Principle of Inheritance and Variation Chapter Notes - Botany PDF Download

Mendel’s Laws of Inheritance

Mendel's work laid the foundation for our understanding of how traits are passed down from one generation to the next. He focused on simple traits in pea plants, like height and seed color, and used careful experiments to uncover the basic rules of inheritance.

Seven pairs of contrasting traits in pea plant studied by Mendel

Introduction

• In the mid-19th century, significant progress was made in understanding how traits are passed from parents to offspring.
• Gregor Mendel, through his hybridization experiments on garden peas over seven years (1856-1863), proposed fundamental laws of inheritance in living organisms.
• Mendel was the first to apply statistical analysis and mathematical logic to biological problems during his investigations into inheritance patterns.
• His experiments involved a large sample size, lending greater credibility to his findings.
• The confirmation of his results through experiments on successive generations of pea plants demonstrated that his conclusions reflected general rules of inheritance rather than mere conjectures.

True-Breeding Lines

• A true-breeding line is a group of organisms that, over many generations of self-pollination, consistently produce offspring with the same stable traits.
• Mendel selected 14 pairs of true-breeding pea plant varieties that were similar except for one contrasting trait. For instance, he might choose one variety with smooth seeds and another with wrinkled seeds.

Contrasting Traits

Some of the contrasting traits Mendel studied included:

• Seed Shape: Smooth vs. Wrinkled
• Seed Color: Yellow vs. Green
• Pod Shape: Inflated (full) vs. Constricted
• Pod Color: Green vs. Yellow
• Plant Height: Tall vs. Dwarf
• Flower colour:  Violet vs. white
•  Flower position: Axial vs. terminal

Inheritance of One Gene

• Trait: A feature or characteristic of an organism, like eye color or height.
• Gene: A unit of heredity, a segment of DNA that determines a specific trait.
• Allele: A variant form of a gene, such as the gene for flower color having a blue or white allele.
• Homozygous: Having two identical alleles for a trait, like two blue alleles for flower color.
• Heterozygous: Having two different alleles for a trait, like one blue and one white allele for flower color.
• Phenotype: The observable physical or physiological traits of an organism, like the color of its flowers.
• Genotype: The genetic makeup of an organism, represented by the alleles it carries, like BB or Bb for flower color.
• Dominant Trait: A trait that appears in the phenotype even when only one allele is present, like blue flowers in a plant with one blue and one white allele.
• Recessive Trait: A trait that only appears in the phenotype when both alleles are recessive, like white flowers in a plant with two white alleles.

Introduction to Mendel's Experiment

• Mendel conducted a hybridization experiment by crossing tall and dwarf pea plants to study the inheritance of one gene.
• The seeds produced from this cross were collected and grown to generate the first hybrid generation, known as the Filial 1 progeny (F1).

Steps in making a cross in pea

Observation of F1 Progeny

• Mendel observed that all the F1 progeny plants were tall, resembling one of the parents, and none were dwarf.
• This pattern was consistent across other pairs of traits, where the F1 generation always resembled one parent and did not show the trait of the other parent.

Self-Pollination of F1 Plants

• When Mendel self-pollinated the tall F1 plants, he found that in the Filial 2 generation (F2), some offspring were dwarf, a trait that was not seen in the F1 generation.
• The F2 generation exhibited a ratio of 3:1, with 3/4th being tall and 1/4th being dwarf.

[Question: 1815006]

Introduction of Concepts

• Mendel proposed that factors(now known as genes) were being passed down unchanged from parent to offspring through gametes.
• Genes are units of inheritance that contain information necessary for expressing specific traits.

Alleles and Genotypes

• Genes coding for contrasting traits are called alleles.
• For example,T represents the tall trait and t represents the dwarf trait.
• Plants can have pairs of alleles like TT(homozygous tall),Tt(heterozygous tall), or tt(homozygous dwarf).

Dominance and Recessiveness

• Mendel found that in a pair of dissimilar factors, one factor dominates the other, as seen in the F1 generation.
• In the case of height,T(tall) is dominant over t(dwarf).

Monohybrid Cross

• The cross between TT and tt plants is called a monohybrid cross.
• Mendel's observations led to the principle of segregation, where alleles segregate randomly into gametes during meiosis.

