Chapter 10: Principles of Inheritance and Variation

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Introduction

Overview

• Genetics studies inheritance and variation of traits from parents to offspring.


• Inheritance: transfer of traits; basis of heredity.


• Variation: differences between progeny and parents.


• Early agriculturists used natural variation and sexual reproduction to breed domesticated plants and animals.


• Example: Sahiwal cows domesticated from wild ancestors.


• Gregor Johann Mendel: Father of genetics.


• William Bateson coined the term \"genetics\".

Memory Aids

• Genetics = Inheritance + Variation.


• Mendel = Father of Genetics.


• Bateson = Named Genetics.

Quick Checkpoints

• What is inheritance?


• Define variation.


• Who is the father of genetics?


• What did early agriculturists exploit?

Mendel's Laws of Inheritance

Key Points

• Gregor Mendel studied inheritance using Pisum sativum (garden pea) from 1856-1863.


• Studied traits with two contrasting forms (e.g., tall/dwarf, yellow/green seeds).


• Used true-breeding pea lines showing stable traits over generations.


• Selected 14 pairs of varieties differing in one contrasting trait.


• Reasons for choosing pea plant:
  
  
  
  • Annual plant with short 2-3 month life cycle.
  
  
  • Many contrasting traits.
  
  
  • Natural self-pollination present.
  
  
  • Artificial cross-pollination possible.
  
  
  • Easy to cultivate.
  
  
  • Produces many seeds.
  
  
  
  

Memory Aids

• Pea plant = Ideal for genetics due to short life and contrasting traits.


• True-breeding lines = Stable traits over generations.


• Dominant traits use capital letters; recessive use lowercase.

Quick Checkpoints

• Why did Mendel choose pea plants?


• What is a true-breeding line?


• What traits did Mendel study?


• How are dominant and recessive traits represented?

Inheritance of One Gene

Observations and Results

• Mendel crossed tall (TT) and dwarf (tt) pea plants to study single-gene inheritance.


• F₁ generation plants were all tall, resembling one parent; dwarf trait was not visible.


• Self-pollination of F₁ produced F₂ generation with 3/4 tall and 1/4 dwarf plants.


• Traits showed no blending; plants were either tall or dwarf.


• Similar 3:1 ratio observed for other traits in F₂ generation.

Memory Aids

• F₁ shows dominant trait only.


• F₂ shows 3:1 dominant to recessive ratio.


• No blending of traits; discrete inheritance.

Quick Checkpoints

• What phenotype appears in F₁ from tall × dwarf cross?


• What is the phenotypic ratio in F₂ generation?


• Does blending of traits occur in F₁ or F₂?

Genetics Terminology

Key Terms

• Alleles: Different forms of the same gene.


• Homozygous: Organism with identical alleles (e.g., TT, tt).


• Heterozygous: Organism with different alleles (e.g., Tt).


• Monohybrid cross: Cross between homozygous tall (TT) and dwarf (tt) producing heterozygous tall (Tt).


• Dominant allele: Expressed in homozygous or heterozygous state (e.g., T for tall).


• Recessive allele: Expressed only in homozygous state (e.g., t for dwarf).


• Phenotype: Observable traits influenced by genotype and environment.


• Genotype: Genetic makeup of an organism for a trait.


• Alleles segregate during gamete formation; each gamete carries one allele.


• Fertilization unites alleles from parents to form zygote genotype.

Memory Aids

• Homozygous = same alleles; heterozygous = different alleles.


• Dominant masks recessive in heterozygotes.


• Phenotype = what you see; genotype = genetic code.


• Alleles segregate during meiosis.

Quick Checkpoints

• Define homozygous and heterozygous.


• What is a monohybrid cross?


• When is a recessive allele expressed?


• What determines phenotype?

Punnett Square Method

Concept and Usage

• Developed by Reginald C. Punnett to predict offspring genotypes.


• Graphical tool showing gamete combinations and genotype probabilities.


• Test cross: Cross between unknown genotype and recessive homozygote to determine genotype.


• Test cross progeny ratios reveal if unknown is homozygous or heterozygous.

Memory Aids

• Punnett square = probability grid for genotypes.


• Test cross uses recessive parent to reveal unknown genotype.

Quick Checkpoints

• What does a Punnett square show?


• Purpose of a test cross?


• How to identify heterozygous genotype using test cross?

Laws of Inheritance

Fundamental Principles

• Law of Dominance: In a pair of alleles, one (dominant) masks the other (recessive) in heterozygotes.


