Principles of Inheritance and Variation - Class 12 Biology

Class 12 Biology | Unit VIII

Chapter 5: Principles of Inheritance and Variation

Mendelism • Deviations • Chromosomal Theory • Linkage • Sex Determination • Mutations • Genetic Disorders

1. Mendel and His Laws

Gregor Johann Mendel (1822–1884), an Augustinian monk at Brno, Austria. Worked with Pisum sativum (garden pea) for 7 years (1856–1863). Published results in 1866 in “Versuche über Pflanzenhybriden” (Experiments on Plant Hybridisation). Rediscovered in 1900 by de Vries, Correns, and Tschermak.

1.1 Why Mendel Succeeded

Chose Pisum sativum — easy to grow, short life span, large number of offspring, many contrasting pairs of characters, naturally self-pollinating.
Studied 7 pairs of contrasting characters simultaneously.
Maintained large sample size — statistical analysis.
Kept records of all generations (P, F₁, F₂, etc.).

1.2 Mendel's 7 Characters in Garden Pea

Character	Dominant Trait	Recessive Trait
Seed shape	Round (R)	Wrinkled (r)
Seed colour (cotyledon)	Yellow (Y)	Green (y)
Pod shape	Inflated (F)	Constricted (f)
Pod colour	Green (G)	Yellow (g)
Flower position	Axial (A)	Terminal (a)
Stem height	Tall (T)	Dwarf (t)
Flower colour	Violet (V)	White (v)

1.3 Law of Dominance

In a cross between parents differing in a pair of contrasting characters, only one character (dominant) appears in the F₁ generation; the other (recessive) disappears but reappears unchanged in F₂.

1.4 Law of Segregation (Law of Purity of Gametes)

The two alleles of a gene that are present in an individual separate/segregate from each other during gamete formation, so each gamete receives only one allele. This is Mendel's First Law.

P: TT x tt → F1: Tt (all tall)
F1 x F1: Tt x Tt → F2: TT : Tt : tt = 1:2:1 (genotypic)
Phenotypic ratio: Tall : Dwarf = 3 : 1

1.5 Law of Independent Assortment

When two pairs of contrasting characters are considered simultaneously, the alleles of different genes assort independently of each other during gamete formation (provided the genes are on different chromosomes = not linked). This is Mendel's Second Law.

P: RRYY x rryy → F1: RrYy (all Round Yellow)
F1 x F1: RrYy x RrYy
F2 Phenotypic ratio: Round Yellow : Round Green : Wrinkled Yellow : Wrinkled Green
= 9 : 3 : 3 : 1

⚠️ NEET Focus (2013, 2016, 2018, 2021, 2022): Monohybrid ratio = 3:1 (phenotypic) | 1:2:1 (genotypic). Dihybrid ratio = 9:3:3:1. Test cross ratio = 1:1. Back cross = cross of F₁ with either parent. Test cross = cross of F₁ with homozygous recessive parent. Law of Segregation = also called Law of Purity of Gametes.

2. Extensions of Mendelism (Deviations)

2.1 Incomplete Dominance

Neither allele is completely dominant; F₁ shows an intermediate phenotype (blending appearance). F₂ ratio changes to 1:2:1 (phenotypic = genotypic).

Example: Flower colour in Antirrhinum majus (snapdragon / dog flower) and Mirabilis jalapa (4 O'Clock plant).
Red (RR) x White (rr) → F₁: Pink (Rr) → F₂: Red : Pink : White = 1:2:1

2.2 Codominance

Both alleles are expressed simultaneously and equally in the heterozygote. No blending — both phenotypes appear together. F₂ ratio: 1:2:1.

Example 1: ABO blood groups in humans (I^AI^B = AB blood type — both A and B antigens expressed). Both I^A and I^B are codominant; i is recessive to both.
Example 2: Coat colour in cattle (roan = red + white hair both present).

