Introduction to Genetics
- Fundamental Questions: Why do elephants always give birth to baby elephants and not other animals? Why do mango seeds grow into mango trees and not other plants? These questions delve into the science of genetics.
- Inheritance and Variation: Genetics explores how traits are passed from parents to offspring (inheritance) and how offspring vary from their parents (variation).
- Historical Insight: Humans have long understood that sexual reproduction causes variation, exploiting these natural variations to breed desirable traits in plants and animals.
Mendel’s Contributions to Genetics
- Gregor Mendel: In the mid-19th century, Gregor Mendel's hybridization experiments with garden peas (1856-1863) laid the foundation for the laws of inheritance.
- Scientific Approach: Mendel applied statistical analysis and mathematical logic to biology, using large sample sizes and confirming his results across successive generations.
- Key Discoveries: Mendel identified traits in pea plants that exhibited clear opposing forms (e.g., tall vs. dwarf plants, yellow vs. green seeds). This allowed him to establish basic rules of inheritance.
- He selected varieties that differed with respect to seven traits with easilydistinguishable contrasting forms, i.e., he selected fourteen true-breeding varieties asshown in the table given below:-
Mendel’s Experiments and Findings
- Artificial Pollination: Mendel used artificial pollination and cross-pollination with true-breeding pea lines to study inheritance.
- True-Breeding Lines: A true-breeding line consistently exhibits a specific trait over multiple generations. Mendel selected 14 such lines, focusing on pairs with contrasting traits like smooth or wrinkled seeds, yellow or green seeds, and tall or dwarf plants.
The Importance of Genetics in Agriculture
- Early Knowledge: Even ancient civilizations understood that variations in plants and animals could be harnessed for breeding desirable traits, as seen in domesticated breeds like the Sahiwal cows in Punjab.
- Scientific Basis: Despite early knowledge of inheritance, the scientific understanding of these processes began with Mendel's work.
Genetics is the scientific study of how traits are inherited and vary in living organisms. Gregor Mendel's pioneering work in the 19th century established foundational laws of inheritance through meticulous experiments with pea plants. His discoveries laid the groundwork for modern genetics, influencing how we understand the transmission of traits and the role of variation in natural and artificial selection. This knowledge has profound implications in agriculture, enabling the breeding of plants and animals with desirable characteristics to enhance productivity and sustainability.
Inheritance of One Gene: Mendel's Hybridization Experiments
Mendel's Experiment on Pea Plants
- Hybridization Experiment: Mendel conducted a hybridization experiment crossing tall and dwarf pea plants to study the inheritance of one gene.
- First Generation (F1): He observed that all F1 progeny were tall, resembling one parent, with no dwarf plants present.
- Second Generation (F2): Self-pollination of F1 plants resulted in a mix of tall and dwarf plants in a 3:1 ratio. This showed that the dwarf trait reappeared, indicating it was not lost but hidden in the F1 generation.
Key Observations and Conclusions
- No Blending: The traits did not blend; F2 plants were either tall or dwarf, mirroring the original parental traits.
- Stable Factors: Mendel proposed that stable factors, now known as genes, are passed unchanged from parents to offspring through gametes.
- Alleles: Genes exist in different forms called alleles, which can be dominant or recessive. Dominant alleles mask the expression of recessive alleles in heterozygous individuals.
Genetic Terminology
- Genotype and Phenotype: The genetic makeup (TT, Tt, tt) is the genotype, while the observable trait (tall or dwarf) is the phenotype.
- Homozygous and Heterozygous: Homozygous individuals have identical alleles (TT or tt), while heterozygous individuals have different alleles (Tt).
Monohybrid Cross
- Allele Segregation: During gamete formation, alleles segregate randomly, giving a 50% chance for each allele to be passed on.
- Punnett Square: A tool to predict the probability of offspring genotypes. For example, crossing TT (tall) and tt (dwarf) results in F1 hybrids with Tt genotypes, all tall due to the dominance of T over t.
Test Cross
- Purpose: To determine the genotype of an organism displaying a dominant phenotype.
- Procedure: Cross the organism with a homozygous recessive individual. Analyzing the offspring can reveal the unknown genotype.