• Law of Segregation: Alleles segregate during gamete formation; each gamete carries one allele.


• Dominance explains F₁ phenotype; segregation explains 3:1 ratio in F₂.


• Law of segregation ensures no blending; traits reappear in F₂.


• Law of segregation also called \"Law of Purity of Gametes\".

Memory Aids

• Dominant allele masks recessive in F₁.


• Alleles separate during meiosis; gametes are pure.


• F₂ shows both traits in 3:1 ratio.

Quick Checkpoints

• What does the Law of Dominance state?


• Explain the Law of Segregation.


• Why is the Law of Segregation called \"Law of Purity of Gametes\"?

Incomplete Dominance

Definition and Example

• Neither allele is completely dominant; heterozygote shows intermediate phenotype.


• Example: Flower color in Antirrhinum (snapdragon).


• Cross of red (RR) and white (rr) flowers produces pink (Rr) F₁ flowers.


• F₂ generation shows 1 red : 2 pink : 1 white ratio in both genotype and phenotype.


• Phenotypic ratio differs from Mendelian 3:1 due to incomplete dominance.

Memory Aids

• Incomplete dominance = blended intermediate trait.


• F₂ phenotypic ratio equals genotypic ratio (1:2:1).

Quick Checkpoints

• What is incomplete dominance?


• How does F₁ phenotype differ from parents in incomplete dominance?


• What is the F₂ phenotypic ratio in incomplete dominance?

Co-dominance

Definition and Example

• Both alleles express simultaneously in heterozygotes.


• Example: ABO blood group in humans controlled by gene with alleles I^A, I^B, and i.


• I^A and I^B are co-dominant; both expressed in AB blood group.


• Allele i is recessive; does not produce sugar antigen.


• Multiple alleles exist in population; individuals have two alleles.


• Single gene can have multiple effects (pleiotropy), e.g., starch synthesis in pea seeds.


• Dominance depends on gene product and phenotype expression.

Memory Aids

• Co-dominance = both alleles visible.


• AB blood group shows co-dominance of I^A and I^B.


• Multiple alleles exist but only two per individual.

Quick Checkpoints

• What is co-dominance?


• Which blood group shows co-dominance?


• Define multiple alleles.


• How does dominance depend on gene product?

Law of Independent Assortment

Explanation

• Mendel crossed pea plants differing in two traits: seed color (yellow dominant over green) and seed shape (round dominant over wrinkled).


• Parental genotypes: RRYY (round yellow) × rryy (wrinkled green).


• F₁ generation genotype: RrYy, all round yellow seeds.


• F₂ generation phenotypic ratio: 9 round yellow : 3 wrinkled yellow : 3 round green : 1 wrinkled green.


• Segregation of one gene pair is independent of the other pair.


• Applicable only when genes are on different chromosomes or far apart on the same chromosome.

Memory Aids

• Independent segregation = traits assort independently in hybrids.


• 9:3:3:1 ratio = hallmark of dihybrid cross.


• Use symbols: Y/y for color, R/r for shape.

Quick Checkpoints

• What is the phenotypic ratio in F₂ of a dihybrid cross?


• When does independent assortment apply?


• What are the parental genotypes in Mendel’s dihybrid cross?

Dihybrid Test Cross

Explanation

• Cross between F₁ dihybrid (TtRr) and homozygous recessive ().


• F₁ produces four gamete types: TR, Tr, tR, tr.


• Recessive parent produces only tr gametes.


• Offspring genotypes: TtRr, Ttrr, ttRr, ttrr in equal proportions.


• Phenotypic ratio: 1 : 1 : 1 : 1.

Memory Aids

• Test cross reveals genotype by crossing with recessive.


• Equal ratio in dihybrid test cross confirms independent assortment.

Quick Checkpoints

• What is the purpose of a test cross?


• What is the expected phenotypic ratio in a dihybrid test cross?

Intergenic Interaction

Explanation

• Interaction between genes at different loci modifying gene effects.


• Complementary genes: Two genes produce new trait together; F₂ ratio 9:7.


• Supplementary genes: One gene’s effect depends on presence of another; F₂ ratio 9:3:4.


• Epistasis: One gene masks expression of another.


• Pleiotropy: One gene affects multiple phenotypic traits.

Memory Aids

• Complementary = both needed for trait.


• Supplementary = one gene supports another.


• Epistasis = gene masking.


• Pleiotropy = one gene, many effects.

Quick Checkpoints

• Define complementary genes with ratio.


• What is epistasis?