2.3 ABO Blood Groups

Blood Group	Genotype(s)	Antigen on RBC	Antibody in Plasma
A	I^AI^A or I^Ai	A	anti-B
B	I^BI^B or I^Bi	B	anti-A
AB (Universal Recipient)	I^AI^B	A and B	None
O (Universal Donor)	ii	None	anti-A and anti-B

2.4 Pleiotropy

A single gene controlling multiple phenotypic traits. Example: Phenylketonuria (PKU) — single gene causes multiple effects (mental retardation, reduced hair/skin pigmentation, eczema). Sickle cell anaemia — single gene causes multiple symptoms.

2.5 Multiple Allelism

More than two alleles exist for a single gene in a population, though an individual can carry only 2. Example: ABO blood groups (3 alleles: I^A, I^B, i).

2.6 Polygenic Inheritance

A single character controlled by multiple genes (polygenes). Shows continuous variation. Examples: Skin colour in humans (3–5 gene pairs), height in humans. Bell-shaped (normal distribution) curve obtained.

⚠️ NEET Focus (2014, 2017, 2019, 2020): Incomplete dominance = intermediate phenotype, F₂ = 1:2:1. Codominance = both alleles expressed (ABO system). AB = Universal Recipient (I^AI^B). O = Universal Donor (ii). Pleiotropy = one gene, many effects. Polygenic = many genes, one character (skin colour = 3:1 ratio does NOT apply).

3. Chromosomal Theory of Inheritance & Linkage

3.1 Chromosomal Theory of Inheritance

Proposed by Sutton and Boveri (1902–1903). Stated that chromosomes are the carriers of genes. Parallels between Mendel's factors and chromosomal behaviour during meiosis. Summarised by Sutton as “Genes are located on chromosomes.”

3.2 Morgan's Work and Linkage

T.H. Morgan worked with Drosophila melanogaster (fruit fly). Advantages of Drosophila:

Short life cycle (~2 weeks), large number of offspring, easy to culture.
Only 4 pairs of chromosomes (3 autosomes + 1 sex chromosome pair).
Clear sexual dimorphism (males vs females easily distinguishable).
Many mutant characters easy to observe.

Linkage: Genes present on the same chromosome tend to be inherited together (linked). They do not assort independently (violate Law of Independent Assortment). Groups of genes on the same chromosome = linkage group. Number of linkage groups = number of haploid chromosome pairs.

Example: Morgan showed in Drosophila that body colour and wing shape genes are linked — they do not give 9:3:3:1 ratio.

Recombination frequency (crossing over): The farther apart two linked genes are, the more likely they are to be separated by crossing over. Recombination frequency used to construct genetic maps (linkage maps). 1 map unit = 1 centimorgan (cM) = 1% recombination.

⚠️ NEET Focus: Chromosomal theory = Sutton & Boveri. Morgan = Drosophila + linkage. Linkage = genes on same chromosome. Crossing over = recombination. Drosophila has 4 pairs of chromosomes. Recombination frequency (%) = map distance in centimorgans.

4. Sex Determination

4.1 Types of Sex Determination

Type	Organism	Mechanism
XX-XY	Human, Drosophila, most mammals	Male = XY, Female = XX. Male is heterogametic. Sex determined by father (Y chromosome).
XX-XO	Grasshoppers (Locusta)	Male = XO (45 chromosomes), Female = XX (46). Male produces two types of sperm (X or O).
ZZ-ZW	Birds, some reptiles, Lepidoptera (butterflies)	Male = ZZ (homogametic), Female = ZW (heterogametic). Opposite of mammals.
Haplodiploid	Honey bee (Apis mellifera)	Female = diploid (fertilised egg). Male (drone) = haploid (unfertilised egg = parthenogenesis).

4.2 Sex-Linked Inheritance

Genes located on the X chromosome show sex-linked inheritance. More commonly expressed in males (hemizygous = only one allele, no Y counterpart).