Punnett Square

• The Punnett Square is a tool developed by Reginald C. Punnett to predict the probability of different genotypes in offspring.
• It helps visualize the gametes produced by parents and the possible combinations in the offspring.

F2 Generation and Ratios

• In the F2 generation, a phenotypic ratio of 3:1 (tall to dwarf) and a genotypic ratio of 1:2:1 (TT:Tt:tt) are observed.
• The 1/4:1/2:1/4 ratio of TT:Tt:tt can be represented using the binomial expression (1/2T + 1/2t) 2.

Test Cross

• To determine the genotype of a tall F2 plant, Mendel performed a test cross by crossing the tall plant with a dwarf plant.
• A typical test cross involves crossing an organism with a dominant phenotype (whose genotype is unknown) with a recessive parent to analyze the offspring.

Conclusion

Mendel's experiments and observations laid the foundation for the understanding of genetic inheritance and the principles of dominance, segregation, and the role of alleles in determining traits.

[Question: 1815002]

Principles or Laws of Inheritance

Based on his observations from monohybrid crosses, Mendel formulated two fundamental rules to articulate his understanding of inheritance. These rules are now recognized as the Principles or Laws of Inheritance: the First Law (Law of Dominance) and the Second Law (Law of Segregation).

Law of Dominance

• According to this law:
• Characters are governed by distinct units known as factors.
• Factors exist in pairs.
• When a pair of factors is dissimilar, one factor dominates over the other. The dominant factor is expressed, while the recessive factor is masked.

The Law of Dominance explains the following:

• In the F1 generation of a monohybrid cross, only one parental character is expressed due to dominance.
• In the F2 generation, both parental characters appear, reflecting the segregation of alleles.
• The 3:1 phenotypic ratio observed in the F2 generation.

Law of Segregation

• This law is based on the principle that alleles do not blend and are recovered intact in the F2 generation, even though one allele is not visible in the F1 generation.
• During gamete formation, the alleles of a pair segregate from each other, so each gamete receives only one of the two alleles.
• A homozygous parent produces identical gametes, while a heterozygous parent produces two types of gametes, each with one allele, in equal proportions.

Incomplete Dominance

When scientists conducted experiments on different traits in various plants, they discovered a phenomenon called incomplete dominance. This occurs when the F1 generation exhibits a phenotype that is a blend of the two parental traits, rather than resembling either parent exactly. A classic example of incomplete dominance is seen in the inheritance of flower color in the dog flower, also known as snapdragon (Antirrhinum sp.).

Example of Incomplete Dominance in Snapdragons

• In a cross between true-breeding red-flowered plants (RR) and true-breeding white-flowered plants (rr), the F1 generation (Rr) displayed a pink phenotype.
• When the F1 plants were self-pollinated, the F2 generation exhibited a phenotypic ratio of 1 Red (RR): 2 Pink (Rr): 1 White (rr).
• Although the genotype ratios followed the expected Mendelian monohybrid cross pattern, the phenotypic ratios differed from the typical 3:1 dominant to recessive ratio.
• This shift occurred because the red allele (R) was not completely dominant over the white allele (r), allowing the heterozygous (Rr) phenotype to be distinctively pink.

Key Points About Dominance:

• Dominance refers to the relationship between alleles and how they influence the expression of traits.
• Not all alleles are created equal; some are dominant while others are recessive based on the traits they produce.
• A gene carries the information necessary to express a specific trait, and in diploid organisms, there are two copies of each gene (alleles).
• Alleles can differ due to changes, leading to variations in the information they provide.
• For example, in a gene responsible for producing an enzyme, one allele might produce a normal enzyme while the other could produce a less effective or non-functional enzyme.

In cases where the modified allele produces a non-functional enzyme or no enzyme at all, the phenotype will depend on the functioning of the unmodified allele. The unmodified allele, which represents the original phenotype, is considered the dominant allele, while the modified allele is typically the recessive one. Therefore, the recessive trait may manifest due to a non-functional enzyme or the absence of enzyme production. This illustrates the concept of dominance and how certain alleles can override others in determining phenotypic traits.

