Mendel's Laws of Inheritance — Revision Notes

⚡ 30-Second Revision

Law of Dominance — One allele (dominant) masks the other (recessive) in heterozygotes.
Law of Segregation — Alleles for a trait separate during gamete formation; each gamete gets one allele.
Law of Independent Assortment — Alleles for different traits assort independently during gamete formation.
Monohybrid Cross F2 Phenotypic Ratio — $3:1$
Monohybrid Cross F2 Genotypic Ratio — $1:2:1$
Dihybrid Cross F2 Phenotypic Ratio — $9:3:3:1$
Test Cross — Cross with homozygous recessive to determine unknown dominant genotype.
Number of Gametes — $2^n$ (where n = number of heterozygous pairs).

2-Minute Revision

Mendel's Laws are the foundation of genetics. The Law of Dominance explains that when two contrasting alleles are present, only the dominant one expresses itself in the phenotype (e.g., tallness over dwarfness).

The Law of Segregation is crucial: during gamete formation, the two alleles for a single trait separate, so each gamete receives only one allele, ensuring gamete purity. This explains the reappearance of recessive traits in the F2 generation of a monohybrid cross, yielding a 3:1 phenotypic ratio.

The Law of Independent Assortment applies to two or more traits, stating that alleles for different traits segregate independently of each other. This leads to new combinations of traits and a 9:3:3:1 phenotypic ratio in a dihybrid F2 cross.

Remember key terms like gene, allele, homozygous, heterozygous, phenotype, and genotype. A test cross (crossing an unknown dominant individual with a homozygous recessive) helps determine the unknown genotype, yielding a 1:1 ratio if the unknown is heterozygous.

While fundamental, be aware of exceptions like incomplete dominance (intermediate phenotype) and co-dominance (both alleles expressed fully).

5-Minute Revision

To master Mendel's Laws for NEET, focus on the core principles and their applications. Start with the Law of Dominance: in a heterozygous individual, the dominant allele's trait is expressed, masking the recessive one.

For example, in pea plants, Tall (T) is dominant over dwarf (t), so Tt plants are tall. Next, the Law of Segregation is paramount: during meiosis, the two alleles for each gene separate, so each gamete receives only one allele.

This 'purity of gametes' ensures that when F1 hybrids (e.g., Tt) self-pollinate, the F2 generation shows both dominant and recessive traits in a predictable 3:1 phenotypic ratio (1:2:1 genotypic ratio).

Finally, the Law of Independent Assortment applies when considering two or more traits. It states that the alleles for different genes (located on different chromosomes or far apart on the same chromosome) segregate independently.

This means, for instance, that seed color inheritance doesn't affect seed shape inheritance. A dihybrid cross (e.g., RrYy x RrYy) demonstrates this, producing a 9:3:3:1 phenotypic ratio in the F2 generation, showing new combinations of traits.

Key Tools & Concepts:

Punnett Square — Essential for predicting offspring genotypes and phenotypes. Practice drawing them for both monohybrid and dihybrid crosses.
Test Cross — Used to determine the genotype of an individual showing a dominant phenotype. Cross the unknown individual with a homozygous recessive. A 1:1 ratio in offspring indicates the unknown parent was heterozygous.
Calculating Gametes — For a given genotype, the number of unique gametes is $2^n$, where 'n' is the number of heterozygous gene pairs.
Exceptions — While fundamental, be prepared for questions on non-Mendelian inheritance. Incomplete dominance (e.g., pink flowers from red x white) results in an intermediate F1 phenotype and a 1:2:1 phenotypic ratio in F2. Co-dominance (e.g., ABO blood groups) shows full expression of both alleles in the heterozygote. Understanding these modifications is crucial for advanced problems.

Prelims Revision Notes

Mendel's Laws of Inheritance - NEET Quick Recall

1. Law of Dominance:

Principle: — In a pair of contrasting traits, one (dominant) masks the other (recessive) in heterozygotes.
Example: — Tall (T) is dominant over dwarf (t); F1 (Tt) are all tall.
Key: — Recessive trait only expressed in homozygous state (tt).

2. Law of Segregation (Purity of Gametes):

Principle: — Alleles for a trait separate during gamete formation; each gamete receives only one allele.
Universality: — Most fundamental and universally applicable law.
Monohybrid Cross (F1 self-cross, Tt x Tt):

* F2 Genotypic Ratio: $1:2:1$ (TT:Tt:tt) * F2 Phenotypic Ratio: $3:1$ (Tall:Dwarf)

3. Law of Independent Assortment:

Principle: — Alleles for different traits assort independently during gamete formation.
Condition: — Genes must be on different chromosomes or far apart on the same chromosome.
Dihybrid Cross (F1 self-cross, RrYy x RrYy):

* F2 Phenotypic Ratio: $9:3:3:1$ (e.g., Round Yellow : Round Green : Wrinkled Yellow : Wrinkled Green)

Key Terminology:

Gene: — Unit of heredity.
Allele: — Alternative form of a gene.
Homozygous: — Identical alleles (TT, tt).
Heterozygous: — Different alleles (Tt).
Phenotype: — Observable trait.
Genotype: — Genetic makeup.

Important Tools & Concepts:

Punnett Square: — Grid for predicting offspring genotypes/phenotypes.
Test Cross: — Cross unknown dominant phenotype with homozygous recessive (e.g., T_ x tt) to determine unknown genotype.

* If all offspring dominant: Unknown is homozygous dominant (TT). * If 1:1 dominant:recessive offspring: Unknown is heterozygous (Tt).

Number of Gametes: — For genotype with 'n' heterozygous pairs, number of gamete types = $2^n$.

Exceptions/Extensions to Mendelian Ratios:

Incomplete Dominance: — F1 shows intermediate phenotype (e.g., Red x White $\rightarrow$ Pink). F2 phenotypic ratio $1:2:1$.
Co-dominance: — Both alleles express fully (e.g., ABO blood groups, Roan cattle). F2 phenotypic ratio $1:2:1$.
Gene Linkage: — Genes on same chromosome, close together, don't assort independently.

Problem Solving Tip: Break down dihybrid problems into two monohybrid problems and use the product rule for probabilities.

Vyyuha Quick Recall

Some Dominant Individuals Assort Segregated Gametes.

Segregation
Dominance
Independent Assortment
Segregated Gametes (Purity of Gametes - another name for Law of Segregation)