• Explain pleiotropy briefly.

Chromosomal Theory of Inheritance

Explanation

• Proposed by Walter Sutton and Theodore Boveri (1902), proved by T.H. Morgan (1933).


• Chromosomes carry genes and behave as units during inheritance.


• Both sperm and egg contribute equally to heredity.


• Chromosomes and genes occur in pairs in diploid cells.


• Alleles of a gene pair are located on homologous chromosomes.


• During meiosis I, chromosome pairs align independently, explaining Mendel’s laws.


• Segregation of alleles corresponds to separation of homologous chromosomes in meiosis.

Memory Aids

• Chromosomes = carriers of genes.


• Meiosis explains segregation and independent assortment.


• Homologous chromosomes separate to form gametes.

Quick Checkpoints

• Who proposed the chromosomal theory?


• How does meiosis explain Mendel’s law of segregation?


• What is the role of homologous chromosomes in inheritance?

Drosophila melanogaster in Genetics

Explanation

• Model organism used by Morgan for genetic studies.


• Advantages: easy to grow, short life cycle (~2 weeks), large progeny, distinguishable sexes, visible hereditary variations.


• Male smaller than female; used to study inheritance patterns.

Memory Aids

• Drosophila = fruit fly, fast genetics.


• Short life cycle = quick results.


• Visible traits = easy observation.

Quick Checkpoints

• Why is Drosophila melanogaster used in genetics?


• What are key features making it suitable for genetic studies?

Linkage and Recombination

Explanation

• Genes located on the same chromosome tend to be inherited together (linkage).


• Parental gene combinations appear more frequently than recombinant types.


• Complete linkage: no crossing over; genes inherited as a block.


• Incomplete linkage: crossing over occurs; produces recombinant offspring.


• Recombination frequency indicates gene distance on chromosome.


• Alfred Sturtevant used recombination frequency to map gene positions.

Memory Aids

• Linkage = genes close on chromosome.


• Recombination = crossing over creates new gene combos.


• Map genes by recombination frequency.

Quick Checkpoints

• What is genetic linkage?


• How does recombination frequency relate to gene distance?


• What is the difference between complete and incomplete linkage?

Polygenic Inheritance

Overview

• Inheritance controlled by three or more genes is called polygenic inheritance.


• Phenotype reflects additive effects of each allele.


• Examples: human skin colour, human height, wheat kernel colour.


• Human skin colour depends on melanin pigment quantity controlled by three gene pairs (A, B, C).


• Genotype AABBCC = darkest skin; aabbcc = lightest skin; mixed dominant and recessive alleles = intermediate skin tone.

Memory Aids

• Polygenic = Many genes add up effects.


• Skin colour intensity = number of dominant alleles.

Quick Checkpoints

• What defines polygenic inheritance?


• How does allele contribution affect phenotype?


• Which traits show polygenic inheritance?


• How do dominant and recessive alleles influence skin colour?

Pleiotropy

Overview

• A single gene influences multiple traits.


• Gene affects two or more related metabolic pathways producing different phenotypes.


• Example: Phenylketonuria caused by mutation in phenylalanine hydroxylase gene.


• Phenylketonuria symptoms: mental retardation, reduced hair and skin pigmentation.

Memory Aids

• Pleiotropy = One gene, many effects.


• Phenylketonuria = single gene mutation, multiple symptoms.

Quick Checkpoints

• What is pleiotropy?


• How can one gene affect multiple traits?


• What causes phenylketonuria?


• What are the phenotypic effects of pleiotropy?

Sex Determination

Overview

• Sex determination establishes male or female identity in species.


• Determined by specific chromosomes called sex chromosomes or allosomes.


• X and Y chromosomes are sex chromosomes; other chromosomes are autosomes.


• Henking discovered X-body in 50% of sperm in some insects.

Memory Aids

• Sex chromosomes = Allosomes; others = Autosomes.


• Henking’s X-body marks sex chromosome presence.

Quick Checkpoints

• What are sex chromosomes?


• Difference between autosomes and allosomes?


• Who discovered the X-body?

Types of Chromosomal Sex Determination

XX–XY type

• Female is homogametic (XX), produces one type of gamete.


• Male is heterogametic (XY), produces two types of gametes: gynosperm (X) and androsperm (Y).


• Both sexes have same number of autosomes.


• Found in humans and Drosophila.

XX–XO type

• Males have one sex chromosome (X), heterogametic; some sperms have X, some none.


• Females are homogametic (XX).


