Elaborate Notes
Analysis of Viral Genomes
The study of viruses, or virology, is crucial in understanding diseases. Viruses are non-cellular entities that require a host cell to replicate. Their genetic material, or genome, is a key characteristic used for classification.
- Genome: The complete set of genetic instructions for an organism, encoded in DNA or RNA.
- Adenoviruses: These are a group of viruses that can cause infections in the respiratory tract, eyes, and intestines. As noted by scientists like Matthew Meselson and Franklin Stahl in their broader work on DNA replication (1958), the structure of genetic material is fundamental. Adenoviruses possess a double-stranded DNA (dsDNA) genome. A common example is their role in causing some cases of the common cold.
- Retroviruses: This is a family of viruses distinguished by their unique replication method. They are enveloped viruses that possess a single-stranded RNA (ssRNA) genome. A defining feature is the enzyme reverse transcriptase, discovered independently by Howard Temin and David Baltimore (1970), which allows the virus to synthesize DNA from its RNA template. This DNA is then integrated into the host cell’s genome. The most prominent example is the Human Immunodeficiency Virus (HIV), which causes Acquired Immunodeficiency Syndrome (AIDS).
Chromosomes and Cellular Genetics
The foundation of heredity lies within the nucleus of eukaryotic cells, in structures known as chromosomes.
- Chromosome Number: Each species has a characteristic, fixed number of chromosomes in its somatic (body) cells. For Homo sapiens (humans), this number is 46, arranged in 23 pairs. This concept of a species-specific count was established by cytogeneticists in the early 20th century.
- Zygote Formation: Sexual reproduction involves the fusion of two specialized cells called gametes. The female gamete is the ovum (egg), and the male gamete is the spermatozoon (sperm). Their fusion results in a single cell called a zygote, which develops into a new organism.
- Somatic vs. Gametic Cells:
- All somatic cells in the human body contain the full set of 46 chromosomes. These cells are diploid (2n).
- Reproductive cells, or gametes, are an exception. They undergo a special type of cell division to contain only half the number of chromosomes, i.e., 23. These cells are haploid (n). This ensures that when an egg (n=23) and a sperm (n=23) fuse, the resulting zygote has the correct diploid number (2n=46).
Cell Division
Cell division is the process by which a parent cell divides into two or more daughter cells. It is fundamental to growth, repair, and reproduction.
- Mitosis: This is the process of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth. It is essential for the growth of an organism from a single-celled zygote, for the replacement of old cells (e.g., skin cells), and for wound healing. The stages were first described by Walther Flemming in the 1880s.
- Meiosis: This is a specialized type of cell division that reduces the chromosome number by half, creating four haploid cells, each genetically distinct from the parent cell and from each other. Meiosis is essential for sexual reproduction and is the source of genetic variation through processes like crossing over (exchange of genetic material between homologous chromosomes).
Ploidy: Haploid and Diploid Cells
- Haploid (n): A cell containing a single set of unpaired chromosomes. In humans, only gametes (egg and sperm) are haploid.
- Diploid (2n): A cell containing two complete sets of chromosomes, one from each parent. Nearly all cells in the human body (somatic cells) are diploid. The term was coined by Eduard Strasburger in 1905.
- Homologous Chromosomes: These are pairs of chromosomes (one from the mother, one from the father) that have the same length, centromere position, and carry genes for the same traits at corresponding loci. For instance, both chromosomes in a pair will carry a gene for eye colour, although the specific versions of the gene (alleles) may differ (e.g., one for blue eyes, one for brown eyes).
Inheritance of Characteristics: Mendel’s Experiments
The principles of heredity were first systematically investigated by Gregor Mendel, an Augustinian friar, whose work with pea plants (Pisum sativum) published in 1866 laid the foundation for modern genetics.
- Rejection of Blending Inheritance: Before Mendel, the prevailing theory was “blending inheritance,” which suggested that offspring were a simple blend of their parents’ traits. Mendel’s experiments demonstrated that traits are inherited as discrete units, which he called “factors” (now known as genes).
- Dominant and Recessive Traits: Mendel observed that for any given trait, an individual inherits two factors, one from each parent.
- A Dominant Trait is one that is expressed in the organism’s appearance (phenotype) even if only one copy of the gene is present.
- A Recessive Trait is one that is masked in the presence of a dominant gene and is only expressed if two copies of the recessive gene are present.
- For example, in pea plants, the gene for green pod colour (G) is dominant over the gene for yellow pod colour (y). A plant with a Gg genotype will have green pods. A plant will only have yellow pods if its genotype is gg.
- Genetic Terminology:
- Homozygous: An organism with two identical alleles for a particular trait (e.g., GG or yy).
- Heterozygous: An organism with two different alleles for a trait (e.g., Gy).
- Genotype: The genetic makeup of an organism for a specific trait (e.g., GG, Gg, gg).
- Phenotype: The observable physical or biochemical characteristics of an organism, as determined by both genetic makeup and environmental influences. For example, the flamingo’s pink colour is a phenotype influenced by its diet (environmental factor), while its underlying genetic code (genotype) would produce white feathers without the specific dietary carotenoids.
