Principles of Inheritance and Variation

Principles of Inheritance and Variation

Overview of the Chapter

• This chapter deals with the principles of inheritance and variation.
• It explains Mendel’s laws of inheritance, gene behavior, dominance, segregation, and the basic genetic terms required to understand how traits are passed from parents to offspring.

Inheritance, Variation, and Genetics

• Inheritance: refers to the transmission of traits from parents to offspring.
• Variation: refers to the differences observed between parents and their offspring.
• Genetics: is the branch of biology that studies inheritance and variation.

• Examples from daily life show inheritance clearly:
  • Elephants always give birth to baby elephants.
  • Mango seeds grow into mango plants.

Early Understanding of Inheritance

• Humans were aware of variation caused by sexual reproduction as early as 8000–1000 B.C.
• Selective breeding: Early agriculturists used selective breeding to improve plants and animals.
  • Selective breeding involves choosing organisms with desirable traits for reproduction.
  • Example: Indian cattle breeds like Sahiwal originated from selective breeding of ancestral wild cows.

Mendel’s Laws of Inheritance

Gregor Mendel and Genetics

• Gregor Mendel is known as the Father of Genetics.
• He conducted experiments between 1856 and 1863 on garden pea plants (Pisum sativum).
• Mendel used statistical analysis and mathematical logic to interpret inheritance patterns.
• Large sample sizes made his results reliable.
• His conclusions were confirmed by repeated experiments across generations.

Why Mendel Selected Pea Plants

• Pea plants have a short life cycle.
• They show many contrasting traits.
• They naturally self-pollinate.
• Artificial cross-pollination is easy to perform.
• They produce a large number of seeds.

True-Breeding Pea Lines

• True-breeding plants produce offspring with the same traits through continuous self-pollination.
• Mendel selected 14 true-breeding pea plant varieties.
• Each pair differed in only one contrasting character.

Seven Pairs of Contrasting Traits Studied by Mendel

7 Character	Dominant Trait	Recessive Trait
Stem Height	Tall	Dwarf
Flower Color	Violet	White
Flower Position	Axial	Terminal
Pod Shape	Inflated	Constricted
Pod Color	Green	Yellow
Seed Shape	Round	Wrinkled
Seed Color	Yellow	Green

Inheritance of One Gene (Monohybrid Cross)

Mendel’s Experiment

• Mendel crossed tall plants (TT) with dwarf plants (tt).

• F₁ Generation
  • All offspring were tall.
  • Only one parental trait was expressed.

• F₂ Generation
  • F₁ plants were self-pollinated.
  • Tall and dwarf plants appeared in the ratio 3:1.

• Key Observations
  • Traits did not blend.
  • Both parental traits reappeared unchanged.
  • The recessive trait was masked in F₁ but reappeared in F₂.

Genes and Alleles

• Genes: Genes are units of inheritance that control traits.
  • Mendel called them “factors”.
• Traits: Traits are observable characteristics of an organism.
• Alleles: Alleles are alternative forms of the same gene.

• Example:
  • T = Tall allele
  • t = Dwarf allele

Genotype and Phenotype

• Genotype refers to the genetic makeup (TT, Tt, tt).
• Phenotype refers to the observable character (tall or dwarf).
• Phenotype depends on genotype and environment.

Dominance and Recessiveness

• Dominant alleles express themselves even in heterozygous condition.
• Recessive alleles express only in homozygous condition.

• Examples:
  • TT → Tall
  • Tt → Tall
  • tt → Dwarf

Monohybrid Cross

• A monohybrid cross involves inheritance of one character.
• Example: TT × tt

• Phenotypic ratio in F₂: 3 Tall : 1 Dwarf
• Genotypic ratio in F₂: 1 TT : 2 Tt : 1 tt

Law of Segregation

• Alleles separate during gamete formation.
• Each gamete receives only one allele.
• Segregation is random and equal.

Punnett Square

• Punnett square is a diagrammatic method to predict offspring genotypes.
• It helps calculate genotype and phenotype ratios.

Backcross and Test Cross

• Backcross
  • Cross between a hybrid and one of its parents.
• Test Cross
  • Cross between an individual with dominant phenotype and a homozygous recessive parent.
  • Purpose: To determine the genotype of the dominant individual.

