🧬 Genetics – Full Breakdown

1) What is Genetics?

The word "Genetics" comes from the Greek word "gen" meaning "to give birth". It was coined by W. Bateson in 1905 — he is also known as the "Father of Modern Genetics".

Key branches you should know:

• Heredity: Transmission of characters (traits) from parents to offspring
• Variation: Differences between individuals of the same species
• Genomics: Study of genes and their functions

2) Variation — Acquired vs Inherited

Variation means differences among individuals of the same species. No two individuals (except identical twins) are exactly alike — not even in looks, height, blood group, or disease resistance. These differences are called variations.

a) Acquired (Somatic) Variation

Definition: Variations that develop during an individual's lifetime due to environmental factors, use/disuse of organs, or habits. They are NOT passed to offspring.

b) Inherited (Germinal) Variation

Definition: Variations that arise due to changes in genes or chromosomes in the germ cells (gametes). These ARE passed from parents to offspring.

3) Basic Terminology — The Foundation

You cannot solve genetics problems unless you know these terms cold. Every board exam starts with 2-3 definition-based 1-mark questions.

4) Gregor Mendel — The Father of Genetics

Gregor Johann Mendel (1822–1884) was an Austrian monk who conducted experiments on pea plants (Pisum sativum) in his monastery garden for 7 years (1856–1863). He published his work in 1866, but nobody understood it until 1900 when three scientists (de Vries, Correns, Tschermak) independently rediscovered his work.

Why Mendel chose the pea plant?

• Short life cycle — could study multiple generations quickly
• True-breeding varieties available — pure lines for 7 distinct traits
• Easy to grow and cross-pollinate
• Many offspring — good for statistical analysis
• 7 distinct, easily observable traits with clear dominant-recessive patterns (no blending):

5) Monohybrid Cross — Law of Dominance & Law of Segregation

The Cross (Tall × Dwarf)

Mendel crossed a true-breeding Tall plant (TT) with a true-breeding Dwarf plant (tt):

Law of Dominance (First Law)

Statement: "When two homozygous individuals with contrasting traits are crossed, the trait that appears in the F₁ generation is called the dominant trait, and the one that remains hidden is the recessive trait."

Explanation: In the F₁ hybrid (Tt), the dominant allele (T) masks the expression of the recessive allele (t). The recessive trait reappears only in the F₂ generation when it is in homozygous condition (tt).

Law of Segregation (Second Law)

Statement: "During gamete formation (meiosis), the two alleles for a trait separate (segregate) from each other so that each gamete receives only one allele."

Explanation: A heterozygous individual (Tt) produces two types of gametes — 50% with T and 50% with t. This is what gives the 3:1 ratio in F₂.

6) Dihybrid Cross — Law of Independent Assortment

The Cross (Round Yellow × Wrinkled Green)

Mendel then studied two traits together — seed shape (Round R / Wrinkled r) and seed colour (Yellow Y / Green y).

F₂ Punnett Square (4 × 4):

F₂ Phenotypic Ratio: 9 : 3 : 3 : 1

• 9 Round Yellow (RRYY, RRYy, RrYY, RrYy)
• 3 Round Green (RRyy, Rryy)
• 3 Wrinkled Yellow (rrYY, rrYy)
• 1 Wrinkled Green (rryy)

F₂ Genotypic Ratio: 1:2:1:2:4:2:1:2:1 (for the 9 genotypic combinations)

Law of Independent Assortment (Third Law)

Statement: "When two pairs of contrasting traits are considered simultaneously, the factors (alleles) for each trait assort independently of each other during gamete formation."

Explanation: The allele for Round (R) can combine equally with Yellow (Y) or Green (y) — giving RY and Ry gametes with equal probability. Similarly, r can combine with Y or y. This only works when the genes are on different chromosomes or far apart on the same chromosome.

7) Test Cross & Back Cross

Test Cross

Definition: Crossing an individual with a dominant phenotype (but unknown genotype — could be TT or Tt) with a homozygous recessive individual (tt). The offspring ratio reveals the unknown genotype.