Utility for Blood Group Inheritance
The ABO blood group system in humans, discovered by Karl Landsteiner in 1901, is a classic example of Mendelian inheritance, specifically demonstrating multiple alleles and co-dominance.
- Genotypes and Phenotypes:
- There are three alleles for this gene: I^A, I^B, and i.
- Alleles I^A and I^B are dominant over i.
- Alleles I^A and I^B are co-dominant, meaning both are expressed if present together.
| Phenotype (Blood Group) | Possible Genotypes |
|---|---|
| A | I^A I^A, I^A i |
| B | I^B I^B, I^B i |
| AB | I^A I^B (Co-dominance) |
| O | ii (Recessive) |
- Paternity/Maternity Application: This system can be used to exclude potential parents. For instance, a couple with blood groups AB (genotype I^A I^B) and O (genotype ii) cannot have a biological child with blood group O. Their possible offspring genotypes are I^A i (Blood group A) and I^B i (Blood group B). Therefore, a child with blood group O must have a different biological parentage. The Rhesus (Rh) factor follows a simpler dominant-recessive pattern but is inherited independently of the ABO group.
Sex Determination in Humans
- The sex of an individual is determined by the sex chromosomes. The work of Thomas Hunt Morgan on fruit flies (Drosophila melanogaster) in the early 1910s was instrumental in confirming the role of chromosomes in heredity, including sex determination.
- Humans have one pair of sex chromosomes. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
- During meiosis, a female produces eggs that all contain a single X chromosome. A male produces two types of sperm in roughly equal numbers: half contain an X chromosome, and half contain a Y chromosome.
- The sex of the offspring is determined by which type of sperm fertilizes the egg:
- If an X-sperm fertilizes the egg (X), the zygote is XX (female).
- If a Y-sperm fertilizes the egg (X), the zygote is XY (male).
- Therefore, the genetic contribution of the father determines the sex of the child. This scientific fact contradicts patriarchal social norms that often blame women for not bearing male children. The probability of having a boy or a girl is approximately 50:50, or a 1:1 phenotypic ratio.
Genetic Abnormalities
Genetic disorders arise from mutations in genes or abnormalities in chromosome structure or number.
Chromosomal Disorders (Aneuploidy): These are caused by an incorrect number of chromosomes, often due to an error called non-disjunction during meiosis.
- (a) Down Syndrome: First described by John Langdon Down in 1866, it is caused by the presence of a full or partial extra copy of chromosome 21 (Trisomy 21). The individual has 47 chromosomes instead of 46. It is characterized by distinct facial features, a small round head, and varying degrees of intellectual disability.
- (b) Klinefelter’s Syndrome: Described by Harry Klinefelter in 1942, this affects males and is caused by the presence of an extra X chromosome (XXY), resulting in 47 chromosomes. Individuals are typically tall, have underdeveloped testes, reduced fertility, and may develop some female characteristics like gynecomastia.
- (c) Turner’s Syndrome: Described by Henry Turner in 1938, this affects females and is caused by the complete or partial absence of one X chromosome (XO), resulting in 45 chromosomes. It is characterized by short stature, rudimentary ovaries leading to sterility, and a “webbed” neck.
Assisted Reproductive Technology
- In-Vitro Fertilization (IVF): A medical procedure where an egg is fertilized by sperm outside the body, in a laboratory dish (“in vitro”). The first successful birth of a child, Louise Brown, through IVF occurred in 1978, pioneered by Robert Edwards and Patrick Steptoe. The resulting embryo is then transferred to the uterus. The genetic characteristics of the baby are determined solely by the DNA from the egg and sperm donors, not from the woman who carries the pregnancy (surrogate or host mother).
Twins
- Identical (Monozygotic) Twins: Originate from a single zygote that splits into two separate embryos early in development. Because they come from the same zygote, they are genetically identical and are always the same sex.
- Non-identical (Dizygotic or Fraternal) Twins: Occur when two separate eggs are ovulated and fertilized by two different sperm. The resulting zygotes develop concurrently. Genetically, they are no more similar than any other siblings, sharing approximately 50% of their genes.
Gene-Related Genetic Defects
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Sickle Cell Anemia (Autosomal Recessive): This is a disorder affecting hemoglobin, the oxygen-carrying protein in red blood cells (RBCs). A mutation in the gene on chromosome 11 causes RBCs to become rigid and sickle-shaped, especially under low oxygen conditions.
- Genetics: An individual with two copies of the sickle cell gene (Hb^sHb^s) has the disease. An individual with one normal gene and one sickle cell gene (HbHb^s) is a carrier. They are generally healthy but can pass the gene to their offspring.
- Heterozygote Advantage: Carriers (HbHb^s) show increased resistance to malaria, a significant evolutionary advantage in regions where malaria is endemic. This is a classic example of natural selection maintaining a harmful allele in a population.
- Consanguinity: Marriage between close relatives increases the probability that both partners carry the same recessive allele (e.g., Hb^s), thereby increasing the risk of having a child with the disorder (e.g., Hb^sHb^s).