• Results:
  • All tall offspring → dominant parent is TT
  • Tall and dwarf offspring in 1:1 ratio → dominant parent is Tt

Mendel’s Laws of Inheritance

Overview of Mendel’s Laws

• Based on his monohybrid cross experiments, Gregor Mendel proposed two fundamental laws of inheritance:
  1. Law of Dominance
  2. Law of Segregation
• These laws explain how traits are inherited from parents to offspring and how alleles behave during reproduction.

1. Law of Dominance

• Statement
  • “In a pair of alleles, one allele can mask the expression of the other in a heterozygous condition.”

Key Principles of the Law of Dominance

• Discrete Units:
  • Characters are controlled by discrete units called factors (now known as genes).
• Paired Nature of Genes:
  • Each character is controlled by a pair of alleles, one inherited from each parent.
• Dominant and Recessive Alleles:
  • In a dissimilar pair, one allele expresses itself (dominant), while the other remains hidden (recessive).

Explanation Using Monohybrid Cross

• Parental Cross (P):
  • Tall plant (TT) × Dwarf plant (tt)
• F₁ Generation:
  • All offspring are tall (Tt).
  • Only the dominant trait is expressed.
• F₂ Generation (Selfing of F₁):
  • Tall : Dwarf = 3 : 1

This confirms that the dominant allele masks the recessive allele only in heterozygous condition.

Significance of the Law of Dominance

• Explains why only one parental trait appears in the F₁ generation.
• Explains reappearance of the recessive trait in the F₂ generation.
• Accounts for the 3:1 phenotypic ratio in monohybrid crosses.

2. Law of Segregation

• Statement
  • “Alleles separate during gamete formation so that each gamete receives only one allele from each pair.”

Key Principles of the Law of Segregation

• No Blending:
  • Alleles do not mix or blend with each other.
• Separation During Gametogenesis:
  • During meiosis, the two alleles of a gene segregate from each other.
• Purity of Gametes:
  • Each gamete carries only one allele and is genetically pure.
• Gamete Types:
  • Homozygous individuals produce one type of gamete.
  • Heterozygous individuals produce two types of gametes in equal proportion.

Alternate Name

• The Law of Segregation is also called the “Law of Purity of Gametes”.

Importance of the Law of Segregation

• Explains reappearance of recessive traits in F₂ generation.
• Has universal applicability with no exception.
• Forms the foundation of modern genetics.

Exceptions to the Law of Dominance

The Law of Dominance is not universal. Certain genetic patterns show deviations.

a. Incomplete Dominance

• Definition
  • “A condition where neither allele is completely dominant, resulting in an intermediate phenotype in heterozygotes.”
• Example: Flower Colour in Snapdragon (Antirrhinum)

• Flower color in snapdragons (dog flowers).
• Parental Cross (P):
  • Red flower (RR) × White flower (rr)
• F₁ Generation:
  • All flowers are Pink (Rr)
• F₂ Generation Ratio:
  • 1 Red (RR) : 2 Pink (Rr) : 1 White (rr)
• 

Explanation

• The dominant allele does not completely mask the recessive allele.
• Heterozygous condition shows an intermediate phenotype.
• Genotypic ratio and phenotypic ratio in F₂ are the same (1:2:1).

b. Co-dominance

• Definition
  • “A condition where both alleles in a heterozygote are fully and simultaneously expressed without blending.”

Example: ABO Blood Group System in Humans

• Gene Involved:
  • Gene I (blood group gene)
• Alleles:
  • Iᴬ, Iᴮ, i
• Co-dominant Alleles:
  • Iᴬ and Iᴮ

Expression Pattern

• Iᴬ Iᴬ or Iᴬ i → Blood group A
• Iᴮ Iᴮ or Iᴮ i → Blood group B
• Iᴬ Iᴮ → Blood group AB (both sugars expressed)

• ABO blood groups in humans.
  • 

Explanation

• Iᴬ and Iᴮ both produce different sugars on RBC surface.
• Neither allele suppresses the other.
• Both traits appear together in the phenotype.