Disease	Inheritance	Key Feature
Haemophilia A	X-linked recessive	X^HX^h = carrier female; X^hY = affected male. Missing Factor VIII. Royal disease (Queen Victoria). Bleeder's disease.
Haemophilia B	X-linked recessive	Missing Factor IX (Christmas Disease).
Colour Blindness	X-linked recessive	Cannot distinguish red from green. X^cX^C = carrier; X^cY = colour blind male. Frequency: 8% males, 0.4% females.

📋 Key Cross — Colour Blindness:
Carrier female x Normal male → X^CX^c x X^CY
Offspring: X^CX^C (normal female), X^CX^c (carrier female), X^CY (normal male), X^cY (colour blind male).
Ratio: 1 normal female : 1 carrier female : 1 normal male : 1 colour blind male.

5. Mutation

Mutation: Sudden heritable changes in the DNA sequence (gene or chromosome number/structure). Term coined by Hugo de Vries. Mutations provide the raw material for evolution.

5.1 Types of Mutations

Type	Description	Example
Point Mutation (Gene Mutation)	Change in a single base pair (substitution, insertion, deletion).	Sickle cell anaemia: GAG → GUG (Glu → Val in β-globin).
Frameshift Mutation	Insertion or deletion of one/two nucleotides shifts the reading frame.	Entire protein changed downstream of mutation.
Chromosomal Aberrations	Changes in chromosome number (aneuploidy, polyploidy) or structure (deletion, duplication, inversion, translocation).	Down syndrome (trisomy 21), Turner syndrome (45, X).

6. Genetic Disorders

6.1 Chromosomal Disorders

Disorder	Karyotype	Features
Down Syndrome (Trisomy 21)	2n+1 = 47 (trisomy of chromosome 21)	Mental retardation, short stature, open mouth, broad forehead, flat nose, protruding tongue, heart defects. More common with advanced maternal age. Discovered by Langdon Down.
Klinefelter's Syndrome	47, XXY	Male but with feminine features (gynaecomastia, less facial hair, sterile). Extra X chromosome. Discovered by Harry Klinefelter.
Turner's Syndrome	45, XO (monosomy X)	Phenotypically female; sterile (ovaries not present), short stature, webbed neck, primary amenorrhea, no secondary sexual characters.
Super Female	47, XXX	Triplo-X, fertile female, usually normal phenotype.
Super Male / XYY	47, XYY	Tall male, aggressive, fertile.

6.2 Mendelian / Autosomal Disorders

Disorder	Inheritance	Cause / Features
Sickle Cell Anaemia	Autosomal recessive	Point mutation: GAG → GUG → Glu → Val in β-globin chain. RBCs sickle-shaped under low O₂. Carriers protected against malaria (balancing selection).
Phenylketonuria (PKU)	Autosomal recessive	Deficiency of phenylalanine hydroxylase → accumulation of phenylalanine → brain damage. Example of pleiotropy.
Thalassaemia	Autosomal recessive	Reduced/absent synthesis of α or β globin chains. Haemolytic anaemia. Mediterranean origin.
Huntington's Disease / Chorea	Autosomal dominant	Degeneration of basal ganglia and cortex. Progressive neurological disorder. Late onset (35–45 yrs). Trinucleotide (CAG) repeat expansion.

⚠️ NEET Focus (2013, 2015, 2017, 2018, 2020, 2022): Down syndrome = trisomy 21 (2n=47). Turner = 45, XO. Klinefelter = 47, XXY. Sickle cell anaemia = autosomal recessive; GAG → GUG (point mutation). Haemophilia = X-linked recessive (missing Factor VIII). Colour blindness = X-linked recessive. Huntington = autosomal dominant. Thalassaemia = autosomal recessive.

🎓 Key NEET Questions (Previous Years)

Q1. [NEET 2022] A cross between a tall and short pea plant gives 50% tall and 50% short offspring. The genotype of tall parent is:

(a) TT (b) Tt (c) tt (d) TTTT

Answer: (b) If tall parent was TT → all tall (100%). If 50:50 → cross is Tt x tt (test cross). Tall parent = Tt (heterozygous). This is a test cross with 1:1 ratio.