- Example: Crossing a tall plant (T_) with a dwarf plant (tt). If any offspring are dwarf, the tall plant must be heterozygous (Tt).
Another example:
Understanding Mendel's Findings
- Genetic Inheritance: Mendel's principles laid the foundation for modern genetics, explaining how traits are inherited and why offspring resemble their parents but also show variation.
- Application in Agriculture: Mendel’s work is crucial in breeding programs, helping to predict and select for desirable traits in crops and livestock.
Practical Application: Test Cross Example
- Example Test Cross: Crossing a violet flower (dominant, W_) with a white flower (recessive, ww) using a Punnett Square to determine the genotype of the violet flower.
- Possible Outcomes: If all offspring are violet, the violet flower is homozygous (WW). If some offspring are white, the violet flower is heterozygous (Ww).
Mendel's Principles of Inheritance
Based on his observations from monohybrid crosses, Gregor Mendel formulated two fundamental rules known as the Principles or Laws of Inheritance: the Law of Dominance and the Law of Segregation. These principles are foundational to understanding genetic inheritance.
Law of Dominance
The Law of Dominance states that:
- Discrete Units: Characters are controlled by discrete units called factors (now known as genes).
- Pair Occurrence: These factors occur in pairs.
- Dominance and Recessiveness: In a pair of dissimilar factors, one factor dominates (dominant) over the other (recessive).
This law explains why, in a monohybrid cross, only one of the parental traits is expressed in the F1 generation and both traits reappear in the F2 generation in a 3:1 ratio. It demonstrates that the dominant trait masks the presence of the recessive trait in the F1 generation, but the recessive trait is not lost and reappears in the F2 generation.
Law of Segregation
The Law of Segregation asserts that:
- No Blending: Alleles do not blend; both characters reappear in the F2 generation as distinct traits, even if one is not visible in the F1 generation.
- Allele Segregation: During gamete formation, the alleles of a pair segregate from each other so that each gamete receives only one allele of the pair.
In simpler terms, even though the parent organisms contain two alleles for each trait, these alleles separate during gamete formation, ensuring that each gamete carries only one allele. Homozygous parents produce identical gametes, while heterozygous parents produce two types of gametes in equal proportion.
Practical Implications
- Predicting Traits: Mendel's laws help predict the inheritance patterns of traits in offspring, crucial for genetic research and breeding programs.
- Genetic Variation: Understanding these principles explains genetic variation and how dominant and recessive traits are passed down through generations.
Incomplete Dominance
Incomplete dominance is a fascinating aspect of genetics where neither allele in a pair is completely dominant over the other, resulting in a blending of traits in the heterozygous phenotype. Let's explore this concept further:
Example of Incomplete Dominance
Consider the inheritance of flower color in dog flowers (snapdragons). When true-breeding red-flowered (RR) plants are crossed with true-breeding white-flowered (rr) plants, the F1 generation (Rr) displays a pink phenotype. Upon self-pollination of the F1 generation, the F2 generation exhibits a ratio of 1 (RR) red : 2 (Rr) pink : 1 (rr) white.
Concept of Dominance
Dominance refers to the expression of a particular allele over another in the heterozygous condition. But why are some alleles dominant while others are recessive?
- Allelic Information: Each gene contains information for expressing a specific trait. In diploid organisms, there are two copies of each gene, forming a pair of alleles.
- Allelic Variation: Alleles can undergo changes, resulting in modified information. These variations can affect the expression of traits.
- Functional Enzymes: Taking the example of an enzyme-producing gene, the normal allele produces a functional enzyme needed for a specific transformation.
- If the modified allele produces:
- a normal/less efficient enzyme: It is equivalent to the unmodified allele and results in the same phenotype.
- a non-functional enzyme or no enzyme: The phenotype is affected, and it depends solely on the functioning of the unmodified allele.
- If the modified allele produces:
Dominant and Recessive Alleles
- Dominant Allele: It produces the original phenotype and is expressed even in the presence of a recessive allele.
- Recessive Allele: It produces a non-functional enzyme or no enzyme, resulting in the expression of the recessive trait only when present in the homozygous condition.
