Board exam focus — Principles of Inheritance and Variation (CBSE & HBSE)

CBSE tests Mendel's laws with monohybrid and dihybrid ratios, exceptions to Mendel (incomplete dominance, codominance, ABO blood groups), linkage, sex-linked inheritance, and genetic disorders. HBSE focuses on definitions, cross diagrams, and application-type questions on blood groups and sex determination.

Mendel's Laws and Exceptions

Gregor Mendel — Father of Genetics

Gregor Johann Mendel (1822-1884), an Augustinian monk, conducted experiments on the garden pea (Pisum sativum) from 1856-1863. His work was published in 1866 but remained unrecognised until 1900 (rediscovered by de Vries, Correns, and Tschermak).

Why Pisum sativum was ideal:

• Well-defined, contrasting traits
• Short life span (one growing season)
• Large number of offspring (statistically reliable)
• Self-pollinating (by default) but can be artificially cross-pollinated
• Available in pure breeding lines

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The 7 Contrasting Traits Studied by Mendel

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Law 1: Law of Dominance

When two contrasting forms of a trait are brought together in an F1 hybrid, only one form (dominant) is expressed; the other (recessive) is hidden.

Monohybrid Cross — TT × tt:

• P: TT (Tall) × tt (Dwarf)
• F1: All Tt (Tall) — only dominant expressed
• F1 × F1: Tt × Tt → F2: 1 TT : 2 Tt : 1 tt
• Phenotypic ratio: 3 Tall : 1 Dwarf
• Genotypic ratio: 1 TT : 2 Tt : 1 tt

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Law 2: Law of Segregation (Purity of Gametes)

"The two alleles of a gene segregate (separate) during gamete formation, and each gamete receives only one allele."

Each gamete is pure for one allele — hence "purity of gametes." The F2 ratio of 3:1 demonstrates this law.

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Law 3: Law of Independent Assortment

"Genes located on different (non-homologous) chromosomes assort independently of each other during gamete formation."

Dihybrid Cross — RRYY × rryy (Round Yellow × Wrinkled Green):

• F1: All RrYy (Round Yellow)
• F2: 9 R_Y_ : 3 R_yy : 3 rrY_ : 1 rryy = 9:3:3:1 ratio
• 4 phenotypic classes: Round Yellow : Round Green : Wrinkled Yellow : Wrinkled Green

This ratio only holds when genes are on different (non-homologous) chromosomes.

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Test Cross

Test cross: Crossing an organism with an unknown genotype with a homozygous recessive parent (tt or rr).

• If all offspring are dominant: unknown parent is homozygous dominant (TT)
• If 1:1 ratio: unknown parent is heterozygous (Tt)

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Exceptions to Mendel's Laws

1. Incomplete Dominance:

• F1 heterozygote shows an intermediate phenotype (not exactly like either parent)
• Neither allele is fully dominant
• Example: Antirrhinum majus (snapdragon) — Red (RR) × White (WW) → Pink (RW) in F1; F2 ratio: 1 Red : 2 Pink : 1 White (1:2:1)
• Also: Mirabilis jalapa (four o'clock plant)

2. Codominance:

• Both alleles are equally expressed in heterozygote
• Example: ABO blood group — I^A I^B genotype → both A and B antigens on RBC → AB blood group (neither A nor B is dominant over the other)
• Multiple alleles: I^A, I^B, i (three alleles for one gene)

• Universal donor: O; Universal recipient: AB

3. Pleiotropy:

• One gene affects multiple phenotypic traits
• Example: Sickle cell anaemia — one gene (HbS) → abnormal Hb → multiple effects (anaemia, pain, organ damage)

Diagram Indicator: [Punnett squares for monohybrid cross (Tt × Tt → 3:1 ratio), dihybrid cross (9:3:3:1), and incomplete dominance in snapdragon (1:2:1); ABO blood group genotype-phenotype table]

Chromosomal Theory, Linkage and Sex-linked Inheritance

Chromosomal Theory of Inheritance

Sutton and Boveri (1902) proposed that chromosomes are the carriers of genes (Sutton studied grasshopper chromosomes; Boveri studied sea urchin eggs). Key observations:

• Chromosomes occur in pairs (like Mendel's alleles)
• Chromosomes segregate during meiosis (like Mendel's factors)
• Chromosomes assort independently (like Mendel's Law 3)

This became the Chromosomal Theory of Inheritance (Sutton-Boveri Theory).

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Morgan's Experiments — Drosophila melanogaster

Thomas Hunt Morgan (1908+) used Drosophila melanogaster (fruit fly) for genetic research. Advantages:

• Very short life cycle (~2 weeks)
• Large number of offspring
• Only 4 pairs of chromosomes (easy to study)
• Clear sexual dimorphism (XX = female, XY = male)
• Many visible mutations

Morgan's key findings:

1. Linkage: Genes located on the same chromosome tend to be inherited together (do not assort independently). This violated Mendel's Law of Independent Assortment.

Morgan crossed: Yellow body, white eyes (Drosophila) × grey body, red eyes and found that body colour and eye colour genes were linked (on the same X chromosome).

Linkage group = all genes on the same chromosome.

