Introduction

Inheritance and variation are core topics in NEET Genetics. They explain how traits are transmitted from parents to offspring and how variations contribute to evolution. NEET frequently asks Mendelian crosses, sex-linked inheritance, human disorders, and mutation-based questions.

Understanding inheritance and variation requires mastering Mendel’s laws, Punnett squares, and genetic formulas.

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1. Mendelian Genetics

Gregor Mendel: Father of genetics

• Conducted pea plant experiments

• Proposed laws of inheritance

Mendel’s Laws:

1. Law of Dominance: Dominant allele expresses over recessive allele

2. Law of Segregation: Allele pairs separate during gamete formation

3. Law of Independent Assortment: Alleles of different genes segregate independently

NEET Tip:

• Monohybrid cross → 3:1 ratio in F2

• Dihybrid cross → 9:3:3:1 ratio in F2

2. Monohybrid Cross (Single Trait Inheritance)

Definition:

• Cross involving one trait with two alleles

Example:

• Mendel’s pea plant: Tall (T) × Dwarf (t)

Punnett Square Formula:

• F1: Tt (all tall)

• F2: Tt × Tt → Genotypic ratio = 1 TT : 2 Tt : 1 tt

• Phenotypic ratio = 3 tall : 1 dwarf

NEET Tip:

• Always list genotypic and phenotypic ratios

• Dominant allele represented by capital letter, recessive by small letter

3. Dihybrid Cross (Two Traits Inheritance)

Definition:

• Cross involving two traits with two alleles each

Example:

• Seed shape and color: Round yellow (R Y) × Wrinkled green (r y)

F2 Genotypic Ratio: 1:2:1:2:4:2:1:2:1
F2 Phenotypic Ratio: 9:3:3:1

NEET Tip:

• Use FOIL method to get gamete combinations

4. Co-dominance and Incomplete Dominance

• Incomplete dominance: Heterozygote shows intermediate phenotype

  • Example: Red × White → Pink (Snapdragon)

• Co-dominance: Both alleles expressed equally

  • Example: ABO blood group → A + B → AB

NEET Tip:

• Blood group problems frequently appear in NEET MCQs

5. Sex-linked Inheritance

Definition:

• Traits located on X or Y chromosome

Key Points:

• X-linked recessive → More common in males (XY)

  • Example: Hemophilia, Color blindness

• Y-linked → Only males affected

  • Example: SRY gene → Male sex determination

NEET Formula:

• Male (XY) × Female (XX) → Determine X-linked phenotype

• Pedigree analysis → Use symbols: Circle (female), Square (male), Shaded (affected)

6. Multiple Alleles and Blood Groups

• Multiple alleles: More than 2 alleles for a gene

• Human ABO blood group → IA, IB, i

Genotypes and Phenotypes:

• IAIA or IAi → Blood group A

• IBIB or IBi → Blood group B

• IAIB → Blood group AB

• ii → Blood group O

NEET Tip:

• Always check co-dominance (AB) and dominance hierarchy

7. Mutation and Variation

Mutation:

• Permanent change in DNA sequence

• Types:

  1. Point mutation: Change in single nucleotide → Sickle cell anemia

  2. Frame shift mutation: Addition/deletion of nucleotide → Protein malfunction

  3. Chromosomal mutation: Deletion, duplication, inversion, translocation

Variation:

• Genetic variation: Caused by mutation, recombination, independent assortment

• Environmental variation: Temperature, nutrition, lifestyle

NEET Tip:

• Mutations → Source of evolution and human disorders

8. Human Disorders and Genetic Formulas

NEET Formula:

• Autosomal dominant: One allele enough to express

• Autosomal recessive: Two alleles needed to express

9. NEET Important Formulas / Memory Aids

1. Monohybrid cross: F2 = 3:1 (phenotype), 1:2:1 (genotype)

2. Dihybrid cross: F2 = 9:3:3:1

3. Sex-linked inheritance: Male = XY, Female = XX

4. Blood group formula: IA, IB, i → A, B, AB, O

5. Mutation types → Point, frame shift, chromosomal

Mnemonic for Mendelian Laws:
“Dominant rules, Segregation splits, Assortment independent”

10. NEET Exam Tips

• Always draw Punnett squares for monohybrid, dihybrid, and sex-linked crosses

• Remember ratios, symbols, and allele notations

• Focus on human disorders, blood groups, and mutation types

• Use formulas for chromosome number and gamete combination MCQs

Conclusion

Principles of Inheritance and Variation are high-yield topics in NEET Biology. Mastery of Mendelian laws, monohybrid/dihybrid crosses, sex-linked inheritance, human disorders, and mutation is essential for scoring in MCQs and diagram-based questions.

With this StudentBro.in guide, students can efficiently revise genetics formulas, Mendelian ratios, human variation, and mutation concepts, enhancing speed and accuracy during NEET exams.

Remember: Linking alleles → gametes → zygote → phenotype → variation is key to mastering Inheritance and Variation for NEET.