Q2. [NEET 2021] A colour-blind woman marries a normal man. What is the probability of colour blindness in their sons?

Cross: X^cX^c x X^CY → Offspring: X^CX^c (carrier daughter), X^cY (colour blind son), X^CX^c, X^cY

Answer: 100% ALL sons will be colour blind (X^cY) because they receive the X^c chromosome from the colour-blind mother and Y from the father. All daughters will be carriers.

Q3. [NEET 2020] Which of the following shows incomplete dominance?

(a) ABO blood group in humans (b) Seed coat colour in Pisum sativum (c) Flower colour in Antirrhinum majus (d) Height in pea

Answer: (c) Antirrhinum majus (snapdragon): Red x White → Pink (intermediate). F₂ gives 1 Red : 2 Pink : 1 White. ABO blood group = codominance (both alleles expressed, not intermediate).

Q4. [NEET 2019] Turner's syndrome is characterised by:

(a) 47, XXY (b) 45, X (c) 47, XX +21 (d) 47, XYY

Answer: (b) Turner's syndrome = 45, XO (monosomy X). Phenotypically female, sterile, short stature, webbed neck. (a) = Klinefelter; (c) = Down syndrome; (d) = XYY syndrome.

Q5. [NEET 2018] The mutation in sickle cell anaemia involves:

(a) Deletion of one nucleotide (b) Inversion of a segment (c) Substitution of one base pair (d) Addition of multiple base pairs

Answer: (c) Sickle cell anaemia = point mutation (substitution of Adenine by Thymine in the 6th codon of β-globin gene): GAG → GUG → Glutamic acid → Valine. One nucleotide substitution.

Q6. [NEET 2017] Which of the following is an example of polygenic inheritance?

(a) Flower colour in snapdragon (b) Skin colour in humans (c) ABO blood groups (d) Haemophilia

Answer: (b) Skin colour in humans is polygenic (controlled by 3–5 genes). It shows continuous variation. Flower colour in snapdragon = incomplete dominance. ABO = codominance. Haemophilia = X-linked recessive.

💡 Rapid Revision — Key Ratios & Facts

Monohybrid: Phenotypic = 3:1 | Genotypic = 1:2:1
Dihybrid: 9:3:3:1 | Test cross: 1:1
Incomplete dominance: 1:2:1 (phenotypic = genotypic)
Codominance example: ABO blood groups | I^AI^B = AB = Universal Recipient
Down syndrome = trisomy 21 (47 chromosomes). Turner = 45, XO. Klinefelter = 47, XXY
Sickle cell = autosomal recessive; point mutation GAG → GUG; Glu → Val
Haemophilia = X-linked recessive | missing Factor VIII (A) or IX (B)
Colour blindness = X-linked recessive | X-linked gene 8% males, 0.4% females
Huntington's chorea = autosomal dominant
ZZ-ZW (birds): female = heterogametic. Haploid males in honey bee = drones

NCERT Solutions - Principles of Inheritance and Variation - Class 12

CLASS 12 BIOLOGY | NCERT SOLUTIONS

Chapter 5 — Principles of Inheritance and Variation

All NCERT Exercise Questions with Detailed Solutions

📋 Note: All questions from NCERT Class 12 Biology Chapter 5 Exercise. Genetic cross solutions with punnet squares and ratios as per NCERT.

NCERT Exercise Questions & Solutions

2 MarksQ1. Mention the advantages of selecting pea plant for experiment by Mendel.

✓ Answer
Advantages of Pisum sativum (Garden pea):

Naturally self-pollinating: Ensures pure breeding lines; easy to obtain pure lines (true breeding) before hybridisation.
Easy cross-pollination: Emasculation is easy to perform for hybridisation experiments.
Short life cycle and easy cultivation: Several generations can be studied in a year.
Large number of offspring: Allows statistical analysis of ratios.
Many contrasting pairs: 7 pairs of contrasting characters that are easy to observe and score.
Distinct, easily observable traits: No ambiguity in scoring seed shape (round vs wrinkled), colour, stem height, etc.
Economical and available: Inexpensive and widely available at the time.