Additional Genetic Concepts Linked to Mendel’s Laws

Multiple Alleles (Multiple Allelism)

• Definition
  • “The presence of more than two alleles for a gene in a population.”
• Key Point
  • An individual can possess only two alleles, but a population may have many.
• Example
  • ABO blood group system has three alleles: Iᴬ, Iᴮ, i.

• Multiple Alleles (Multiple Allelism)
  • 

Single Gene, Multiple Effects (Pleiotropy)

• Definition
  • “A phenomenon in which a single gene influences multiple phenotypic traits.”

Example: Starch Synthesis in Pea Seeds

• BB genotype:
  • Efficient starch synthesis → Large starch grains → Round seeds
• bb genotype:
  • Less starch synthesis → Small starch grains → Wrinkled seeds
• Bb genotype:
  • Intermediate starch grain size, but seeds appear round

Key Insight

• Dominance depends on the trait being observed.
• A gene may show complete dominance for one character and incomplete dominance for another.

• Single Gene, Multiple Effects (Pleiotropy)
• 

Final Points

• Law of Dominance explains typical Mendelian inheritance.
• Law of Segregation is universal and fundamental.
• Incomplete dominance and co-dominance modify phenotypic outcomes without violating segregation.
• Dominance is not an absolute property of a gene but depends on gene product and phenotype examined.

Inheritance of Two Genes

Mendel’s Experiments with Two Traits (Dihybrid Cross)

Mendel studied the inheritance of two contrasting traits simultaneously to understand how different genes behave during inheritance.

Example Traits Studied

1. Seed shape: Round (R) / Wrinkled (r)
2. Seed colour: Yellow (Y) / Green (y)

• Parental Generation (P)
  • Round, yellow seeds: RRYY
  • Wrinkled, green seeds: rryy

• Dominant traits: Round (R), Yellow (Y)
• Recessive traits: Wrinkled (r), Green (y)

F₁ Generation

• Gametes formed:
  • RRYY → RY
  • rryy → ry
• All F₁ offspring: RrYy
• Phenotype: All round and yellow seeds

• Observation
  • F₁ generation resembles only the dominant parental traits, similar to monohybrid crosses.

F₂ Generation (Selfing of F₁)

• F₁ plants (RrYy) produce four types of gametes in equal proportion:
  • RY, Ry, rY, ry
• Phenotypic Ratio in F₂ Generation
  • 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green
• This ratio is written as 9 : 3 : 3 : 1.

3. Law of Independent Assortment

• Statement
  • “When two pairs of traits are combined in a hybrid, segregation of one pair of alleles is independent of the segregation of the other pair.”

Key Points of Law of Independent Assortment

• Traits governed by different genes assort independently during gamete formation.
• Inheritance of one character does not influence the inheritance of another character.
• Applicable in dihybrid crosses involving two different genes.
• Explains the 9:3:3:1 phenotypic ratio in F₂ generation.

Dihybrid Test Cross

• Definition
  • A cross between a dihybrid organism and a double recessive parent.
• Example
  • RrYy × rryy
• Gametes from RrYy: RY, Ry, rY, ry
• Gametes from rryy: ry only
• Phenotypic Ratio
  • 1 : 1 : 1 : 1
• Significance
  • Confirms independent assortment of genes.

Explanation Using Dihybrid Cross

• Segregation of seed shape alleles (R and r) is independent of segregation of seed colour alleles (Y and y).
• 50% gametes carry R and 50% carry r.
• 50% gametes carry Y and 50% carry y.
• Independent combinations result in four types of gametes and four phenotypes.

• Punnett square helps visualize the independent assortment.
• 

Law of Segregation vs. Law of Independent Assortment

Feature	Law of Segregation	Law of Independent Assortment
Definition	Alleles of a single gene separate during gamete formation.	Alleles of different genes assort independently during gamete formation.
Applies to	One gene	Two or more genes
Cross type	Monohybrid cross	Dihybrid cross
Key Principle	Each gamete receives only one allele of a gene.	Alleles of different genes behave/assort independently.

Limitations of Law of Independent Assortment

• The law is applicable only when:
  • Genes are located on different chromosomes, or
  • Genes are located far apart on the same chromosome.

• Genes that are close together on the same chromosome tend to be inherited together. This phenomenon is called linkage.