2. Crossing Over and Recombination:

• During meiosis I prophase (pachytene), homologous chromosomes undergo crossing over at points called chiasmata
• This exchanges segments between non-sister chromatids → recombinant (non-parental) chromosomes
• Recombination frequency = number of recombinant offspring / total offspring × 100%
• 1% recombination = 1 centiMorgan (cM) = 1 map unit — measure of genetic distance

3. Chromosomal Mapping (Genetic Maps): Recombination frequencies used to construct genetic/linkage maps showing relative positions of genes on chromosomes.

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Sex Determination

Human (XX-XY type):

• Females: XX (homogametic — only X gametes produced)
• Males: XY (heterogametic — X and Y gametes, 50:50)
• Sex determined at fertilisation:
• Egg (X) + X sperm → XX = Female
• Egg (X) + Y sperm → XY = Male
• Father determines sex of the child (since X from mother is constant)

Other mechanisms:

In honeybees (haplodiploidy):

• Fertilised eggs (2n) → Females (queens/workers)
• Unfertilised eggs (n, by parthenogenesis) → Males (drones)

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Sex-linked Inheritance

Genes located on the sex chromosomes (X or Y) show sex-linked inheritance.

X-linked recessive inheritance:

• Gene is on X chromosome
• Affected individuals: males with one copy of recessive allele (X^a Y)
• Females need two copies (X^a X^a) to show the trait
• Carrier females (X^A X^a) — carriers, not affected, but can pass gene to sons

Classic Examples:

1. Colour Blindness (Red-Green):

• Gene for colour vision on X chromosome
• Normal vision: X^N; Colour blind: X^c
• Carrier female: X^N X^c (normal vision, carrier)
• Colour-blind female: X^c X^c (rare)
• Colour-blind male: X^c Y (common; ~8% of males, ~0.4% females)

Criss-cross inheritance: X-linked trait from grandfather → daughter (carrier) → grandson (affected). Sons of carrier females have 50% chance of being colour blind.

2. Haemophilia:

• Deficiency of clotting factor VIII (Haemophilia A) or IX (Haemophilia B — Christmas disease)
• X-linked recessive
• Queen Victoria was a carrier; disease appeared in royal families of Europe
• Haemophilia in females possible only if father affected + mother carrier (extremely rare)
• "Bleeder's disease": blood doesn't clot normally; prolonged bleeding

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Comparison: Autosomal vs Sex-linked Inheritance

Diagram Indicator: [Diagram showing criss-cross inheritance of haemophilia: X^H X^h (carrier mother) × X^H Y (normal father) → showing 4 offspring with X^H X^H (normal female), X^H X^h (carrier female), X^H Y (normal male), X^h Y (affected male haemophiliac); AND chromosomal sex determination in humans (XX = female, XY = male)]

Sex Determination, Mutation and Genetic Disorders

Mutation — Definition and Types

Mutation = any heritable change in the nucleotide sequence of DNA (or in chromosome structure/number).

Types of Mutations:

1. Gene/Point Mutations (changes in single base pairs):

2. Chromosomal Mutations:

Aneuploidy:

• Monosomy (2n-1): one chromosome of a pair missing
• Trisomy (2n+1): one extra chromosome

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Genetic Disorders

1. Autosomal Chromosomal:

Down Syndrome (Mongolism — Trisomy 21):

• 47 chromosomes; extra chromosome 21
• Caused by non-disjunction during meiosis (failure of chromosomes to separate)
• Features: round face, flat nasal bridge, narrow slanting eyes, short stature, IQ 25-50, congenital heart defects, susceptibility to leukaemia, short metacarpals
• Incidence: 1 in 800 live births; risk increases with maternal age (>35 years)

2. Sex Chromosomal Disorders:

3. Autosomal Genetic Disorders:

Phenylketonuria (PKU):

• Autosomal recessive
• Deficiency of phenylalanine hydroxylase enzyme
• Cannot convert phenylalanine → tyrosine
• Accumulation of phenylalanine → phenylpyruvate in urine, blood → brain damage
• Features: Intellectual disability, seizures, light skin/hair (tyrosine needed for melanin)
• Treatment: Diet low in phenylalanine from birth; detected by Guthrie test at birth

Sickle Cell Anaemia:

• Autosomal recessive (but codominance in some contexts — HbS/HbA = sickle cell trait)
• Mutation: GAG → GTG (codon 6 of β-globin) → Glu → Val
• HbS: forms long fibres when O₂ is low → distorts RBCs into sickle shape
• Features: anaemia, pain crises (vaso-occlusion), organ damage, increased susceptibility to malaria (carrier advantage)
• Heterozygous (HbA HbS): Sickle cell trait — mild; some protection against malaria
• Homozygous (HbS HbS): Sickle cell anaemia — severe

Thalassaemia:

• Reduced or absent globin chain synthesis (α or β thalassaemia)
• Autosomal recessive
• Transfusion-dependent anaemia

Diagram Indicator: [Karyotype diagram of Down syndrome (47 chromosomes, three chromosome 21s); AND chromosomes showing Turner's syndrome (45,XO) and Klinefelter's (47,XXY); diagram of sickle cell mutation: normal RBC vs sickle-shaped RBC]

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