3 MarksQ2. Differentiate between the following: (a) Dominance and Recessiveness (b) Homozygous and Heterozygous (c) Monohybrid and Dihybrid.

✓ Answer
(a) Dominance vs Recessiveness:

Dominant: An allele that expresses itself in both homozygous (TT) and heterozygous (Tt) conditions. Example: Tall in pea (T).
Recessive: An allele that expresses itself only in the homozygous condition (tt). Masked by dominant allele in heterozygote. Example: Dwarf in pea (t).

(b) Homozygous vs Heterozygous:

Homozygous: Individual with identical alleles for a gene (TT or tt). Breeds true (pure breeding). Produces only one type of gamete.
Heterozygous: Individual with different alleles for a gene (Tt). Hybrid. Produces two types of gametes.

(c) Monohybrid vs Dihybrid:

Monohybrid: Cross involving one pair of contrasting characters. F₂ ratio: 3:1 (phenotypic), 1:2:1 (genotypic).
Dihybrid: Cross involving two pairs of contrasting characters simultaneously. F₂ ratio: 9:3:3:1.

5 MarksQ3. A diploid organism is heterozygous for 4 loci. How many types of gametes can it produce?

✓ Answer
For a heterozygous locus (e.g., Aa), it produces 2 types of gametes (A and a).
For n heterozygous loci: number of gamete types = 2ⁿ

Given: n = 4 heterozygous loci
Number of gamete types = 2⁴ = 16 types of gametes

Example: If genotype is AaBbCcDd, gametes include: ABCD, ABCd, ABcD, ABcd, AbCD, AbCd, AbcD, Abcd, aBCD, aBCd, aBcD, aBcd, abCD, abCd, abcD, abcd = 16 types.

3 MarksQ4. Explain the law of dominance using a monohybrid cross.

✓ Answer
Law of Dominance: Characters are controlled by discrete units called factors (genes). When two contrasting types are crossed, only one character (dominant) appears in F₁. The other (recessive) disappears but reappears in F₂.

P generation: TT (Tall) x tt (Dwarf) | | Gametes: T only t only ↓ F1 generation: Tt (All Tall) ↓ Self (Tt x Tt) F2 generation: TT : Tt : tt = 1 : 2 : 1 Phenotype: Tall : Tall : Dwarf = 3 Tall : 1 Dwarf

Observations:

F₁: All tall (dominant character appears; dwarf = recessive disappears).
F₂: 3 Tall : 1 Dwarf. Dwarf reappears unchanged (confirming it was not lost, just masked).

5 MarksQ5. In a dihybrid cross between two pea plants bearing round yellow seeds and wrinkled green seeds, find the phenotypic and genotypic ratios of F2.

✓ Answer
Parents: Round Yellow (RRYY) x Wrinkled Green (rryy)
F₁: RrYy (Round Yellow) — all seeds Round Yellow
F₁ x F₁: RrYy x RrYy (Self)

Gametes from RrYy: RY, Ry, rY, ry (4 types each parent) F2 Phenotypic Ratio: Round Yellow (R_Y_) : Round Green (R_yy) : Wrinkled Yellow (rrY_) : Wrinkled Green (rryy) = 9 : 3 : 3 : 1

F₂ Genotypic Ratio:
RRYY : RRYy : RRyy : RrYY : RrYy : Rryy : rrYY : rrYy : rryy
= 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1
(9 genotypic classes total)

This ratio (9:3:3:1) illustrates Mendel's Law of Independent Assortment — R/r and Y/y genes assort independently as they are on different chromosomes.

3 MarksQ6. What is incomplete dominance? Give an example and the genotypic and phenotypic ratios in F2.

✓ Answer
Incomplete Dominance: When neither allele is completely dominant, the F₁ shows an intermediate (blending) phenotype between the two parents.