Chromosomal Theory of Inheritance

Historical Background

• Mendel’s work remained unrecognized for many years.
• Rediscovered in 1900 by:
  • Carl Correns
  • Hugo de Vries
  • Erich von Tschermak

• Chromosomal Theory proposed by:
  • Walter Sutton and Theodore Boveri (1902)
  • Experimentally verified by Thomas Hunt Morgan (1933)

Core Ideas of Chromosomal Theory of Inheritance

• Genes on Chromosomes: Genes are located on chromosomes.
• Chromosome Pairs: Chromosomes occur in homologous pairs in diploid cells.
  • One chromosome of each pair is inherited from each parent.
  • Alleles of a gene are located on homologous chromosomes.
• Meiosis and Inheritance:
  • During meiosis, homologous chromosomes segregate, explaining Mendel’s laws.
  • Chromosomes and genes retain their individuality through generations.

Chromosomal Explanation of Mendel’s Laws

• Law of Segregation
  • Homologous chromosomes separate during Anaphase I of meiosis.
  • Chromatids separate during Anaphase II.
  • Each gamete receives only one allele.
• Law of Independent Assortment
  • Different homologous chromosome pairs align independently at metaphase I.
  • This leads to independent distribution of different gene pairs.

Role of Thomas Hunt Morgan

• Worked on fruit fly Drosophila melanogaster.
• Provided experimental proof of chromosomal theory.

Final Points

• Dihybrid crosses demonstrate inheritance of two genes simultaneously.
• Law of Independent Assortment explains the 9:3:3:1 ratio.
• Independent assortment depends on chromosomal position of genes.
• Chromosomal theory provides physical basis for Mendel’s laws.
• Linkage and crossing over modify Mendelian ratios but do not contradict Mendel’s principles.

Linkage and Recombination

Linkage

• Definition:
  • Linkage is the physical association of genes located on the same chromosome, due to which they tend to be inherited together.
  • It is an exception to Mendel’s Law of Independent Assortment.
• Discovery:
  • Thomas Hunt Morgan discovered linkage while working on fruit fly Drosophila melanogaster.
• Experimental Evidence:
  • Morgan performed dihybrid crosses in Drosophila and observed that certain gene combinations did not follow the expected 9:3:3:1 ratio.
  • Instead, parental combinations appeared more frequently than recombinant combinations.
• Explanation:
  • Genes located on the same chromosome do not assort independently.
  • Their physical proximity causes them to be transmitted together from one generation to the next.
• Linkage Group:
  • All genes present on a single chromosome form a linkage group.
  • The number of linkage groups in a species equals its haploid chromosome number.
  • Example: Humans have 23 linkage groups.

Types of Linkage

1. Complete Linkage
   • Genes are inherited together without any crossing over.
   • Only parental combinations are produced.
   • Rare in nature.
2. Incomplete Linkage
   • Occasional crossing over occurs between homologous chromosomes.
   • Both parental and recombinant types are produced.
   • Parental types are more than 50%, recombinant types are less than 50%.

Linkage explains deviation from Mendelian ratios.

Recombination

• Definition
  • Recombination is the formation of new gene combinations due to crossing over between homologous chromosomes during meiosis.
• Cause
  • Crossing over occurs during pachytene stage of prophase I of meiosis.
• Effect
  • Recombination breaks linkage between genes.
  • Produces non-parental (recombinant) combinations.
• Relationship with Linkage
  • Strong linkage → Low recombination frequency
  • Weak linkage → High recombination frequency
• Examples from Morgan’s Experiments
  • White eye and yellow body genes showed very tight linkage (low recombination).
  • White eye and miniature wing genes showed loose linkage (high recombination).

Recombination generates genetic variation.

Genetic Mapping

• Discovery
  • Alfred Sturtevant developed genetic maps.
• Principle
  • Distance between genes on a chromosome is directly proportional to recombination frequency.
• Recombination Frequency
  • Percentage of recombinant offspring produced between two genes.
• Map Unit
  • Centimorgan (cM)
  • 1 cM = 1% recombination frequency
• Application
  • Genetic maps are used to determine gene order and relative distance on chromosomes.
  • They serve as a foundation for genome sequencing.