Back Cross

Definition: Crossing an F₁ hybrid with either of its parents. If crossed with the dominant parent → called outcross. If crossed with the recessive parent → same as test cross.

8) Exceptions to Mendel's Laws

Mendel was lucky — the 7 traits he chose were all on different chromosomes and showed complete dominance. But many traits don't follow these simple rules. ICSE loves asking about these exceptions.

a) Incomplete Dominance (1:2:1 Phenotypic Ratio)

Definition: Neither allele is fully dominant over the other. The heterozygote shows an intermediate/blended phenotype.

Classic example — Snapdragon (Antirrhinum) flower colour:

• RR = Red flower
• Rr = Pink flower (intermediate)
• rr = White flower

Cross: RR (Red) × rr (White) → F₁: Rr (All Pink) → F₂: 1 Red : 2 Pink : 1 White

Why different from Mendel? In Mendel's monohybrid cross, F₁ showed dominant trait (Tall) and F₂ gave 3:1. Here, F₁ shows blend (Pink) and F₂ gives 1:2:1 — phenotypic ratio matches genotypic ratio.

b) Co-dominance (1:2:1 Phenotypic Ratio)

Definition: Both alleles express themselves fully and simultaneously in the heterozygote. Neither is dominant or recessive.

Classic example — ABO Blood Group:

• Iᴬ and Iᴮ are co-dominant → IᴬIᴮ gives AB blood type (both A and B antigens present)
• Both Iᴬ and Iᴮ are dominant over i (recessive allele)

Other example: Roan cattle (RR = red, Rr = roan/red+white interspersed, rr = white)

c) Multiple Alleles

Definition: When more than two alternative forms (alleles) of a gene exist in a population. However, any individual can have only two alleles (one from each parent).

Classic example — ABO Blood Group System:

• 3 alleles: Iᴬ, Iᴮ, i
• 6 possible genotypes: IᴬIᴬ, Iᴬi, IᴮIᴮ, Iᴮi, IᴬIᴮ, ii
• 4 possible phenotypes: A, B, AB, O

d) Lethal Genes (2:1 Ratio)

Definition: A gene that causes death of the organism when present in a particular genotype (usually homozygous dominant or recessive).

Example — Yellow mouse in mice: The yellow coat colour allele (Y) is dominant for coat colour but recessive lethal in homozygous condition (YY).

• Yy (Yellow) × Yy (Yellow) → 1 YY : 2 Yy : 1 yy
• YY = dies before birth → only 2 Yellow : 1 Non-yellow survive

Result: Phenotypic ratio becomes 2:1 instead of 3:1

9) Sex Determination

In humans, sex is determined by the 23rd pair of chromosomes (sex chromosomes):

• Females: XX (homogametic — produces only X-bearing eggs)
• Males: XY (heterogametic — produces X and Y sperm in 1:1 ratio)

The Cross for Sex Determination:

Result: 50% probability of female (XX), 50% probability of male (XY). The sex of the child is determined by the father because he contributes either X or Y chromosome.

10) Sex-Linked (X-Linked) Inheritance

Some genes are present only on the X chromosome and have no corresponding allele on the Y chromosome. Such genes are called sex-linked or X-linked. Because males have only one X, they express the trait even if it's recessive.

Key Characteristics of X-Linked Recessive Inheritance:

• More males affected than females
• Criss-cross inheritance: Affected grandfather → carrier daughter → affected grandson
• A father cannot pass X-linked trait to his son (son gets Y from father, X from mother)
• A female needs two recessive alleles (homozygous) to express the trait; a male needs only one (hemizygous)

a) Colour Blindness

Inability to distinguish between red and green colours. Caused by a recessive allele (c) on the X chromosome.

Common cross types:

b) Haemophilia ("Royal Disease")

Blood clotting disorder caused by deficiency of clotting Factor VIII (Haemophilia A) or Factor IX (Haemophilia B). Recessive X-linked.

• Affected person bleeds excessively even from minor cuts
• Famous history: Queen Victoria was a carrier, passed it to European royal families
• Treatment: Factor VIII/IX injections (replacement therapy)

Same inheritance pattern as colour blindness. Cross types are identical — just replace c with h.