P: RR (Red) x rr (White) [Antirrhinum majus] F1: Rr (Pink) F1 x F1: Rr x Rr F2: RR : Rr : rr = 1 : 2 : 1 Red : Pink : White = 1 : 2 : 1

F₂ Phenotypic ratio = 1:2:1 (not 3:1 as in complete dominance)
F₂ Genotypic ratio = 1:2:1
In incomplete dominance, the phenotypic ratio equals the genotypic ratio (same as with codominance).

Other examples: Mirabilis jalapa (4 o'clock plant).

3 MarksQ7. What are the different types of chromosomal disorders? Describe Turner's syndrome and Klinefelter's syndrome.

✓ Answer
Types of chromosomal disorders:

Aneuploidy: Gain (trisomy) or loss (monosomy) of one or more chromosomes. Examples: Down syndrome (2n+1=47), Turner syndrome (2n-1=45).
Polyploidy: Gain of one or more complete sets of chromosomes (3n, 4n, etc.). Common in plants.
Structural abnormalities: Deletion, duplication, inversion, translocation of chromosomal segments.

Turner's Syndrome (45, XO):

Karyotype: 45 chromosomes (2n−1 = 45); monosomy of sex chromosomes (XO).
Phenotypically female but sterile (ovaries absent/rudimentary).
Features: short stature, webbed neck (pterygium colli), primary amenorrhea (no menstruation), lack of secondary sexual characters, low-set ears, shield-shaped chest.

Klinefelter's Syndrome (47, XXY):

Karyotype: 47 chromosomes; XXY sex chromosomes.
Phenotypically male but infertile (sterile).
Features: gynaecomastia (breast development in male), sparse body/facial hair, long legs, small testes, female-like fat distribution.

5 MarksQ8. A man with haemophilia marries a normal woman. What will be the genotypes and phenotypes of their children?

✓ Answer
Haemophilia is X-linked recessive. Let X^H = normal allele, X^h = haemophilia allele.
Haemophilic father: X^hY Normal mother: X^HX^H

X(h)Y x X(H)X(H) | | Gametes: X(h), Y x X(H), X(H) ↓ Offspring: X(H)X(h) : Carrier daughter (normal phenotype, carrier) X(H)Y : Normal son (not affected)

Results:

All daughters will be carriers (X^HX^h) — phenotypically normal but carry the recessive allele.
All sons will be normal (X^HY) — they receive X^H from mother and Y from father.
No child will be haemophilic in this cross.

Note: If carrier daughter (X^HX^h) marries normal male → 50% sons will be haemophilic.

3 MarksQ9. Explain the chromosomal theory of inheritance.

✓ Answer
Chromosomal Theory of Inheritance was proposed by Sutton (USA) and Boveri (Germany) in 1902–1903, independently.

Key observations and parallels with Mendel's Laws:

Both chromosomes and Mendelian factors (genes) occur in pairs.
Both separate (segregate) during gamete formation (meiosis).
One member of each pair comes from each parent — same as genes.
The behaviour of chromosomes during meiosis provides a physical basis (mechanism) for Mendel's Laws of Segregation and Independent Assortment.

Conclusion: Genes are located on chromosomes. Different genes are on different chromosomes (explaining independent assortment). Genes on the same chromosome tend to be linked (explaining deviation from 9:3:3:1).

Facts Capsule - Principles of Inheritance and Variation - Class 12

CLASS 12 BIOLOGY | NEET RAPID CAPSULE

Facts & High-Yield Points

Chapter 5 — Principles of Inheritance and Variation | 30 Key Facts for NEET

🌿 Mendelism

FACT #01 — Mendel Background

Gregor Johann Mendel (1822–1884), Augustinian monk, Brno, Austria. Worked with Pisum sativum for 7 years (1856–1863). Published 1866. Rediscovered 1900 by de Vries, Correns, Tschermak.