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Sex-Chromosome-Linked Diseases (X-linked Recessive): These are caused by mutations on the X chromosome.
- Hemophilia: A disorder where the blood’s ability to clot is severely reduced. The genes for clotting factors VIII and IX are on the X chromosome. Since males (XY) have only one X chromosome, a single recessive allele for hemophilia will cause the disease. Females (XX) must have two copies of the allele to be affected and are more often asymptomatic carriers. It is famously known as the “royal disease” due to its prevalence among the descendants of Queen Victoria.
- Color Blindness: The most common form, red-green color blindness, is also an X-linked recessive trait. The genes for red and green light-sensitive proteins are located on the X chromosome. As with hemophilia, it is far more common in males. A female carrier (X^C X^c) will have normal vision, but can pass the colorblind allele (X^c) to her son, who will be colorblind (X^c Y).
Prelims Pointers
- Adenoviruses have double-stranded DNA (dsDNA) genomes.
- Retroviruses (like HIV) have single-stranded RNA (ssRNA) genomes and use the enzyme reverse transcriptase.
- Humans have 46 chromosomes (23 pairs) in somatic cells.
- Human gametes (sperm and egg) are haploid, containing 23 chromosomes.
- Somatic cells are diploid (2n); gametes are haploid (n).
- Mitosis: Cell division for growth and repair; produces two identical diploid cells.
- Meiosis: Cell division for making gametes; produces four genetically unique haploid cells.
- Genotype: The genetic constitution of an individual organism.
- Phenotype: The set of observable characteristics of an individual.
- Homozygous: Having two identical alleles for a trait (e.g., TT).
- Heterozygous: Having two different alleles for a trait (e.g., Tt).
- Blood group AB is an example of co-dominance.
- Blood group O is recessive to both A and B.
- The sex of a human child is determined by the father’s sperm (carrying either an X or a Y chromosome).
- Down Syndrome: Trisomy 21 (total 47 chromosomes).
- Klinefelter’s Syndrome: XXY genotype (total 47 chromosomes), affects males.
- Turner’s Syndrome: XO genotype (total 45 chromosomes), affects females.
- Identical (Monozygotic) Twins: Develop from one zygote. Genetically identical.
- Fraternal (Dizygotic) Twins: Develop from two separate zygotes. Genetically like siblings.
- Sickle Cell Anemia: An autosomal recessive disorder. Carriers have resistance to malaria.
- Hemophilia: An X-linked recessive disorder related to blood clotting.
- Color Blindness: An X-linked recessive disorder affecting vision. It is more common in males.
Mains Insights
GS Paper I (Indian Society)
- Science and Social Norms: The scientific understanding of sex determination (that the male partner is responsible) directly challenges deep-rooted patriarchal norms in Indian society, where women are often blamed and stigmatized for giving birth to daughters. This knowledge can be a powerful tool for social reform and combating practices like female foeticide.
- Consanguinity and Community Health: The practice of marriage among close relatives, prevalent in certain communities and tribes in India, has significant genetic implications. While strengthening social bonds, it increases the incidence of autosomal recessive disorders like Sickle Cell Anemia and Thalassemia. This presents a unique public health challenge requiring culturally sensitive genetic counseling and awareness campaigns.
GS Paper III (Science & Technology; Economy)
- Assisted Reproductive Technologies (ART): Technologies like IVF offer solutions for infertility but also raise complex issues. These include the high cost making them inaccessible to many, the lack of robust regulation leading to exploitation, and the legal and emotional complexities of surrogacy. The Surrogacy (Regulation) Act, 2021, and the ART (Regulation) Act, 2021, are steps towards addressing these issues.
- Genetics and Public Health Policy: The prevalence of genetic disorders like Sickle Cell Anemia (especially high among tribal populations in Central India) necessitates targeted public health interventions. This includes universal screening programs, genetic counseling facilities, and investing in research for affordable treatments like gene therapy. This forms a critical part of achieving Sustainable Development Goal 3 (Good Health and Well-being).
- Future of Genetic Technology: Advances like CRISPR-Cas9 gene editing hold the promise of curing inherited genetic disorders. However, this also opens up a debate on “designer babies” and the potential for new forms of social inequality based on genetic enhancement. India needs to develop a strong bioethical framework to guide research and application in this field.
GS Paper IV (Ethics, Integrity, and Aptitude)
- Ethical Dilemmas in ART: IVF and surrogacy involve profound ethical questions. What are the rights of the surrogate mother? What is the moral status of an embryo? Should there be a limit on the number of IVF cycles? These questions require a balance between individual autonomy, the welfare of the child, and societal values.
- Genetic Information and Privacy: With the decreasing cost of genetic testing, there is a risk of genetic discrimination by employers or insurance companies. There is an ethical imperative to protect the privacy of an individual’s genetic information and ensure it is not used to create a “genetic underclass.”
- The Responsibility of “Carriers”: An individual who is a carrier for a serious recessive genetic disorder faces an ethical choice regarding reproduction. Genetic counseling plays a vital role in providing non-directive information to help individuals make informed decisions that align with their own values and principles, without coercion.