• Fertilisation with X sperm = female (2A + XX); without X = male (2A + XO).


• Found in grasshopper, cockroach, roundworms.

ZW–ZZ type

• Female is heterogametic (ZW), produces Z and W gametes.


• Male is homogametic (ZZ), produces only Z gametes.


• Found in birds and some reptiles.


• In ZO–ZZ type, females have single Z (AA + Z), males have ZZ; found in butterflies and moths.

Haploid–Diploid Mechanism

• Observed in Hymenoptera (ants, honeybees, wasps).


• Sex determined by chromosome sets: females diploid (fertilised egg), males haploid (unfertilised egg).


• Males produce sperms by mitosis, no meiosis.


• Males have no father, cannot have sons; have grandfather and can have grandsons.

Memory Aids

• XX–XY: Female XX, Male XY.


• XX–XO: Male missing one sex chromosome.


• ZW–ZZ: Female heterogametic, male homogametic.


• Haploid–Diploid: Females diploid, males haploid.

Quick Checkpoints

• Which sex is homogametic in XX–XY system?


• What defines male in XX–XO system?


• In ZW–ZZ, which sex produces two gamete types?


• How are males produced in haplodiploid system?

Sex Determination in Humans

Mechanism

• Humans have 23 pairs of chromosomes: 22 autosomes, 1 pair sex chromosomes.


• Females have XX; males have XY chromosomes.


• During spermatogenesis, males produce 50% X-bearing and 50% Y-bearing sperms.


• Females produce only X-bearing ova.


• Fertilisation with X sperm = female (XX); with Y sperm = male (XY).


• Sex of child determined by sperm chromosome.


• Equal probability of male or female child in each pregnancy.


• Social misconception wrongly blames women for female births.


• Sex-linked inheritance involves genes on sex chromosomes, more apparent in males.


• Females often carriers of recessive sex-linked disorders (e.g., colour blindness, haemophilia).


• Criss-cross inheritance: traits passed to grandchild of same sex via opposite sex offspring.

Memory Aids

• Sperm chromosome decides sex.


• Females XX, males XY.


• Sex-linked traits show more in males.


• Criss-cross: father to grandson via daughter; mother to granddaughter via son.

Quick Checkpoints

• Which chromosome determines human sex?


• What is the role of sperm in sex determination?


• Define sex-linked inheritance.


• Explain criss-cross inheritance.

Mutation

Types and Effects

• Mutation alters DNA sequences, changing genotype and phenotype.


• Both mutation and recombination cause DNA variation.


• Chromosomal changes occur by loss or gain of DNA segments, causing abnormalities.


• Chromosomal aberrations are common in cancer cells.


• Chromosomal mutations include:
  
  
  
  • Deletion: Loss of chromosome segment (terminal or intercalary).
  
  
  • Duplication: Repetition of gene blocks in haploid set.
  
  
  • Translocation: Exchange of segments between non-homologous chromosomes.
  
  
  • Inversion: Chromosome segment breaks, rotates 180°, and rejoins reversed.
  
  
  • Euploidy: Gain or loss of complete chromosome sets; types: monoploidy, polyploidy.
  
  
  • Aneuploidy: Addition or loss of individual chromosomes; types: trisomy, tetrasomy, monosomy, nullisomy.
  
  
  
  


• Gene mutations can occur naturally or internally.


• Point mutation: Change in single base pair (e.g., sickle cell anaemia).


• Frame-shift mutation: Caused by insertions or deletions of base pairs.


• Mutagens are chemical or physical agents inducing mutations (e.g., UV radiation).

Memory Aids

• Chromosomal mutations: Deletion, Duplication, Translocation, Inversion, Euploidy, Aneuploidy (D-D-T-I-E-A).


• Aneuploidy types: trisomy (+1), tetrasomy (+2), monosomy (-1), nullisomy (-2).


• Point mutation affects one base; frame-shift alters reading frame.


• Mutagens = Mutation inducers (UV, chemicals).

Quick Checkpoints

• What causes chromosomal aberrations?


• Define point mutation and give an example.


• Difference between euploidy and aneuploidy.


• Types of chromosomal mutations and their effects.


• What is a frame-shift mutation?


• Examples of mutagens.

Genetic Disorders

Overview

• Genetic disorders arise from altered genes or chromosomes.


• Two main categories: Mendelian disorders and Chromosomal disorders.


• Pedigree charts track inheritance across generations.

Memory Aids

• Mendelian = single gene mutation.


• Chromosomal = chromosome number/structure changes.


