Mendelian Genetics — Dominance, Segregation, Independent Assortment

Gregor Mendel, a monk in 19th-century Austria, spent years cross-breeding pea plants and counting seeds. From that systematic work, he uncovered the fundamental rules of heredity — rules that we now know govern all sexually reproducing life.

Mendelian genetics is not just history; it’s the bedrock of everything from genetic counseling to crop improvement. For NEET and CBSE, it’s one of the highest-weightage topics in Class 12 Biology, appearing in 4-6 questions per exam. And unlike many biology topics, it rewards systematic thinking over pure memorization.

Key Terms and Definitions

Gene: A segment of DNA that determines a specific trait. Genes come in pairs (one on each homologous chromosome).

Allele: Alternative forms of a gene. For a gene controlling flower color, alleles might be “purple” or “white.” Represented by letters: $T$ (tall), $t$ (dwarf).

Dominant allele: The allele that expresses its trait even when present as just one copy (paired with a different allele). Usually represented with a capital letter (T).

Recessive allele: The allele whose effect is “masked” when a dominant allele is present. Expresses only in homozygous recessive condition (tt). Represented with lowercase (t).

Homozygous: Having two identical alleles for a trait. $TT$ (homozygous dominant) or $tt$ (homozygous recessive).

Heterozygous (hybrid): Having two different alleles for a trait. $Tt$ (heterozygous for height). The dominant trait is expressed.

Genotype: The actual genetic makeup. $TT$ , $Tt$ , or $tt$ .

Phenotype: The observable characteristic produced by the genotype. $TT$ and $Tt$ both show “tall” phenotype; $tt$ shows “dwarf.”

Monohybrid cross: A genetic cross involving one pair of contrasting characters.

Dihybrid cross: A genetic cross involving two pairs of contrasting characters simultaneously.

Test cross: Crossing an individual showing the dominant phenotype with a homozygous recessive individual to determine the unknown genotype.

True-breeding (pure line): Organisms that are homozygous and consistently produce offspring with the same phenotype when self-bred.

Mendel’s Laws

Law 1: Law of Dominance

When two organisms with contrasting traits (one dominant, one recessive) are crossed, only the dominant trait appears in the F₁ generation. The recessive trait re-appears in the F₂ generation.

Example: Cross $TT$ (tall) × $tt$ (dwarf)

F₁ generation: All $Tt$ → All tall (dominant T masks recessive t)

F₁ × F₁: $Tt$ × $Tt$

F₂ generation: $TT : Tt : tt = 1 : 2 : 1$ (genotype ratio) Phenotype: Tall : Dwarf = $3 : 1$

This 3:1 phenotype ratio in F₂ of monohybrid cross is a classic exam result.

Law 2: Law of Segregation (Law of Purity of Gametes)

During gamete formation, the two alleles of a gene segregate (separate) from each other so that each gamete carries only one allele for each gene. This is the explanation for the 3:1 ratio.

Each gamete is pure — it carries either $T$ or $t$ , never $Tt$ .

Molecular basis: alleles on homologous chromosomes separate during Meiosis I (anaphase I).

Law 3: Law of Independent Assortment

During gamete formation, genes for different traits assort (distribute) independently of each other — as long as the genes are on different chromosomes (non-homologous chromosomes).

This is the basis of the dihybrid cross and its 9:3:3:1 ratio.

Example: Round, Yellow ( $RRYY$ ) × Wrinkled, Green ( $rryy$ )

F₁: All $RrYy$ → Round, Yellow (both dominants expressed)

F₁ × F₁: $RrYy$ × $RrYy$

F₂ phenotype ratio: 9 (Round Yellow) : 3 (Round Green) : 3 (Wrinkled Yellow) : 1 (Wrinkled Green)

The 9:3:3:1 ratio is the product of two independent 3:1 ratios: (3 Round : 1 Wrinkled) × (3 Yellow : 1 Green) = 9:3:3:1. If you know this, you can derive the dihybrid ratio without drawing a 16-box Punnett square every time.

Punnett Square Method

A Punnett square systematically shows all possible gamete combinations and predicts offspring ratios.

Monohybrid Cross Example

Cross: $Tt \times Tt$

Gametes: $T, t$ from each parent

	T	t
T	TT	Tt
t	Tt	tt

Genotype ratio: $TT : Tt : tt = 1 : 2 : 1$ Phenotype ratio: Tall : Dwarf = $3 : 1$

Dihybrid Cross Example

Cross: $RrYy \times RrYy$

Gametes: $RY, Ry, rY, ry$ from each parent (4 types)

The full 4×4 Punnett square gives 16 combinations:

9 R_Y_ (Round Yellow)
3 R_yy (Round Green)
3 rrY_ (Wrinkled Yellow)
1 rryy (Wrinkled Green)

Ratio: 9:3:3:1

The Test Cross

A test cross reveals whether a dominant-phenotype individual is homozygous ( $TT$ ) or heterozygous ( $Tt$ ).