Practical Implications
- Phenotypic Variation: Incomplete dominance contributes to phenotypic diversity by creating intermediate phenotypes.
- Breeding Programs: Understanding incomplete dominance helps in selective breeding to achieve desired traits in plants and animals.
Co-dominance
Co-dominance is a fascinating genetic phenomenon where the offspring in the F1 generation display traits from both parents rather than resembling one or the other. Let's delve deeper into this concept:
Example of Co-dominance
An excellent example of co-dominance is observed in the ABO blood group system in humans, controlled by the gene I. This gene has three alleles: IA, IB, and i. While IA and IB produce slightly different forms of sugars on the red blood cell surface, allele i does not produce any sugar. When IA or IB is present with i, it expresses fully. However, when IA and IB are present together, they both express their own types of sugars, resulting in red blood cells having both A and B types of sugars.
Understanding Multiple Alleles
The ABO blood grouping exemplifies multiple alleles, where more than two alleles govern the same trait. With three alleles (IA, IB, and i), there are six possible genotypes for the ABO blood types. Phenotypically, this results in various blood group combinations.
Implications of Co-dominance
Starch Synthesis in Pea Seeds: Another example is starch synthesis in pea seeds controlled by a single gene with two alleles (B and b). BB homozygotes produce large starch grains, bb homozygotes produce smaller grains, and Bb heterozygotes produce intermediate-sized grains. This showcases how a single gene can have multiple effects, leading to different phenotypes.
Understanding Dominance: Dominance is not solely determined by the gene itself but also depends on the gene product and the specific phenotype being examined. Co-dominance and incomplete dominance highlight the complexity of genetic interactions.
Practical Applications
- Medical Diagnosis: Understanding co-dominance aids in interpreting genetic traits, such as blood types, for medical diagnoses and blood transfusions.
- Selective Breeding: In agriculture, knowledge of co-dominance helps breeders develop crops with desired traits by selectively crossing plants with specific genetic compositions.
Dihybrid Crosses and Mendel's Laws
In his pioneering work with pea plants, Gregor Mendel ventured beyond monohybrid crosses to investigate the inheritance patterns of two different traits simultaneously. Let's unravel the complexities of dihybrid crosses and Mendel's Laws:
Understanding Dihybrid Crosses
Mendel crossed pea plants with distinct traits for seed color and shape: yellow and round seeds (dominant traits) versus green and wrinkled seeds (recessive traits). Surprisingly, the F1 generation exhibited dominant traits, indicating that yellow color and round shape were dominant over green color and wrinkled shape, respectively.
Mendel's Second Law: Independent Assortment
In his dihybrid crosses, Mendel observed that the segregation of one pair of traits was independent of the segregation of another pair. This led to the formulation of Mendel's Law of Independent Assortment. According to this law, when two pairs of traits are combined in a hybrid, the segregation of one pair of characters is unrelated to the segregation of the other pair.
Using Punnett Squares for Analysis
Punnett squares are invaluable tools for understanding the inheritance patterns of dihybrid crosses. By depicting the segregation of alleles for each pair of traits, we can predict the genotypes and phenotypes of offspring in the F2 generation.
Deriving Genotypic and Phenotypic Ratios
In dihybrid crosses, the phenotypic ratio of 9:3:3:1 emerges, representing the combinations of dominant and recessive traits. This ratio can be derived from the independent assortment of alleles during gamete formation and fertilization.
Practical Applications
- Selective Breeding: Understanding dihybrid crosses assists breeders in developing crops with desired combinations of traits, such as color, shape, and size.
- Genetic Research: Mendel's Laws continue to serve as foundational principles in genetic research, guiding studies on inheritance patterns and gene interactions.
Chromosomal Theory of Inheritance
Mendel's Revolutionary Work
- Gregor Mendel's groundbreaking research on inheritance, published in 1865, remained unnoticed for several decades due to various reasons.
- Mendel's concept of genes as discrete units controlling traits was initially met with skepticism, and his mathematical approach was unconventional at the time.
- While Mendel proposed the existence of genes, he lacked physical evidence to support his theories.