11) Linkage & Crossing Over

Linkage

Definition: The tendency of genes located close together on the same chromosome to be inherited together. Linked genes do not follow Mendel's Law of Independent Assortment.

Key point: All genes on a chromosome form a linkage group. The number of linkage groups = the number of chromosome pairs. In humans, there are 23 linkage groups.

Crossing Over

Definition: Exchange of genetic material between non-sister chromatids of homologous chromosomes during Prophase I of meiosis (pachytene stage).

• Significance: Creates new combinations of genes (recombination) → increases genetic variation
• Recombination frequency is used to determine distance between genes on a chromosome (gene mapping) — higher frequency = farther apart
• Complete linkage: No crossing over occurs (e.g., male Drosophila)
• Incomplete linkage: Crossing over occurs, producing some recombinant types

12) Mutation

Definition: A sudden, stable, heritable change in the DNA sequence or chromosome structure. Mutations are the ultimate source of all genetic variation.

Types of Mutations:

a) Gene Mutation (Point Mutation): Change in the DNA sequence of a single gene.

• Substitution: One base pair replaced by another (e.g., GAG → GTG in sickle cell — Glu → Val)
• Insertion/Deletion: Addition or loss of a base → frameshift mutation (shifts entire reading frame)

b) Chromosomal Mutation: Changes in chromosome structure or number.

Mutagens (Causes of Mutation):

• Physical: UV radiation, X-rays, gamma rays, radioactive decay
• Chemical: Mustard gas, nitrous acid, certain pesticides
• Biological: Viruses (some insert their DNA into host genome)

13) Punnett Square Mastery

This is the most practical skill you need for boards. Here's my step-by-step approach:

Quick Reference — Gamete Formation

14) Pedigree Analysis

A pedigree is a family tree showing the inheritance pattern of a trait across multiple generations. ICSE often gives a pedigree chart and asks you to identify the inheritance pattern.

Standard Symbols:

• □ = Normal male
• ○ = Normal female
• ■ = Affected male
• ● = Affected female
• ◐ or half-shaded = Carrier (heterozygous) — usually for recessive disorders
• Horizontal line = Marriage
• Vertical line = Offspring

How to Identify Inheritance Pattern:

15) Common ICSE Board Questions

1-Mark Definitions:

1. Define: Gene, Allele, Genotype, Phenotype, Homozygous, Heterozygous, Dominant, Recessive
2. What is a test cross?
3. Define linkage and crossing over.
4. What is mutation? Name one mutagen.
5. Define variation. Differentiate acquired vs inherited.

3-Mark Reasoning Questions:

1. "The F₁ progeny of Mendel's monohybrid cross showed only dominant traits." Explain.
2. Why did Mendel choose pea plants for his experiments? (Give 3 reasons)
3. State and explain the Law of Segregation with a suitable cross.
4. Differentiate between incomplete dominance and co-dominance with examples.
5. A colour-blind man marries a normal woman. What are the chances of their son being colour-blind? (Show working)
6. Why are males more likely to suffer from X-linked disorders than females?
7. What is criss-cross inheritance? Explain with an example.

5-Mark Numericals / Crosses:

1. A pure tall pea plant is crossed with a pure dwarf plant. Show the F₁ and F₂ generations with genotypic and phenotypic ratios.
2. In pea plants, yellow seed colour (Y) is dominant over green (y), and round seed shape (R) is dominant over wrinkled (r). Show a dihybrid cross between YYRR and yyrr. Give the F₂ phenotypic ratio.
3. A woman with blood group A (heterozygous) marries a man with blood group B (heterozygous). Show the possible blood groups of their children.
4. A carrier woman for haemophilia marries a normal man. Show the probability of their children inheriting the disorder.
5. Analyse a given pedigree chart and identify the pattern of inheritance. (Justify your answer.)

16) Quick Reference Tables (Print This)

Mendel's Laws at a Glance

Ratios Cheat Sheet

Inheritance Patterns Compared