FACT #02 — Mendel's 7 Characters

Seed shape (R/r), Seed colour (Y/y), Pod shape (F/f), Pod colour (G/g), Flower position (A/a), Stem height (T/t), Flower colour (V/v). Dominant always listed first.

FACT #03 — Key Ratios

Monohybrid F2 phenotypic: 3:1. Monohybrid F2 genotypic: 1:2:1. Dihybrid F2 phenotypic: 9:3:3:1. Test cross: 1:1. Back cross: F₁ x either parent.

FACT #04 — Gametes Formula

Number of types of gametes produced by an organism heterozygous at n loci = 2ⁿ. Heterozygous at 4 loci = 2⁴ = 16 gamete types.

FACT #05 — Law of Segregation

Also called Law of Purity of Gametes. Alleles segregate during gamete formation (anaphase I of meiosis). Each gamete gets one allele. All three Mendel's laws are based on meiotic behaviour of chromosomes.

📈 Deviations from Mendelism

FACT #06 — Incomplete Dominance

F₁ = intermediate phenotype. F₂ phenotypic = genotypic = 1:2:1. Examples: Antirrhinum majus (dog flower / snapdragon) and Mirabilis jalapa (4 o'clock plant). Red x White → Pink.

FACT #07 — Codominance

Both alleles expressed equally. No blending. F₂ = 1:2:1. Example: ABO blood group (I^AI^B = AB type). Roan cattle. IA and IB are codominant; i is recessive to both.

FACT #08 — ABO Blood Groups

Universal Donor: O (ii) — no antigens. Universal Recipient: AB (I^AI^B) — no antibodies. IgM = ABO antibodies. 3 alleles (multiple allelism). Controlled by single gene with 3 alleles: I^A, I^B, i.

FACT #09 — Pleiotropy

One gene → multiple phenotypic effects. Examples: Sickle cell anaemia (multiple symptoms from single Hb-S gene), PKU (mental retardation + pigmentation loss). Contrasts with polygenic.

FACT #10 — Polygenic Inheritance

Multiple genes → single character. Continuous variation. Examples: Skin colour, height, eye colour, intelligence in humans. Bell-shaped distribution. Does NOT follow 3:1 or 9:3:3:1 ratios.

🦀 Chromosomal Theory & Linkage

FACT #11 — Sutton and Boveri

Walter Sutton (USA) and Theodor Boveri (Germany), 1902–1903. Observed parallels between Mendel's factors and chromosome behaviour during meiosis. Genes on chromosomes.

FACT #12 — Morgan & Drosophila

T.H. Morgan worked with Drosophila melanogaster. Advantages: 4 chromosome pairs, short life cycle (~2 weeks), large offspring, clear sexual dimorphism. Established concept of linkage and recombination.

FACT #13 — Linkage

Genes on same chromosome = linked. Violate Law of Independent Assortment. Number of linkage groups = haploid chromosome number. In humans = 23 linkage groups. Crossing over → recombination; frequency gives genetic map distance (centimorgan).

♾️ Sex Determination & Sex-Linked

FACT #14 — XX-XY Type

Human, Drosophila, most mammals. Male = XY (heterogametic — produces X and Y sperm). Female = XX. Sex of offspring determined by father's sperm (X or Y).

FACT #15 — ZZ-ZW Type

Birds, butterflies (Lepidoptera), some reptiles. Male = ZZ (homogametic). Female = ZW (heterogametic). Opposite to mammals. Sex determined by mother (Z or W egg).

FACT #16 — Honey Bee Sex

Haplodiploidy in honey bee (Apis mellifera). Female (queen/worker) = diploid (from fertilised egg). Male (drone) = haploid (from unfertilised egg = parthenogenesis). No sex chromosomes.

FACT #17 — Haemophilia

X-linked recessive. Missing Factor VIII (Haemophilia A) or Factor IX (Haemophilia B = Christmas disease). Famous in European royal families (Queen Victoria was carrier). X^HX^h = carrier female; X^hY = affected male.