Rediscovery and Advancements
- In 1900, scientists De Vries, Correns, and von Tschermak independently rediscovered Mendel's work on inheritance, coinciding with advancements in microscopy.
- Microscopy revealed structures in the nucleus called chromosomes, which appeared to align and divide during cell division.
- Walter Sutton and Theodore Boveri correlated chromosome behavior with Mendel's laws, laying the foundation for the chromosomal theory of inheritance.
Understanding Chromosome Behavior
- Chromosomes, like genes, occur in pairs, with alleles located on homologous chromosomes.
- During meiosis, chromosomes segregate independently, leading to the segregation of gene pairs.
- Sutton and Boveri's synthesis of chromosomal behavior and Mendelian principles culminated in the chromosomal theory of inheritance.
Experimental Verification
- Thomas Hunt Morgan and his colleagues conducted experiments with fruit flies (Drosophila melanogaster) to verify the chromosomal theory.
- Fruit flies were ideal for genetic studies due to their rapid life cycle, clear sexual differentiation, and observable hereditary variations.
- Morgan's experiments provided empirical evidence for the role of chromosomes in inheritance, elucidating the mechanisms of variation in sexual reproduction.
Linkage and Recombination in Genetics
Morgan's Drosophila Experiments
- Thomas Hunt Morgan conducted dihybrid crosses in Drosophila to investigate sex-linked genes, akin to Mendel's pea experiments.
- Crossing yellow-bodied, white-eyed females with brown-bodied, red-eyed males revealed a deviation from the expected 9:3:3:1 ratio in the F2 generation.
Understanding Linkage
- Genes situated on the same chromosome tend to be inherited together, a phenomenon termed "linkage."
- Morgan observed that parental gene combinations were more prevalent than non-parental types, indicating physical association on the chromosome.
Introducing Recombination
- Recombination, the generation of non-parental gene combinations, occurs through the exchange of genetic material during crossing over.
- Morgan coined the terms "linkage" and "recombination" to describe these phenomena, elucidating the dynamics of gene inheritance.
Variability in Linkage
- Some genes exhibit tight linkage, with minimal recombination, while others show loose linkage, resulting in higher recombination frequencies.
- For instance, genes like white and yellow were tightly linked, showing only 1.3% recombination, whereas white and miniature wing exhibited 37.2% recombination.
Mapping Genetic Distance
- Alfred Sturtevant utilized recombination frequencies to map gene positions on chromosomes, establishing genetic maps.
- Genetic mapping serves as a foundational tool in genome sequencing projects, facilitating the understanding of gene organization and function.
Polygenic Inheritance
Beyond Mendel's Pea Experiments
- While Mendel focused on traits with distinct forms like flower color, many traits exhibit a continuum of variation.
- Traits like human height and skin color are not binary but exist across a spectrum, reflecting polygenic inheritance.
Understanding Polygenic Traits
- Polygenic traits are governed by three or more genes, with each gene contributing to the trait's expression.
- Environmental factors also influence polygenic traits, adding complexity to inheritance patterns.
Exploring Skin Color as an Example
- Human skin color, controlled by multiple genes, serves as a prime example of polygenic inheritance.
- Dark skin results from the dominance of certain alleles (e.g., A, B, C), while light skin is associated with recessive alleles (e.g., a, b, c).
Additive Allelic Effects
- In polygenic inheritance, the phenotype reflects the additive effects of each allele.
- For instance, individuals with genotypes containing a mix of dominant and recessive alleles exhibit intermediate skin tones.
Determining Phenotypic Variation
- The combination of alleles in an individual's genotype dictates the darkness or lightness of their skin.
- Genotypes with more dominant alleles tend to produce darker skin tones, while those with more recessive alleles yield lighter tones.
Pleiotropy: One Gene, Multiple Effects
Unveiling Pleiotropic Genes
- While many genes influence a single phenotype, pleiotropy occurs when a single gene affects multiple phenotypic traits.
Understanding the Mechanism
- Pleiotropy often arises from a gene's impact on various metabolic pathways, leading to diverse phenotypic expressions.