[Question: 1815003]

Co-Dominance

Co-dominance is a genetic phenomenon where the offspring (F1 generation) exhibit traits from both parents equally, rather than resembling just one parent (dominance) or showing a blend of traits (incomplete dominance).

Example: ABO Blood Groups in Humans

• ABO blood groups are determined by the gene I, which has three alleles: I_A, I_B, and i.
• The I_Aand I_Balleles produce different sugar types on the surface of red blood cells, while the i allele does not produce any sugar.
• In humans, who are diploid, each individual has two of these three alleles.
• Dominance Hierarchy:
  • I_Aand I_Bare co-dominant to each other and dominant over i.
  • When I_Aand i are present, only I_Ais expressed.
  • When I_Band i are present, only I_Bis expressed.
• Co-Dominance:
  • When both I_Aand I_Bare present, both sugars are produced, resulting in AB blood type.

Because there are three alleles, there are six possible combinations, leading to six different genotypes and four phenotypes:

Multiple Alleles

The ABO blood group system also illustrates multiple alleles, as more than two alleles (I_A, I_B, and i) control the same trait.

Example of One Gene with Multiple Effects:

• Starch synthesis in pea seeds is controlled by one gene with two alleles (B and b).
• BB homozygotes produce large starch grains and round seeds.
• bb homozygotes produce smaller starch grains and wrinkled seeds.
• Bb heterozygotes produce intermediate-sized starch grains and round seeds.

This example shows that dominance can vary depending on the specific phenotype being observed.

Inheritance of two genes

Gregor Mendel conducted experiments on pea plants to understand the inheritance of two traits: seed color and seed shape. He crossed plants with yellow, round seeds with those having green, wrinkled seeds. 

Dominance of Traits

•  Mendel discovered that the offspring from this cross exhibited yellow, round seeds, indicating that yellow color and round shape are dominant traits. 
• Dominant Traits: Yellow color (Y) and round shape (R) 
• Recessive Traits: Green color (y) and wrinkled shape (r) 

Genotypes of Parent Plants

•  Mendel used the genotypic symbols Y for dominant yellow seed color, y for recessive green seed color, R for round-shaped seeds, and r for wrinkled seed shape. 
•  The genotypes of the parent plants were RRYY (homozygous dominant for round yellow seeds) and rryy (homozygous recessive for wrinkled green seeds). 

Cross and F1 Generation

•  The cross between the parent plants resulted in the F1 hybrid RrYy, showing a combination of traits from both parents. 
• Gametes: The gametes RY (from RRYY) and ry (from rryy) unite during fertilization to produce the F1 hybrid RrYy. 

F2 Generation and Segregation Ratios

•  When Mendel self-hybridized the F1 plants, he observed that the traits segregated in the F2 generation. 
• Segregation Ratios: Yellow to green seeds segregated in a 3:1 ratio, and round to wrinkled seed shape also segregated in a 3:1 ratio. 

Conclusion

•  Mendel's experiments demonstrated that the inheritance of two genes follows a pattern of dominance and segregation, similar to monohybrid crosses. 
• Key Findings: Yellow color and round shape are dominant traits, while green color and wrinkled shape are recessive traits. 

Law of Independent Assortment

In a dihybrid cross, when Mendel crossed plants differing in two traits, he observed four phenotypes in the ratio 9:3:3:1. This ratio can be explained by the combination of two sets of traits: 3 Round to 1 Wrinkled and 3 Yellow to 1 Green.

• The ratio of 9:3:3:1 can be derived as a combination series of 3 yellow: 1 green, with 3 round: 1 wrinkled.
• Mendel's Law of Independent Assortment states that the segregation of one pair of traits is independent of the other pair in a hybrid.
• The Punnett square illustrates this independent segregation, showing how gametes are formed with different combinations of alleles.
• In the example of the F1 plant RrYy, four types of gametes are produced: RY, Ry, rY, and ry, each with a frequency of 25%.
• When these gametes are crossed in a Punnett square, it reveals the different genotypes and phenotypes of the F2 plants.

Dihybrid Crosses

The dihybrid cross is an experimental method used to determine the inheritance of two traits in offspring. Here’s a breakdown of the process:

Step 1:

Mendel conducted a dihybrid cross between pea plants with round, yellow seeds (RRYY) and wrinkled, green seeds (rryy).

