the procedure for PTC tasting ability test.

The ability to taste phenylthiocarbamide (PTC) is a classic genetic polymorphism in human populations, governed by the taste receptor gene TAS2R38. A standard laboratory procedure to detect this ability is the

sorting method of Harris and Kalmus

.

Procedure:

1. Materials:
   • A set of serial dilutions of PTC solution, typically 13-14 solutions, ranging from the highest concentration (Solution 1) to the most dilute (Solution 13).
   • Distilled water as a control.
   • Small cups for each solution and a cup of water for rinsing.
2. Administration:
   • The subject is asked to rinse their mouth with plain water first.
   • The test starts with the most dilute solution. The subject is presented with a pair of cups – one containing the PTC solution and one with water.
   • The subject takes a sip from each and is asked to identify which cup has a bitter taste.
   • If they cannot correctly identify it, they move to the next higher concentration. This process is repeated until the subject can consistently differentiate the PTC solution from water.
3. Interpretation:
   • The lowest concentration at which an individual can reliably detect the taste is called their threshold value .
   • When a population is tested, a bimodal distribution of threshold values is observed.
   • Individuals with a low threshold value (can taste very weak solutions) are classified as ‘tasters’ .
   • Those who can only taste very high concentrations or none at all are classified as ‘non-tasters’ . The dividing point between the two groups in the distribution defines the boundary between tasters and non-tasters.

A simpler screening method uses a strip of PTC-impregnated paper, which the subject chews and reports if they taste bitterness. This is less quantitative but useful for large-scale studies.