Phenylketonuria: A Case Study
- Phenylketonuria (PKU) exemplifies pleiotropy, stemming from mutations in the gene encoding phenylalanine hydroxylase.
- This single gene mutation results in a cascade of phenotypic effects, including mental retardation and reduced hair and skin pigmentation.
Complex Phenotypic Manifestations
- Pleiotropy underscores the intricate interplay between genes and phenotypes, showcasing the far-reaching consequences of genetic mutations.
- Understanding pleiotropy enhances our comprehension of genetic disorders and the complexities of human biology.
Sex Determination: Genetic Insights
The Puzzle of Sex Determination
- Geneticists have long puzzled over the mechanism of sex determination, with early clues emerging from experiments in insects.
Henking's Discovery
- In 1891, Henking observed a peculiar nuclear structure during spermatogenesis in insects, later identified as the X-chromosome.
- Investigations revealed that in insects like grasshoppers, sex determination follows the XO type, with females bearing an additional X-chromosome.
XY Type: Mammalian Model
- Many mammals, including humans, follow the XY type of sex determination.
- Males possess one X and one Y chromosome, while females have a pair of X-chromosomes alongside autosomes.
Male Heterogamety: Dual Gametes
- In both XO and XY mechanisms, males produce two types of gametes: with or without X-chromosome, or a mix of X and Y chromosomes.
- This pattern, known as male heterogamety, underscores the diverse genetic contributions to offspring.
Female Heterogamety: Avian Anomaly
- Birds exhibit a unique mechanism of sex determination, where females produce two different gametes in terms of sex chromosomes.
- Unlike mammals, both males and females have the same total number of chromosomes, but females possess Z and W chromosomes, while males have a pair of Z-chromosomes.
Sex Determination Mechanisms
Introduction
- The mechanism of sex determination has intrigued geneticists for ages, with initial insights emerging from insect experiments.
Henking's Discovery
- Henking's cytological observations in insects revealed a peculiar nuclear structure during spermatogenesis, later identified as the X chromosome.
XO and XY Types
- In insects like grasshoppers, the XO type prevails, while mammals, including humans, exhibit the XY type.
- XO indicates females with an additional X chromosome, while males lack one X chromosome.
- In XY type, males carry XY chromosomes, while females bear XX chromosomes.
Male Heterogamety
- In both XO and XY types, males produce gametes with varying sex chromosomes, exemplifying male heterogamety.
Female Heterogamety
- Birds showcase a different mechanism, termed female heterogamety, where females produce distinct Z and W sex chromosomes.
Human Sex Determination
- Humans follow the XY type, with males possessing an X and a Y chromosome, while females have a pair of X chromosomes.
- Sperm carrying either X or Y chromosomes determine the offspring's sex upon fertilization.
Misconceptions and Realities
- Despite scientific clarity, societal biases persist, wrongly attributing female births to women and subjecting them to discrimination.
Honey Bee Sex Determination
Unique Chromosomal Dynamics
- Honey bees exhibit haplodiploid sex determination, where females are diploid (32 chromosomes) and males are haploid (16 chromosomes).
- Males develop from unfertilized eggs through parthenogenesis, showcasing a distinctive reproductive system.
Peculiar Traits
- Honey bee males, or drones, lack fathers but have grandfathers, highlighting the intriguing dynamics of haplodiploidy.
Mutation
Introduction
- Mutation, a fundamental biological phenomenon, alters DNA sequences, thereby affecting both genotype and phenotype.
Source of Variation
- Alongside recombination, mutation plays a pivotal role in introducing genetic variation within organisms.
Chromosomal Alterations
- Deletions or insertions in DNA segments lead to chromosomal aberrations, often observed in cancer cells.
- Such alterations can disrupt gene function due to changes in chromosome structure.
Point Mutations
- Point mutations involve the alteration of a single base pair in DNA, exemplified by diseases like sickle cell anemia.
- Deletions or insertions of base pairs can result in frame-shift mutations, contributing to genetic disorders.
Inducing Factors
- Various chemical and physical agents, known as mutagens, can induce mutations in DNA.
- UV radiation stands as a notable mutagen, capable of causing genetic alterations in organisms.