Step 2:

The resulting F1 generation consisted of round, yellow seeds (RrYy), displaying the dominant traits.

Step 3:

When the F1 plants were self-pollinated, Mendel observed the phenotypic ratio in the F2 generation.

Step 4:

• 9 (Round, Yellow)
• 3 (Wrinkled, Yellow)
• 3 (Round, Green)
• 1 (Wrinkled, Green)

Step 5:

• This led to the formulation of Mendel’s Law of Independent Assortment, which states that the segregation of one pair of traits is independent of the other pair in a hybrid.

Conclusion

• The dihybrid cross and the resulting phenotypic ratio provide evidence for the independent inheritance of multiple traits in offspring.
•  Mendel’s experiments laid the foundation for the understanding of genetic inheritance and the principles governing it.

Chromosomal Theory of Inheritance

Mendel’s Work and Early Challenges

•  Mendel published his research on inheritance in 1865, but it went unrecognized until 1900 due to several reasons. 
•  Communication was difficult at the time, preventing widespread publicity of his work. 
•  Mendel’s idea of genes as stable, discrete units was not accepted by his contemporaries, who struggled to understand how these units could explain the continuous variation observed in nature. 
•  His mathematical approach to biology was novel and rejected by many biologists of his era. 
•  Although Mendel suggested the existence of discrete factors (genes), he lacked physical evidence for their existence and could not explain their composition. 

 Rediscovery and Advancements 

•  In 1900, scientists de Vries, Correns, and von Tschermak independently rediscovered Mendel’s findings on inheritance. 
•  Advances in microscopy allowed scientists to observe cell division more closely, leading to the discovery of chromosomes—structures in the nucleus that doubled and divided before cell division. 
•  By 1902, the process of chromosome movement during meiosis was understood. 

 Sutton and Boveri’s Contribution 

•  Walter Sutton and Theodore Boveri observed that the behavior of chromosomes during cell division was similar to the behavior of genes. 
•  They used chromosome movement to explain Mendel’s laws of inheritance. 
•  Sutton proposed that the pairing and separation of chromosomes during meiosis corresponded to the segregation of the genetic factors (alleles) they carried. 
•  Sutton combined the knowledge of chromosomal segregation with Mendelian principles, formulating the chromosomal theory of inheritance. 
•  This theory proposed that genes are located on chromosomes, and the behavior of chromosomes during cell division accounts for the inheritance of traits. 

Thomas Hunt Morgan’s Experiments

•  Thomas Hunt Morgan and his colleagues experimentally verified the chromosomal theory of inheritance, providing a basis for understanding the variation produced by sexual reproduction. 
•  Morgan conducted his experiments using Drosophila melanogaster (fruit flies), which were ideal for genetic studies due to their short life cycle, ease of breeding, and clear sexual differentiation. 
•  The fruit flies also exhibited various hereditary variations that could be observed with low-power microscopes. 

Linkage and Recombination

Morgan's Experiments

• Morgan conducted dihybrid crosses in Drosophila to investigate sex-linked genes.
• He crossed yellow-bodied, white-eyed females with brown-bodied, red-eyed males and studied their F1 progeny.

Observations

• Morgan found that the two genes did not segregate independently, resulting in a F2 ratio that deviated significantly from the expected 9:3:3:1 ratio.
• He realized that when genes are on the same chromosome, parental gene combinations are more common than non-parental ones.

Linkage and Recombination

• Morgan introduced the term "linkage" to describe the physical association of genes on a chromosome.
• He used "recombination" to explain the formation of non-parental gene combinations.

Variability in Linkage

• Morgan's team discovered that some genes on the same chromosome are tightly linked (e.g., white and yellow with 1.3% recombination), while others are loosely linked (e.g., white and miniature wing with 37.2% recombination).

Gene Mapping

• Morgan's student, Alfred Sturtevant, used recombination frequencies to estimate the distance between genes on a chromosome and create genetic maps.
•  These genetic maps are now widely used in whole genome sequencing, such as in the Human Genome Sequencing Project.