Cross the individual with a homozygous recessive ( $tt$ ):

If offspring are all tall: parent was $TT$ (all offspring $Tt$ ) If offspring are half tall, half dwarf: parent was $Tt$ (offspring $Tt$ and $tt$ in 1:1 ratio)

The 1:1 ratio in test cross offspring is characteristic of a heterozygous parent.

Monohybrid F₂: Phenotype 3:1 | Genotype 1:2:1

Dihybrid F₂: Phenotype 9:3:3:1 | Genotype 1:2:1:2:4:2:1:2:1

Test cross (heterozygous × homozygous recessive): 1:1

Back cross (F₁ × either parent): depends on parents

Number of gamete types from $n$ heterozygous gene pairs: $2^n$

Number of genotypes in F₂ from n genes: $3^n$

Number of phenotypes in F₂ from n dominant genes: $2^n$

Deviations from Mendelian Ratios

Mendel’s ratios are modified by several phenomena:

Incomplete Dominance

The heterozygote shows an intermediate phenotype — neither allele is fully dominant.

Example: Antirrhinum (snapdragon) flower color

$RR$ = Red, $R'R'$ = White, $RR'$ = Pink

F₁: $RR'$ = Pink (intermediate) F₂: $RR : RR' : R'R'$ = 1 Red : 2 Pink : 1 White → 1:2:1 phenotype ratio (same as genotype ratio!)

Codominance

Both alleles are fully expressed simultaneously in the heterozygote.

Example: ABO blood groups (partially) — $I^A I^B$ individuals have both A and B antigens → Blood group AB

Another example: Spotted cattle — $BB'$ has both black and white patches.

Multiple Allelism

More than two alleles exist for a gene in the population (though any individual still only has two alleles).

Classic example: ABO blood groups — three alleles: $I^A$ , $I^B$ , $I^O$

$I^A I^A$ or $I^A I^O$ → Blood group A
$I^B I^B$ or $I^B I^O$ → Blood group B
$I^A I^B$ → Blood group AB (codominance)
$I^O I^O$ → Blood group O

NEET PYQ Pattern: The ABO blood group system appears almost every year. Questions ask about possible blood groups of offspring, determining parental blood groups from offspring, and the genetics of transfusion compatibility. Know that O is recessive ( $I^O I^O$ only), and AB individuals have both A and B antigens and are universal recipients. Blood group genetics questions require applying Punnett squares with three alleles.

Solved Examples

Example 1 (Easy — CBSE Class 10)

Q: In a cross between tall ( $Tt$ ) pea plant and dwarf ( $tt$ ) pea plant, what proportion of offspring will be tall?

Solution: Cross: $Tt \times tt$

Gametes: $T, t$ (from tall plant) × $t$ (from dwarf plant)

Offspring: $Tt, Tt, tt, tt$ → 2 Tall : 2 Dwarf = 1:1 ratio

Half the offspring (50%) will be tall.

Example 2 (Medium — CBSE Class 12/NEET)

Q: In a dihybrid cross between two plants both heterozygous for seed color (Y = yellow, y = green) and seed shape (R = round, r = wrinkled), how many offspring out of 160 would be expected to show round yellow seeds?

Solution: Cross: $RrYy \times RrYy$

From dihybrid cross, Round Yellow frequency = 9/16

Expected number = $(9/16) \times 160 = 90$ offspring

Example 3 (Hard — NEET)

Q: A man with blood group A and a woman with blood group B have a child with blood group O. What are the genotypes of the parents?

Solution: Child with blood group O must be $I^O I^O$ — received one $I^O$ from father and one $I^O$ from mother.

Father (blood group A) must be $I^A I^O$ (to give $I^O$ to child) Mother (blood group B) must be $I^B I^O$ (to give $I^O$ to child)

Possible offspring from $I^A I^O \times I^B I^O$ : $I^A I^B$ (AB), $I^A I^O$ (A), $I^B I^O$ (B), $I^O I^O$ (O) — all four blood groups possible!

Exam-Specific Tips

NEET Weightage: Principles of Inheritance and Variation (Chapter 5, Class 12 Biology) contributes 4-6 questions per year. This is the highest-weightage genetics chapter. Most tested: (1) working out dihybrid ratios, (2) blood group inheritance, (3) incomplete dominance vs codominance distinctions, (4) test cross outcomes. Drawing a correct Punnett square earns marks even if the final ratio is slightly off.