Genetic Disorders
Pedigree Analysis
Hereditary Understanding: The concept of inherited disorders has long persisted in human society, based on identifiable characteristics passed down through families.
- Mendel's Influence: Following Mendel's groundbreaking work, the examination of inheritance patterns in humans gained momentum.
- Pedigree Analysis: Unlike controlled crosses in simpler organisms, humans rely on studying familial history to discern inheritance patterns, termed pedigree analysis.
- Family Tree Representation: Pedigree analysis entails observing traits across several generations of a family, depicted in a family tree format.
- Utilitarian Tool: Pedigree studies in human genetics serve as a robust tool for tracing the inheritance of specific traits, abnormalities, or diseases.
- Standard Symbolism: Various symbols are employed in pedigree analysis to represent distinct aspects of inheritance patterns.
Mendelian Disorders
Genetic Classification: Genetic disorders are broadly categorized into Mendelian disorders and Chromosomal disorders.
- Single Gene Alterations: Mendelian disorders primarily arise from mutations or alterations in single genes, following Mendelian principles of inheritance.
- Pedigree Traceability: The inheritance patterns of Mendelian disorders can be traced within families through pedigree analysis.
- Common Examples: Prevailing Mendelian disorders include Haemophilia, Cystic fibrosis, Sickle-cell anemia, Colour blindness, Phenylketonuria, and Thalassemia.
- Dominant or Recessive: These disorders may manifest as dominant or recessive traits, ascertainable through pedigree analysis.
- Sex Chromosome Linkage: Some Mendelian disorders, like haemophilia, are linked to the sex chromosome, demonstrating transmission from carrier females to male progeny.
- Illustrative Pedigrees: Figure 4.14 depicts representative pedigrees showcasing dominant and recessive traits, serving as models for further study and discussion.
Representative pedigree analysis of (a) Autosomal dominant trait(eg. Myotonic
dystrophy) (b) Autosomal recessive trait (eg. Sickle cell anaemia)
Representative pedigree analysis of (a) Autosomal dominant trait(eg. Myotonic
dystrophy) (b) Autosomal recessive trait (eg. Sickle cell anaemia)
Colour Blindness
- Nature of Disorder: Colour blindness is a sex-linked recessive disorder characterized by the inability to distinguish between red and green colors.
- Genetic Basis: It arises from a defect in either the red or green cone of the eye due to mutations in certain genes on the X chromosome.
- Prevalence: Afflicting approximately 8% of males and only 0.4% of females, it is more common in males due to the genes for color blindness being on the X chromosome.
- Inheritance Pattern: Sons of carrier women have a 50% chance of inheriting the disorder, as they receive the affected X chromosome from their mother.
- Manifestation in Females: While carrier mothers do not exhibit color blindness themselves, they can pass on the gene to their sons. Daughters are typically unaffected unless both parents are carriers or the father is color blind.
Haemophilia
- Genetic Background: Haemophilia, another sex-linked recessive disorder, affects the clotting of blood due to a deficiency in a specific protein involved in blood clotting.
- Inheritance Dynamics: Unaffected carrier females can transmit the disease to some of their male offspring, who inherit the affected X chromosome.
- Clinical Manifestation: Afflicted individuals experience prolonged bleeding from even minor cuts due to the impaired clotting process.
- Rare Occurrence in Females: The likelihood of females developing haemophilia is rare, requiring both parents to contribute specific genetic factors.
- Historical Context: The family lineage of Queen Victoria serves as an illustrative example, showcasing several descendants affected by haemophilia due to her status as a carrier.
Sickle-Cell Anaemia
- Nature of Disorder: Sickle-cell anaemia is an autosomal recessive trait transmitted from carrier parents to offspring when both partners are heterozygous for the gene.
- Genetic Control: Controlled by a single pair of alleles, HbA and HbS, with only homozygous individuals for HbS (HbS/HbS) displaying the diseased phenotype.
- Carrier Status: Heterozygous individuals (HbA/HbS) appear unaffected but carry the disease, with a 50% chance of transmitting the mutant gene to their offspring, resulting in sickle-cell trait.