Polygenic Inheritance

Mendel studied traits with distinct forms, like purple or white flowers. However, many traits, such as human height, show a gradual range and are not just one or the other. These traits are controlled by three or more genes, known as polygenic traits. Polygenic inheritance also considers environmental factors. For instance, human skin color is influenced by multiple genes and environmental factors.

• In polygenic traits, the phenotype is the result of the additive effect of each allele. For example, if three genes A, B, and C control skin color, with dominant alleles A, B, and C causing dark skin and recessive alleles a, b, and c causing light skin, the genotype AABBCC would have the darkest skin, while aabbcc would have the lightest.
• The number of each type of allele in a genotype determines the darkness or lightness of an individual’s skin.

Pleiotropy

A single gene can affect multiple traits, known as pleiotropy. This often occurs when a gene impacts metabolic pathways that contribute to different phenotypes. For example, phenylketonuria is caused by a mutation in the gene for the enzyme phenylalanine hydroxylase. This mutation leads to mental retardation and reduced hair and skin pigmentation.

Sex Determination

Sex Determination in Insects:

• Henking's Discovery: In 1891, Henking observed a specific nuclear structure during spermatogenesis in certain insects. He found that this structure, later known as the X body, was present in some sperm but not others.
• Cytological Observations: Further research showed that Henking's X body was actually a chromosome, leading to the identification of the X-chromosome.
• XO Type Sex Determination: In many insects, sex is determined by the presence of an extra X-chromosome in eggs. Sperm may carry an X-chromosome or not, leading to female (XX) or male (XO) offspring.

Examples:

• Grasshoppers: Males have one X-chromosome (XO), while females have two X-chromosomes (XX).
• Drosophila and Humans: Both exhibit XY type sex determination, where males have one X and one smaller Y chromosome (XY), and females have two X-chromosomes (XX).

XY Type Sex Determination:

• Chromosome Composition: In XY type, both males and females have the same number of chromosomes. Males have autosomes plus XY, while females have autosomes plus XX.
• Example Organisms: Human beings and Drosophila are examples where males have one X and one Y chromosome, and females have a pair of X-chromosomes along with autosomes.

Male Heterogamety:

• Definition: Male heterogamety refers to the production of two different types of gametes by males in terms of sex chromosomes.
• Examples: (a) In XO type, males produce gametes with or without an X-chromosome. (b) In XY type, males produce gametes with an X-chromosome or a Y-chromosome.

Female Heterogamety in Birds:

• Mechanism: In birds, females produce two different types of gametes with respect to their sex chromosomes.
• Z and W Chromosomes: Females have one Z and one W chromosome (ZW), while males have a pair of Z-chromosomes (ZZ) along with autosomes.

Sex Determination in Humans

Sex Determination Mechanism: In humans, sex is determined by the XY type mechanism. Chromosome Pairs: Humans have 23 pairs of chromosomes, out of which 22 pairs are the same in both males and females. These are called autosomes. The 23rd pair determines the sex: females have XX chromosomes, while males have XY.

Sperm and Egg Production:Males: During sperm production (spermatogenesis ), males produce two types of sperm: 50% carry the X chromosome and 50% carry the Y chromosome.Females: Females produce only one type of egg (ovum ), which always carries the X chromosome.

Fertilization and Sex Determination: The sex of the child is determined at fertilization: If an egg is fertilized by an X sperm, the zygote becomes female (XX).If an egg is fertilized by a Y sperm, the zygote becomes male (XY).Genetic Makeup: It is the genetic makeup of the sperm that determines the sex of the child. Probability: There is always a 50% probability of having either a male or female child in each pregnancy.

Social Misconceptions: Despite the scientific explanation, there are social misconceptions where women are unfairly blamed for giving birth to female children.

Sex Determination in Honey Bees

Mechanism: The sex of honey bees is determined by the number of sets of chromosomes an individual receives. Offspring Development:Fertilised Egg: An offspring developed from the union of a sperm and an egg becomes a female (either a queen or a worker). Unfertilised Egg: An unfertilised egg develops into a male (drone) through a process called parthenogenesis. Chromosome Number:Females: Honey bee females are diploid, meaning they have two sets of chromosomes (32 chromosomes). Males: Males are haploid, having only one set of chromosomes (16 chromosomes). This difference in chromosome number is the basis of the haplodiploid sex-determination system.