• Pedigree symbols: squares (males), circles (females), filled = affected.

Quick Checkpoints

• What are the two main types of genetic disorders?


• How does pedigree analysis help in genetics?

Mendelian Disorders

Key Points

• Caused by mutation in a single gene.


• Inheritance patterns traced by pedigree analysis.


• Can be dominant or recessive; some are sex-linked (e.g., haemophilia).


• Common examples: haemophilia, cystic fibrosis, sickle-cell anaemia, colour blindness, phenylketonuria, thalassemia.

Memory Aids

• Dominant traits appear every generation.


• Recessive traits may skip generations.


• X-linked recessive traits affect mostly males.

Quick Checkpoints

• How to distinguish dominant vs recessive traits in pedigrees?


• What is the inheritance pattern of haemophilia?

Colour Blindness

Key Points

• Sex-linked recessive disorder affecting red or green cones.


• Mutation in genes on X chromosome.


• ~8% males affected; ~0.4% females affected.


• Carrier mothers have 50% chance of affected sons.


• Daughters affected only if mother is carrier and father is affected.

Memory Aids

• X-linked recessive = mostly males affected.


• Carrier females usually unaffected.

Quick Checkpoints

• Why are males more affected by colour blindness?


• How is colour blindness inherited?

Sickle-cell Anaemia

Key Points

• Autosomal recessive disorder caused by mutation in beta-globin gene.


• Alleles: Hbᴬ (normal), Hbˢ (mutant).


• Only HbˢHbˢ homozygotes show disease; HbᴬHbˢ heterozygotes are carriers.


• Mutation: glutamic acid replaced by valine at 6th position of beta-globin.


• Causes hemoglobin polymerisation under low oxygen, deforming RBCs into sickle shape.

Memory Aids

• Hbˢ = sickle mutation.


• Valine replaces glutamic acid at position 6.


• Carriers are asymptomatic but can pass gene.

Quick Checkpoints

• What amino acid substitution causes sickle-cell anemia?


• How does sickle-cell trait differ from disease?

Haemophilia

Key Points

• Sex-linked recessive disorder affecting blood clotting protein.


• Carrier females transmit disease to sons.


• Rare for females to be affected (requires carrier mother and affected father).


• Leads to prolonged bleeding from minor injuries.


• Famous pedigree: Queen Victoria's descendants.

Memory Aids

• X-linked recessive, affects males mostly.


• Carrier females usually healthy.

Quick Checkpoints

• How is haemophilia inherited?


• Why are females rarely affected?

Phenylketonuria

Key Points

• Autosomal recessive metabolic disorder.


• Lack enzyme converting phenylalanine to tyrosine.


• Phenylalanine accumulates, converted to phenylpyruvic acid.


• Accumulation causes brain damage and mental retardation.


• Excess excreted in urine.

Memory Aids

• PKU = phenylalanine buildup due to enzyme deficiency.


• Leads to mental retardation if untreated.

Quick Checkpoints

• What enzyme is deficient in PKU?


• What causes mental retardation in PKU?

Thalassemia

Key Points

• Autosomal recessive blood disorder due to reduced globin chain synthesis.


• Caused by mutation or deletion in globin genes.


• Types: α-thalassemia (chromosome 16), β-thalassemia and δ-thalassemia (chromosome 11).


• Results in abnormal hemoglobin and anemia.


• Quantitative defect vs sickle-cell qualitative defect.

Memory Aids

• α-thalassemia: defective α-globin genes on chr16.


• β-thalassemia: defective β-globin genes on chr11.


• Thalassemia = less globin; sickle-cell = abnormal globin.

Quick Checkpoints

• Which chromosomes carry globin genes affected in thalassemia?


• How does thalassemia differ from sickle-cell anemia?

Chromosomal Disorders

Key Points

• Caused by absence, excess, or abnormal arrangement of chromosomes.


• Aneuploidy: gain or loss of chromosomes (e.g., trisomy, monosomy).


• Examples: Down's syndrome (trisomy 21), Turner's syndrome (monosomy X).


• Polyploidy: whole set chromosome increase, common in plants.


• Normal human cells have 46 chromosomes (23 pairs).


• Trisomy = extra chromosome; Monosomy = missing chromosome.

Memory Aids

• Aneuploidy = abnormal chromosome number.


• Trisomy 21 = Down's syndrome.


• Monosomy X = Turner's syndrome.

Quick Checkpoints

• What is aneuploidy?


• Which chromosome abnormality causes Down's syndrome?

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