- Molecular Basis: Caused by a substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the beta globin chain of the haemoglobin molecule.
- Pathophysiology: The mutant haemoglobin molecule undergoes polymerisation under low oxygen tension, leading to the distortion of red blood cells into elongated sickle-like structures.
Micrograph of the red blood cells and the amino acid composition of the relevant portion of
B-chain of haemoglobin: (a) from a normal individual; (b) from an individual with sickle-cell
anaemia
Phenylketonuria
|
Micrograph of the red blood cells and the amino acid composition of the relevant portion of
B-chain of haemoglobin: (a) from a normal individual; (b) from an individual with sickle-cell
anaemia |
- Inborn Metabolic Disorder: Phenylketonuria (PKU) is an autosomal recessive trait characterized by the absence of an enzyme that converts phenylalanine into tyrosine.
- Biochemical Consequences: Accumulation of phenylalanine and its derivatives, such as phenylpyruvic acid, occurs due to enzyme deficiency, leading to mental retardation.
- Clinical Manifestations: The accumulation of metabolic by-products in the brain results in cognitive impairment, while urinary excretion occurs due to poor kidney absorption.
Thalassemia
Nature of Disorder
- Autosomal Recessive Blood Disease: Thalassemia is a hereditary condition transmitted from unaffected carrier parents to offspring when both partners are heterozygous for the gene.
- Genetic Defect: The disease arises from mutations or deletions causing a reduced rate of synthesis of one of the globin chains (α and β chains) that form haemoglobin.
- Clinical Manifestations: Reduced synthesis of normal haemoglobin leads to the formation of abnormal haemoglobin molecules, resulting in anaemia, a hallmark of thalassemia.
Classification
- Based on Affected Globin Chain: Thalassemia can be classified according to which chain of the haemoglobin molecule is affected.
- α Thalassemia: Involves the impaired production of α globin chains.
- β Thalassemia: Involves the impaired production of β globin chains.
- α Thalassemia: Involves the impaired production of α globin chains.
- β Thalassemia: Involves the impaired production of β globin chains.
Molecular Basis
- α Thalassemia: Controlled by two closely linked genes, HBA1 and HBA2, on chromosome 16, with mutations or deletions of one or more genes leading to decreased alpha globin production.
- β Thalassemia: Controlled by a single gene, HBB, on chromosome 11, with mutations of one or both genes resulting in reduced beta globin production.
Distinction from Sickle-Cell Anaemia
- Qualitative vs. Quantitative Problem: Thalassemia involves a quantitative issue, with insufficient synthesis of globin molecules, whereas sickle-cell anaemia entails a qualitative problem, resulting in the production of dysfunctional globin.
Chromosomal Disorders
Overview
- Nature of Disorders: Chromosomal disorders stem from abnormalities in chromosome number or structure, including absence, excess, or abnormal arrangement of chromosomes.
- Types of Abnormalities:
- Aneuploidy: Gain or loss of one or more chromosomes due to segregation errors during cell division.
- Polyploidy: Presence of an extra set of chromosomes, often observed in plants.
- Human Chromosome Composition: Typically, humans have 46 chromosomes (23 pairs), including 22 pairs of autosomes and one pair of sex chromosomes.
- Aneuploidy: Gain or loss of one or more chromosomes due to segregation errors during cell division.
- Polyploidy: Presence of an extra set of chromosomes, often observed in plants.
Consequences of Abnormalities
- Trisomy and Monosomy: Rarely, individuals may have an extra or missing chromosome, leading to serious consequences.
- Examples: Down's syndrome (Trisomy 21), Turner's syndrome (Monosomy X), Klinefelter's syndrome (Trisomy XXY).
Specific Disorders
Down's Syndrome
- Cause: Presence of an extra copy of chromosome 21 (Trisomy 21).
- Characteristics: Short stature, distinctive facial features, developmental delays, and intellectual disabilities.
A representative figure showing an individual inflicted with Down’s syndrome and the corresponding Chromosomes of the individual
A representative figure showing an individual inflicted with Down’s syndrome and the corresponding Chromosomes of the individual
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