Unique Features:Male Reproduction: Males produce sperm by mitosis. Ancestry: Males do not have a father and cannot have sons, but they do have a grandfather and can have grandsons.

Sex Determination in Birds

 In birds, sex determination is different from humans and honey bees. It is known as the ZW sex-determination system. Female birds have ZW chromosomes, while male birds have ZZ chromosomes. 

Role of Sperm and Egg: In birds, the egg is responsible for determining the sex of the chicks. When a female bird lays an egg, the sex of the chick is determined by the combination of sex chromosomes in the egg.

Mutation

• Mutation involves changes in the DNA sequence, leading to alterations in an organism's genotype and phenotype.
• Along with recombination, mutation contributes to genetic variation.
• DNA in each chromatid runs continuously and is supercoiled. Changes such as deletions or insertions of DNA segments can alter chromosomes.
• Genes are located on chromosomes, so any alteration in chromosomes can lead to abnormalities.
• Chromosomal aberrations are often seen in cancer cells.
• Mutations can also occur due to a change in a single base pair of DNA, known as point mutation.
• An example of point mutation is sickle cell anemia.
• Deletions and insertions of base pairs can cause frame-shift mutations.
• The mechanisms behind mutations are complex and involve various chemical and physical factors called mutagens.
• For instance, UV radiation is a known mutagen that can induce mutations in organisms.

Genetic Disorders

Pedigree Analysis

• The concept of inheriting disorders has been part of human society for a long time, based on the observation that certain characteristic features are passed down within families.
• After Mendel's work was rediscovered, the practice of analyzing how traits are inherited in humans began.
• Since controlled crosses, like those done with pea plants, cannot be performed with humans, studying the family history of a particular trait provides an alternative method.
• This analysis of traits across several generations of a family is known as pedigree analysis.
• Pedigree analysis involves representing the inheritance of a specific trait in a family tree over multiple generations.
• In human genetics, pedigree studies are a valuable tool for tracing the inheritance of specific traits, abnormalities, or diseases.
• Each feature in an organism is controlled by genes located on DNA within chromosomes. DNA carries genetic information and is passed down from one generation to the next without changes.
• However, changes can occur occasionally, known as mutations. Many disorders in humans are associated with inherited altered genes or chromosomes.

Mendelian Disorders

Mendelian disorders are genetic disorders caused by alterations or mutations in a single gene. These disorders are inherited from parents to offspring following the principles of inheritance. The pattern of inheritance for Mendelian disorders can be traced within a family using pedigree analysis.

Common Mendelian Disorders

• Haemophilia
• Cystic Fibrosis
• Sickle Cell Anaemia
• Colour Blindness
• Phenylketonuria
• Thalassemia

Inheritance Patterns

• Mendelian disorders can be dominant or recessive.
• Pedigree analysis helps determine whether a trait is dominant or recessive.
• Some traits, like haemophilia, are linked to the sex chromosomes. Haemophilia is an X-linked recessive trait, meaning it is passed from carrier females to their male offspring.

A representative pedigree for dominant and recessive traits is shown in Figure 4.14.

Pedigree Analysis Exercise

•  Discuss with your teacher and design pedigrees for characters linked to both autosomes and the sex chromosome.

2. Sex-linked Disorders

(i) Colour Blindness

• It is a sex-linked recessive disorder caused by a defect in either the red or green cone of the eye, leading to an inability to differentiate between red and green colors.
• This condition arises from mutations in specific genes located on the X chromosome.
• Approximately 8% of males and 0.4% of females are affected due to this genetic mutation.
• Males are more commonly affected because they have only one X chromosome, while females have two.
• A son has a 50% chance of being color blind if his mother carries the gene, even though she is not color blind herself because the gene is recessive.
• A daughter will typically not be color blind unless her mother is a carrier and her father is color blind.

(ii) Haemophilia

• Haemophilia is a sex-linked recessive disorder where a crucial protein in the blood clotting process is deficient, leading to continuous bleeding from minor cuts.
• The condition is passed from an unaffected carrier female to some of her male offspring.
• A heterozygous female (carrier) can transmit haemophilia to her sons, but a daughter becoming a haemophilic is highly unlikely.
• Queen Victoria's family pedigree is a famous example of haemophilia transmission, as she was a carrier of the disease.

