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BOT-10 / ZOO-06 Botany + Zoology 6–8 Q/year · High Priority

Genetics & Evolution

Mendel's laws, inheritance patterns, chromosomal theory, sex-linked traits, Hardy-Weinberg law, and Darwinian evolution.

Concept Core

Fact & Formula Vault

Worked Examples

Mistake DNA

Chapter Intelligence

Genetics — Concept Core

Mendel's Laws

Law of Dominance: In a heterozygote (Aa), the dominant allele (A) masks the recessive allele (a). Phenotype shows dominant trait.

Law of Segregation: Allele pairs separate during gamete formation. Each gamete gets one allele. Monohybrid cross ratio = 3:1 (phenotype), 1:2:1 (genotype).

Law of Independent Assortment: Allele pairs for different traits assort independently (applies only to genes on different chromosomes). Dihybrid cross ratio = 9:3:3:1.

Exceptions to Mendel's Laws

Exception	F1	F2 Ratio	Example
Incomplete Dominance	Intermediate (pink)	1:2:1	Antirrhinum (4 o'clock flower)
Codominance	Both expressed	1:2:1	ABO blood groups (IA and IB)
Multiple Alleles	Depends on alleles	Various	ABO: IA, IB, i (3 alleles)
Pleiotropy	One gene → many traits	—	Sickle cell anaemia (HbS)
Polygenic Inheritance	Continuous variation	Bell curve	Skin colour, height in humans

Sex Determination & Sex-Linked Inheritance

XY type (humans): Females = XX, Males = XY. Sex determined by father's sperm (X or Y).

X-linked recessive: Haemophilia, colour blindness — carrier females (XᴴX), affected males (XᴴY). Males affected more frequently (no second X to compensate).

X-linked dominant: Vitamin D-resistant rickets (hypophosphataemia). Affected father passes to ALL daughters, no sons.

Chromosomal Disorders

DOWN SYNDROME

Trisomy 21 (2n+1 = 47)

Mongolism. Extra chromosome 21. Caused by non-disjunction.

KLINEFELTER SYNDROME

47, XXY

Male phenotype, gynaecomastia, infertile. Extra X from non-disjunction.

TURNER SYNDROME

45, XO (monosomy)

Female phenotype, sterile, short stature, webbed neck.

PATAU SYNDROME

Trisomy 13

Severe defects, usually fatal within months.

Hardy-Weinberg Equilibrium

In an ideal large population: p² + 2pq + q² = 1 and p + q = 1. Allele frequencies remain constant if: no mutation, no selection, random mating, no genetic drift, no gene flow. Disturbances = evolutionary forces.

Evolution Key Concepts

Darwinism: Natural selection — survival of the fittest. Variation exists, more offspring than environment supports, struggle for existence, favourable variations are selected.

Evidence for Evolution: Homologous organs (same structure, different function — common ancestry), Analogous organs (different structure, same function — convergent evolution), Fossils (chronological record).

Human Evolution: Dryopithecus → Ramapithecus → Australopithecus → Homo habilis → Homo erectus → Homo sapiens neanderthalensis → Homo sapiens sapiens.

Genetics — Key Facts & Numbers

MONOHYBRID RATIO

3:1 (phenotype) · 1:2:1 (genotype)

DIHYBRID RATIO

9:3:3:1

9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb

TESTCROSS

Cross with homozygous recessive (aa)

Reveals genotype of organism

EPISTASIS

One gene suppresses another

Duplicate dominant: 15:1; Recessive epistasis: 9:3:4

LINKAGE

Genes on same chromosome

Discovered by Morgan in Drosophila. Reduces recombination frequency.

CROSSING OVER

Pachytene stage of meiosis I

Creates recombinant chromosomes. Frequency ∝ distance between genes.

ABO BLOOD GROUPS

IA and IB are codominant over i

6 genotypes, 4 phenotypes (A, B, AB, O)

RH FACTOR

Rh+ dominant over Rh−

Rh− mother + Rh+ foetus → Erythroblastosis foetalis

COLOUR BLINDNESS

X-linked recessive

Carrier female (Xᶜ X) × Normal male (X Y) → 50% sons affected

HAEMOPHILIA A

X-linked recessive (Factor VIII deficiency)

Queen Victoria was a carrier. "Royal disease"

SICKLE CELL ANAEMIA

Autosomal recessive (pleiotropy)

HbA → HbS: Glu → Val at position 6 of β-chain. Protects against malaria (heterozygote advantage)

PHENYLKETONURIA (PKU)

Autosomal recessive

Phenylalanine → tyrosine pathway blocked. Mental retardation if untreated.

Mutation Types

Type	Change	Effect	Example
Point (Missense)	One nucleotide changed	Wrong amino acid	Sickle cell (Glu→Val)
Nonsense	Codon → stop codon	Truncated protein	β-thalassaemia
Silent	Codon change, same aa	No effect (degenerate code)	—
Frameshift	Insertion or deletion	All downstream changes	Duchenne MD
Chromosomal	Structure/number change	Often severe	Trisomy 21

Genetics — Worked Examples

Q1. In a cross between two heterozygous tall pea plants (Tt × Tt), what is the probability of getting a homozygous recessive plant?