Genetic mapping is based on recombination frequency.

Why Drosophila melanogaster Was Used by Morgan

• Easy laboratory culture: They grow on simple synthetic media.
• Short life cycle: They have a short life cycle (about 12–14 days).
• Breeding is possible throughout the year.
• Large number of offspring: Each mating produces hundreds of offspring.
• Clear sexual dimorphism: Males and females are easily distinguishable (males are smaller).
• They show many easily observable hereditary variations & visible mutations.

Polygenic Inheritance

• Definition:
  • Inheritance of a trait controlled by three or more genes is called polygenic inheritance.
• Key Feature:
  • Each allele contributes a small, additive effect to the phenotype.
• Examples:
  • Human height
  • Human skin color
  • Kernel colour in wheat

Polygenic Inheritance of Human Skin Colour

• Pigment Involved:
  • Melanin
• Genes Involved:
  • Three pairs of polygenes: A, B and C
• Genotype–Phenotype Relationship
  • AABBCC → Darkest skin colour
  • aabbcc → Lightest skin colour
  • Intermediate combinations → Intermediate skin colour
• Nature of Variation:
  • Produces continuous variation rather than discrete classes.
• Environmental Influence:
  • Polygenic traits are also influenced by environmental factors such as nutrition and climate.

Polygenic inheritance produces continuous variation.

Mutation

• Definition:
  • Mutation is a sudden change in DNA sequence that causes alteration in genotype and phenotype.
• Recombination and mutation are the two main sources of genetic variation.

DNA Organisation

• DNA runs continuously along each chromatid.
• It exists in a highly supercoiled and compact form.

Chromosomal Mutations (Aberrations)

• Deletion
  • Loss of a chromosome segment (terminal or intercalary).
• Duplication
  • Repetition of a chromosome segment.
• Translocation
  • Exchange of segments between non-homologous chromosomes.
• Inversion
  • A chromosome segment breaks, rotates 180°, and rejoins.

• Euploidy
  • Change in complete sets of chromosomes.
  • Includes monoploidy and polyploidy.
• Aneuploidy
  • Gain or loss of one or more chromosomes.
  • Types:
    • Trisomy (2n+1)
    • Tetrasomy (2n+2)
    • Monosomy (2n−1)
    • Nullisomy (2n−2)

Chromosomal aberrations are common in cancer cells.

Gene Mutations

• Point Mutation
  • Change in a single base pair.
  • Example: Sickle-cell anaemia.
• Frame-shift Mutation
  • Caused by insertion or deletion of base pairs.
  • Alters the reading frame of DNA.

Mutagens

• Definition
  • Agents that induce mutations.
• Types
  1. Chemical mutagens: Mustard gas, phenol, formalin
  2. Physical mutagens: UV radiation

Inheritance of Mutations

• Germ cell mutations → Heritable
• Somatic cell mutations → Non-heritable

Genetic Disorders

Pedigree Analysis

• Definition:
  • A pedigree is a diagrammatic family tree showing inheritance of a trait across generations.
• Purpose:
  • To trace inheritance of traits, diseases, or abnormalities.
  • To predict genotypes from phenotypes.
• Importance:
  • Human crosses cannot be experimentally controlled.
  • Pedigree analysis helps study dominant, recessive, and sex-linked traits.

Types of Genetic Disorders

Genetic Disorders: Two types – Mendelian and Chromosomal.

1. Mendelian Disorders
   • Caused by mutation in a single gene.
   • Follow Mendelian inheritance.
   • Traced using pedigree analysis.
   • Examples
     • Haemophilia
     • Sickle-cell anaemia
     • Cystic fibrosis
     • Colour blindness
     • Phenylketonuria
     • Thalassemia
   • May be dominant or recessive.
   • May be autosomal or sex-linked.
2. Chromosomal Disorders
   • Caused by change in chromosome number or structure.
     • Results from abnormalities during meiosis.
   • Example
     • Down’s syndrome (Trisomy 21)

Key Points

• Sex determination in humans follows XY system.
• Sperm determines the sex of the child.
• Mutation causes genetic variation.
• Pedigree analysis is essential for studying human inheritance.
• Genetic disorders may be Mendelian or chromosomal.