For CBSE Board Exams: 5-mark questions often say “With the help of a suitable cross, explain the Law of Segregation.” This means: write the cross (P × P), show F₁, cross F₁ × F₁, show F₂ with a Punnett square, state the genotype and phenotype ratios, and end with a sentence stating the law.

A productive practice pattern: Don’t just write crosses — also explain in words what each generation shows and why. This is what the marking scheme rewards.

Common Mistakes to Avoid

Mistake 1: Drawing a 2×2 Punnett square for a dihybrid cross. Dihybrid crosses require a 4×4 Punnett square (16 boxes) because each parent produces 4 types of gametes ( $RY, Ry, rY, ry$ ). Using a 2×2 gives completely wrong ratios.

Mistake 2: Confusing incomplete dominance and codominance. In incomplete dominance, the heterozygote is an intermediate (blend) between the two phenotypes — neither allele “wins.” In codominance, both alleles are fully and simultaneously expressed — both are visible, not blended. Pink flower = incomplete dominance (blend). $I^A I^B$ = codominance (both A and B antigens present, not a “blend” antigen).

Mistake 3: Saying “tall is dominant over short” and stopping there. State it precisely: the allele $T$ (tall) is dominant over allele $t$ (dwarf). It’s alleles, not traits, that are dominant or recessive. Phenotypic terms like “tall” and “short” describe the visible expression; the dominance relationship is between the alleles.

Mistake 4: Expecting the Law of Independent Assortment to apply when genes are on the same chromosome (linked genes). Mendel’s third law holds only for genes on different chromosomes (or far apart on the same chromosome). Linked genes do not assort independently — they tend to be inherited together, producing ratios that deviate from 9:3:3:1.

Mistake 5: Confusing genotype ratios and phenotype ratios. In a monohybrid F₂ cross: genotype ratio is $1 TT : 2 Tt : 1 tt$ (1:2:1), but phenotype ratio is $3$ tall $: 1$ dwarf (3:1) because $TT$ and $Tt$ look the same. Similarly, in incomplete dominance F₂, the genotype ratio equals the phenotype ratio (both 1:2:1) because each genotype shows a different phenotype.

Practice Questions

Q1: A black guinea pig ( $Bb$ ) is crossed with a white guinea pig ( $bb$ ). What proportion of offspring will be white?

Cross: $Bb \times bb$

Gametes: $B, b$ × $b$

Offspring: $Bb$ (black), $Bb$ (black), $bb$ (white), $bb$ (white) → 1:1 ratio

50% of offspring will be white ( $bb$ ).

Note: This is a test cross — crossing a heterozygous individual with a homozygous recessive to determine genotype.

Q2: Two plants both heterozygous for two genes ( $AaBb$ ) are crossed. What fraction of the offspring will show the recessive phenotype for both traits?

From the dihybrid ratio (9:3:3:1), the class “recessive for both traits” ( $aabb$ ) represents 1/16 of the offspring.

Alternatively: $P(aa) = 1/4$ and $P(bb) = 1/4$ (from monohybrid ratios), and since genes assort independently:

$P(aabb) = 1/4 \times 1/4 = 1/16$

Q3: A woman with blood group O (genotype $I^O I^O$ ) and a man with blood group AB (genotype $I^A I^B$ ) have children. What blood groups are possible in the offspring?

Cross: $I^O I^O \times I^A I^B$

Possible offspring genotypes: $I^A I^O$ and $I^B I^O$

Possible blood groups: A ( $I^A I^O$ ) and B ( $I^B I^O$ )

Blood groups O and AB are NOT possible from this cross. This is a classic NEET-type question.

FAQs

Q: Why did Mendel choose pea plants for his experiments? Pea plants offered several experimental advantages: they naturally self-pollinate (ensuring pure lines), but can be cross-pollinated manually; they have a short generation time (one season); they produce many offspring per cross; and crucially, the 7 traits Mendel chose each happen to be on separate chromosomes, so they all show independent assortment — a remarkable coincidence that made his laws “clean.”

Q: If traits are dominant, why don’t recessive traits disappear over generations? Because recessive alleles persist in heterozygous individuals ( $Tt$ ) who show the dominant phenotype but carry the recessive allele. The recessive allele is only expressed when an individual inherits two recessive alleles ( $tt$ ). As long as heterozygous individuals exist and mate with each other, recessive alleles are maintained in the population — this is Hardy-Weinberg equilibrium.

Q: What is the difference between a monohybrid cross and a test cross? A monohybrid cross involves crossing two heterozygous individuals ( $Tt \times Tt$ ) — both parents are heterozygous. A test cross involves crossing an individual with an unknown or dominant genotype with a homozygous recessive ( $tt$ ) to determine the unknown genotype. The 1:1 offspring ratio confirms the parent is heterozygous; an all-dominant offspring ratio confirms homozygous dominant.