A) 1/2

B) 1/4

C) 3/4

D) 0

Tt × Tt → 1 TT : 2 Tt : 1 tt. Genotype ratio = 1:2:1. Homozygous recessive (tt) = 1/4 = 25%. This is the classic monohybrid cross — Mendel's Law of Segregation.

Q2. A colour-blind father (XᶜY) and carrier mother (XᶜX) have children. What percentage of their daughters will be colour blind?

A) 25%

B) 50%

C) 100%

D) 0%

Cross: Xᶜ X × Xᶜ Y → daughters: Xᶜ Xᶜ (colour blind) and Xᶜ X (carrier). Each daughter gets Xᶜ from father. 50% get Xᶜ from mother too → colour blind daughters. Sons: 50% colour blind (XᶜY), 50% normal (XY).

Q3. If in a population the frequency of recessive allele (q) = 0.3, what is the frequency of carriers (heterozygotes)?

A) 0.09

B) 0.49

C) 0.42

D) 0.21

q = 0.3, so p = 1 − 0.3 = 0.7. Carriers = 2pq = 2 × 0.7 × 0.3 = 0.42. Hardy-Weinberg: p² (AA) + 2pq (Aa) + q² (aa) = 1. This is a classic NEET question type — always apply H-W formula.

Q4. Antirrhinum majus (snapdragon) shows incomplete dominance. RR (red) × WW (white) → F1 pink. F1 × F1 phenotype ratio in F2 is:

A) 3:1

B) 1:2:1 (red:pink:white)

C) 9:3:3:1

D) 1:1

Incomplete dominance: RW (F1 pink). RW × RW → RR (red) : 2 RW (pink) : WW (white) = 1:2:1. In incomplete dominance, phenotype ratio = genotype ratio (both are 1:2:1). This exception to Mendel's law appears every year in NEET.

Genetics — Mistake DNA

❌ "In codominance, F2 ratio is 3:1 like Mendel's law"

✅ WRONG. Codominance F2 ratio = 1:2:1 (phenotype = genotype ratio). Both alleles are fully expressed in heterozygote (e.g., AB blood group, where both A and B antigens are present). The 3:1 ratio applies only when one allele is fully dominant.

❌ "Sex-linked traits are only found on X chromosome"

✅ WRONG. Sex-linked traits can be on X OR Y. Y-linked (holandric) traits pass from father to ALL sons only (e.g., hypertrichosis of ear pinna). X-linked traits: colour blindness, haemophilia, Duchenne MD. Know which is which!

❌ "Linkage always violates the Law of Independent Assortment"

✅ PARTIALLY WRONG. Linkage reduces (not eliminates) recombination. Crossing over during pachytene still produces recombinants. Morgan showed that genes on the same chromosome can still assort if far apart (frequency of recombination up to 50%).

❌ "Turner syndrome has 45 chromosomes and male phenotype"

✅ WRONG. Turner syndrome (45, XO) has FEMALE phenotype. They are sterile, with webbed neck, short stature, and poorly developed secondary sexual characters. Klinefelter (47, XXY) has male phenotype.

❌ "Hardy-Weinberg equilibrium means evolution is occurring"

✅ WRONG. H-W equilibrium means NO evolution is occurring (allele frequencies stable). When H-W conditions are VIOLATED (mutation, selection, drift, gene flow, non-random mating), evolution occurs. H-W equilibrium = evolutionary stasis.

Genetics — Chapter Intelligence

AVG QUESTIONS

6–8

Per year in NEET. Combined BOT + ZOO

TOTAL MARKS

24–32

Out of 720. One of the highest yield chapters.

NCERT CHAPTERS

BOT: Ch 5 (Principles of Inheritance), Ch 6 (Molecular Basis)

DIFFICULTY

Medium — requires problem solving

Punnett square questions need practice

2025Codominance in ABO blood groups tested (multiple alleles), Turner syndrome identification from karyotype

2024Hardy-Weinberg calculation (q value given, asked for 2pq). X-linked recessive cross problem. Pleiotropy definition

2023Dihybrid cross ratio, Mendel's choice of pea plant (why garden pea), sex determination in humans vs. birds (ZW/ZZ)

2022Chromosomal disorders (trisomy 21 vs. monosomy), linkage and recombination frequency, Morgan's Drosophila experiments

2021Point mutation in sickle cell (Glu→Val), incomplete dominance in 4 o'clock flower, epistasis ratios

🎯 High-Probability 2026 Topics

✦ H-W equilibrium numerical (calculate allele frequencies from phenotype data)

✦ Pedigree analysis (autosomal vs. X-linked, dominant vs. recessive)

✦ Sex-linked cross calculation (haemophilia or colour blindness)

✦ Incomplete dominance vs. codominance distinction

✦ Chromosomal disorder identification from symptoms or karyotype