Types of Mendelian Disorders

Classification

• Mendelian disorders are caused by mutation in a single gene.
• They can be classified as:
  1. Dominant or Recessive
  2. Autosomal or Sex-linked
• Inheritance: Follow Mendel’s principles & Inheritance patterns can be traced using pedigree analysis.

Representative Pedigrees

• (A) Autosomal dominant trait – example: Myotonic dystrophy
• (B) Autosomal recessive trait – example: Sickle-cell anaemia

Common Mendelian Disorders

1. Colour Blindness

• Type: Sex-linked recessive disorder
• Cause: Mutation in genes located on the X chromosome affecting red or green cone cells.
• Effect: Inability to distinguish between red and green colours.
• Prevalence: More common in males (≈8%) than females (≈0.4%) due to single X chromosome in males.
• Genotypes:
  • XCXC → Affected female (rare)
  • XCX → Carrier female
  • XCY → Affected male

2. Haemophilia

• Type: Sex-linked recessive disorder
• Cause: Defect in a clotting protein of blood.
• Effect: Excessive bleeding even from minor cuts or injuries.
• Transmission/Inheritance: Transmitted from unaffected carrier female to male offspring.
• Female haemophilia is extremely rare because:
  • Mother must be a carrier
  • Father must be haemophilic (often unviable)
• Historical Example: Queen Victoria was a carrier and passed the trait to her descendants.

3. Sickle-Cell Anaemia

• Type: Autosomal recessive disorder
• Cause: Point mutation in β-globin gene (HBB gene).
• Molecular Defect: Substitution of glutamic acid (Glu) by valine (Val) at the 6th position of β-globin chain.
  • Codon change: GAG → GUG
• Effect:
  • Haemoglobin polymerises under low oxygen tension.
  • RBCs become sickle-shaped instead of biconcave.
• Genotypes:
  • HbA HbA → Normal
  • HbA HbS → Carrier (sickle-cell trait)
  • HbS HbS → Affected
• Key Note: This is a qualitative defect in haemoglobin structure.

4. Phenylketonuria (PKU)

• Type: Autosomal recessive disorder
• Nature: Inborn error of metabolism
• Cause: Absence of enzyme phenylalanine hydroxylase.
• Effect:
  • Phenylalanine cannot convert into tyrosine.
  • Phenylalanine accumulates and forms phenylpyruvic acid.
• Impact:
  • Accumulation in brain causes mental retardation.
  • Phenylpyruvic acid detected in urine (diagnostic feature).

5. Thalassemia

• Type: Autosomal recessive disorder
• Cause: Mutation or deletion affecting synthesis of globin chains of haemoglobin.
• Effect: Reduced haemoglobin synthesis leading to anaemia.
• Types:
  1. α Thalassemia:
     • Defective α-globin formation
     • Genes HBA1 and HBA2 on chromosome 16
  2. β Thalassemia:
     • Reduced β-globin synthesis
     • HBB gene on chromosome 11
  3. δ-thalassemia: (not mentioned in NCERT)
     • Defect in δ-globin chain
     • HBD gene on chromosome 11
• Key Difference from Sickle-Cell Anaemia:
  • Thalassemia → Quantitative defect (amount of globin)
  • Sickle-cell anaemia → Qualitative defect (structure of globin)

Chromosomal Disorders

Definition: Disorders caused due to absence, excess, or abnormal arrangement of chromosomes.

Causes

1. Aneuploidy: Gain or loss of one or more chromosomes due to failure of segregation during cell division.
   • Example: Down’s syndrome (extra copy of chromosome 21).
   • Example: Turner’s syndrome (loss of an X chromosome in females).
2. Polyploidy: Increase in complete chromosome sets due to failure of cytokinesis after telophase.
   • Common in plants, rare in humans.

Human Chromosome Number

• Total chromosomes: 46 chromosomes (23 pairs).
  • Autosomes: 22 pairs.
  • Sex Chromosomes: 1 pair (XX or XY).

Types of Chromosomal Abnormalities

1. Trisomy: Extra copy of a chromosome (2n+1)
2. Monosomy: Loss of one chromosome from a pair (2n−1)
   • Both lead to severe developmental abnormalities.