FACT #18 — Colour Blindness

X-linked recessive. Cannot distinguish red from green. Frequency: 8% males, 0.4% females. Colour blind father x Normal mother → all daughters carriers. Colour blind woman x Normal man → all sons colour blind.

🧣 Genetic Disorders

FACT #19 — Down Syndrome

Trisomy 21 (2n+1 = 47 chromosomes). Discovered by Langdon Down. Features: mental retardation, short stature, flat nose, heart defects, protruding tongue. More common with advanced maternal age (>40 yrs).

FACT #20 — Turner's Syndrome

45, XO (monosomy X). Phenotypically female, sterile (ovaries absent). Features: short stature, webbed neck, primary amenorrhea, no secondary sexual characters, shield-shaped chest.

FACT #21 — Klinefelter's Syndrome

47, XXY. Phenotypically male, sterile. Discovered by Harry Klinefelter. Features: gynaecomastia (breast development), sparse hair, small testes, infertile, female-like fat distribution.

FACT #22 — Sickle Cell Anaemia

Autosomal recessive. Point mutation in β-globin gene: GAG → GUG (6th codon). Glu → Val substitution. RBCs sickle-shaped under low O₂. Carriers protected against malaria (balancing selection).

FACT #23 — Huntington's Disease

Autosomal dominant. Trinucleotide repeat (CAG) disease. Progressive neurological degeneration. Late onset (35–45 yrs). Even one copy causes disease. Cannot be suppressed by a normal allele.

FACT #24 — Thalassaemia

Autosomal recessive. Reduced/absent α or β globin chains. Haemolytic anaemia. α-thalassaemia: gene on chromosome 16. β-thalassaemia: gene on chromosome 11. Mediterranean and Asian origin. Differ from sickle cell in that structure normal, quantity reduced.

🧠 Mnemonics — Remember Fast

Genetic Ratios: “3:1, 9:3:3:1, 1:1” Mono F2 = 3:1. Dihy F2 = 9:3:3:1. Test cross = 1:1. Incomplete/Codominance F2 = 1:2:1.

Chromosomal Disorders: “DKT” Down = trisomy 21 (47). Klinefelter = 47, XXY. Turner = 45, XO. All three are most common in NEET. Down = autosomal; K and T = sex chromosome.

Autosomal Dominant: “HHD” Huntington's chorea = Dominant. Hypercholesterolaemia (familial) = Dominant. All others (sickle cell, PKU, thalassaemia, cystic fibrosis) = Recessive. Exception rule for NEET.

📊 Genetic Disorders Quick Reference

Disorder	Type	Karyotype / Cause	Key Feature
Down Syndrome	Chromosomal (autosomal)	47 (trisomy 21)	Mental retardation, flat nose
Klinefelter's	Chromosomal (sex)	47, XXY	Male, sterile, gynaecomastia
Turner's	Chromosomal (sex)	45, XO	Female, sterile, webbed neck
Sickle Cell Anaemia	Mendelian (A. recessive)	GAG→GUG in β-globin	Sickle RBCs, malaria protection
Haemophilia A	X-linked recessive	Factor VIII deficiency	Bleeder's disease
Colour Blindness	X-linked recessive	8% males affected	Can't distinguish red/green
Huntington's	Mendelian (A. dominant)	CAG repeat on chr 4	Progressive neurological, late onset
PKU	Mendelian (A. recessive)	Phe hydroxylase absent	Mental retardation (pleiotropy)

🔢 Critical Numbers — Never Forget

7 — characters studied by Mendel 3:1 — monohybrid F2 phenotypic ratio 9:3:3:1 — dihybrid F2 ratio 1:1 — test cross ratio 1:2:1 — incomplete dominance F2 2ⁿ — gamete types from n heterozygous loci 47, trisomy 21 — Down syndrome 45, XO — Turner's syndrome 47, XXY — Klinefelter's 4 pairs — Drosophila chromosomes 8% — colour blindness in males 0.4% — colour blindness in females

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