(iii) Sickle-cell Anaemia

• Sickle-cell anaemia is an autosomal recessive disorder caused by a mutation in the haemoglobin gene.
• The disease is linked to the HbA and HbS alleles, with homozygous HbS individuals (HbSHbS) exhibiting the disease.
• Heterozygous individuals (HbAHbS) are asymptomatic carriers, with a 50% chance of passing the mutant gene to their offspring.
• The mutation involves the substitution of Glutamic acid with Valine at the sixth position of the beta globin chain, due to a single base change in the beta globin gene.
• This leads to the polymerisation of mutant haemoglobin under low oxygen conditions, causing red blood cells to change from their normal biconcave shape to an elongated sickle shape.

(iv) Phenylketonuria

• Phenylketonuria is an autosomal recessive disorder characterized by the absence of an enzyme necessary for converting phenylalanine into tyrosine.
• This leads to the accumulation of phenylalanine, which is converted into phenylpyruvic acid and other derivatives.
• The buildup of these substances in the brain results in mental retardation, and they are also excreted in urine due to poor kidney absorption.

(v) Thalassemia

• Thalassemia is an autosomal recessive blood disorder that occurs when both parents are carriers of the gene.
• The condition results from a mutation or deletion affecting the synthesis rate of one of the globin chains (α or β) that make up haemoglobin.
• It leads to the production of abnormal haemoglobin molecules, causing anaemia, which is a hallmark of the disease.
• Thalassemia is classified into α and β Thalassemia based on which globin chain production is impaired.
• α Thalassemia is controlled by two genes (HBA1 and HBA2) on chromosome 16 and is caused by the mutation or deletion of one or more of the four genes controlling α globin production.
• β Thalassemia is controlled by a single gene (HBB) on chromosome 11 and occurs due to the mutation of one or both genes.
• Unlike sickle-cell anaemia, which is a qualitative issue of producing faulty globin molecules, thalassemia is a quantitative problem of producing insufficient globin molecules.

Chromosomal Disorders

Chromosomal disorders occur due to the absence, excess, or abnormal arrangement of one or more chromosomes.

• Aneuploidy. This condition arises from the failure of chromatid segregation during cell division, leading to the gain or loss of chromosomes. For instance:
  • Down’s syndrome is caused by an extra copy of chromosome 21 (trisomy 21).
  • Turner’s syndrome results from the loss of one X chromosome in females.
• Polyploidy. This occurs when there is an increase in a whole set of chromosomes due to the failure of cytokinesis after the telophase stage of cell division. Polyploidy is commonly observed in plants.

Chromosomal Composition in Humans

• A normal human cell contains 46 chromosomes, arranged in 23 pairs.
• Out of these, 22 pairs are autosomes, and 1 pair comprises sex chromosomes.

Aneuploidy: Trisomy and Monosomy

• Trisomy. This condition involves an additional copy of a chromosome, leading to serious developmental consequences.
• Monosomy. This condition involves the absence of one chromosome from a pair, also resulting in severe impacts on the individual’s development.

Common Examples of Chromosomal Disorders

• Down’s Syndrome
• Turner’s Syndrome
• Klinefelter’s Syndrome

Down’s Syndrome

• Cause: An extra copy of chromosome 21 (trisomy 21).
• Physical Features: Affected individuals are typically short with a small, round head, furrowed tongue, and a partially open mouth.
  • Palm: Broad with a characteristic palm crease.
• Development: Physical, psychomotor, and mental development is delayed.

Klinefelter’s Syndrome

• Cause: An extra copy of the X chromosome, resulting in a karyotype of 47, XXY.
• Physical Features: Individuals have typical masculine development, but also exhibit some feminine characteristics, such as gynecomastia (breast development).
• Fertility: Affected individuals are typically sterile.

Turner’s Syndrome

• Cause: The absence of one X chromosome, resulting in a karyotype of 45, X0.
• Physical Features: Affected females are sterile, with rudimentary ovaries and a lack of secondary sexual characteristics.