Examples of Chromosomal Disorders

1. Down’s Syndrome

• Cause: Extra copy of chromosome 21 (trisomy of 21^th).
• Karyotype: 47, +21
• First described by: Langdon Down (1866)
• Major Features:
  • Short stature
  • Small round head
  • Partially open mouth
  • Furrowed tongue
  • Broad palm with characteristic palmar crease
  • Mental and psychomotor retardation

2. Klinefelter’s Syndrome

• Cause: Extra X chromosome in males
• Karyotype: 47, XXY
• Features:
  • Overall masculine appearance
  • Gynaecomastia (breast development)
  • Small testes
  • Sterility
  • Mild mental deficiency
  • Long limbs

3. Turner’s Syndrome:

• Cause: Loss of one X chromosome in female.
• Karyotype: 45, XO
• Features:
  • Sterile female
  • Rudimentary ovaries
  • Absence of secondary sexual characteristics
  • Short stature
  • Webbed neck
  • Cardiovascular abnormalities

Key Points

• Mendelian disorders arise from single-gene mutations.
• They may be autosomal or sex-linked, dominant or recessive.
• Chromosomal disorders result from numerical or structural chromosomal changes.
• Pedigree analysis is essential for studying inheritance patterns in humans.

Chapter Summary

Introduction to Genetics

• Genetics is a branch of biology that studies inheritance and variation.
• The resemblance of progeny to parents attracted the attention of biologists and led to systematic studies of inheritance patterns.

Mendel and Laws of Inheritance

• Gregor Mendel studied inheritance in pea plants and proposed principles known as Mendel’s Laws of Inheritance.
• His experiments showed that inheritance follows definite patterns rather than random blending.

Genes and Alleles

• Traits are controlled by genes, which occur in pairs called alleles.
• One allele comes from each parent.
• The genetic constitution of an organism is called genotype, while the observable expression is called phenotype.

Law of Dominance

• In a heterozygous condition, the dominant allele expresses itself and masks the recessive allele.
• Recessive traits appear only in homozygous conditions. There is no blending of traits in heterozygotes.

Law of Segregation

• During gamete formation, the two alleles of a gene separate from each other so that each gamete carries only one allele.
• This explains the reappearance of recessive traits in later generations.

Exceptions to Dominance

• Not all traits follow complete dominance.
• Incomplete dominance shows an intermediate phenotype in heterozygotes.
• Co-dominance shows simultaneous expression of both alleles in heterozygotes.

Law of Independent Assortment

• Genes controlling different traits assort independently during gamete formation.
• This results in new combinations of traits in offspring. Punnett squares are used to represent these combinations.

Chromosomal Theory of Inheritance

• Mendel’s laws are explained by chromosome behavior during meiosis.
• Chromosomes occur in pairs, segregate during meiosis, and independently assort, supporting Mendel’s principles.
• Genes are located on chromosomes.

Linkage and Recombination

• Genes present close together on the same chromosome tend to be inherited together (linkage).
• Genes far apart can be separated due to crossing over, leading to recombination and variation.

Sex Determination and Sex-Linked Inheritance

• Sex-linked genes are located on sex chromosomes.
• Humans have 22 pairs of autosomes and one pair of sex chromosomes (XX in females, XY in males).
• In birds like chickens, males are ZZ and females are ZW.

Mutation

• Mutation is a sudden change in genetic material.
• Point mutation involves change in a single base pair.
• Sickle-cell anaemia is caused by a single base substitution in the haemoglobin gene.

Pedigree Analysis

• Pedigree analysis is used to study inheritance of traits and genetic disorders in families.
• It helps trace dominant, recessive, and sex-linked traits across generations.

Chromosomal Mutations and Disorders

• Changes may involve whole chromosome sets (polyploidy) or individual chromosomes (aneuploidy).
• Down’s syndrome is caused by trisomy of chromosome 21 (47 chromosomes).
• Turner’s syndrome has a missing X chromosome (XO).
• Klinefelter’s syndrome has an extra X chromosome (XXY).

Karyotyping

• Karyotypes are used to study chromosome number and structure and help identify chromosomal abnormalities.