Principles of Inheritance and Variation + Molecular Basis of Inheritance Notes and Questions for NEET by PW

Genetics can be challenging to revise because it combines inheritance patterns, molecular processes, numerical problems and exceptions to general rules. From calculating genetic ratios and identifying blood groups to understanding DNA replication, gene expression and chromosomal disorders, each topic requires both conceptual clarity and regular question practice.

PW’s Principles of Inheritance and Variation + Molecular Basis of Inheritance full-chapter video focuses on building concepts and practising a wide range of questions from different topics. Use the notes below to revise the key concepts, then attempt the 20 worked-out questions to check your understanding and prepare for NEET Biology.

Principles of Inheritance and Variation: Important Notes

 Mendelian Inheritance and Genetic Crosses

Mendel's laws explain how hereditary factors pass from parents to offspring.

• Law of Segregation: Alleles separate during gamete formation, so each gamete receives one allele for a character.

• Monohybrid cross: The typical genotypic ratio is 1:2:1, while the phenotypic ratio under complete dominance is 3:1.

• Dihybrid cross: The typical phenotypic ratio is 9:3:3:1 under independent assortment.

• Test cross: An individual with a dominant phenotype and unknown genotype is crossed with a homozygous recessive individual. A heterozygous test cross produces a 1:1 phenotypic ratio.

• Gamete types: For n heterozygous gene pairs, the number of possible gamete types is 2ⁿ.

ABO Blood Group Inheritance

The ABO blood group system demonstrates multiple allelism and codominance.

The Iᴬ and Iᴮ alleles are codominant, while Iᴼ is recessive.

Linkage, Recombination and Inheritance Patterns

• Linkage: Genes on the same chromosome tend to be inherited together.

• Recombination: The formation of new allele combinations can occur through crossing over.

• Gene mapping: Recombination frequency helps estimate distances between genes. One per cent recombination corresponds to approximately one map unit or centimorgan.

• Pleiotropy: One gene affects multiple traits.

• Polygenic inheritance: Multiple genes contribute to one character, as in human skin colour.

• Incomplete dominance: The heterozygote displays an intermediate phenotype.

• Codominance: Both alleles are expressed in the heterozygote.

A recombination frequency of 50% is consistent with independent assortment, although it cannot distinguish independently assorting genes from genes that are very far apart on the same chromosome.

Genetic and Chromosomal Disorders

Non-disjunction, the failure of chromosomes to separate correctly, can result in aneuploidy.

Molecular Basis of Inheritance: Important Notes

 DNA Structure and Chargaff's Rules

DNA consists of two polynucleotide strands with complementary base pairing. Adenine pairs with thymine, and guanine pairs with cytosine.

In double-stranded DNA:

• A = T

• G = C

• Total purines = total pyrimidines

If cytosine makes up 17% of the DNA, guanine is also 17%. The remaining 66% is divided equally between adenine and thymine, giving 33% each.

DNA Packaging and Replication

DNA wraps around a histone octamer to form a nucleosome. The octamer contains two copies each of H2A, H2B, H3 and H4. H1 is a linker histone.

During DNA replication:

• DNA polymerase requires an RNA primer.

• DNA synthesis proceeds in the 5′ to 3′ direction.

• The leading strand is synthesised continuously.

• The lagging strand is synthesised discontinuously.

• Okazaki fragments are joined by DNA ligase through phosphodiester bonds.

Transcription and RNA Processing

Transcription is the synthesis of RNA from a DNA template. A transcription unit includes a promoter, structural gene and terminator.

In eukaryotes, the primary RNA transcript, called hnRNA, undergoes processing before translation:

• Splicing: Introns are removed.

• 5′ capping: A modified guanosine cap is added.

• 3′ tailing: An adenine-rich tail is added.

RNA synthesis proceeds in the 5′ to 3′ direction.

Translation and the Genetic Code

Translation is the synthesis of a protein using the information carried by mRNA.

• AUG: Start codon that codes for methionine.

• UAA, UAG and UGA: Stop codons.

• tRNA: Carries amino acids and contains an anticodon.

• rRNA: Provides catalytic activity for peptide-bond formation.

• Degenerate genetic code: More than one codon can specify the same amino acid.

• Nearly universal genetic code: Most codons have the same meaning across organisms.

Aminoacylation attaches an amino acid to its corresponding tRNA.

Lac Operon and Gene Regulation

The lac operon is an inducible system under negative control.

When lactose is absent, the active repressor binds to the operator and prevents transcription. When lactose is present, its inducer form, allolactose, binds to the repressor, allowing transcription of the structural genes.

The trp operon is a repressible system regulated by its end product, tryptophan.

 DNA Fingerprinting and Genetic Experiments

DNA fingerprinting uses polymorphic repetitive DNA regions, such as VNTRs, for identification.

The main steps are DNA digestion, gel electrophoresis, Southern blotting, hybridisation with a probe and detection.

Important facts to remember:

• BAC: Bacterial Artificial Chromosome, used as a cloning vector.

• EST: Expressed Sequence Tag, representing a sequence expressed as RNA.

• Hershey–Chase experiment: Used radioactive phosphorus to label DNA and radioactive sulfur to label protein, supporting DNA as the genetic material.

Worked-Out Questions on Inheritance and Molecular Biology

1. In a double-stranded DNA molecule, cytosine accounts for 18% of the bases. Calculate the percentages of adenine, thymine and guanine.

Answer: G = 18%. A + T = 100 − 36 = 64%, so A = T = 32%.

2. How many types of gametes can an organism with three heterozygous gene pairs produce?

Answer: Number of types = 2ⁿ = 2³ = 8.

3. A heterozygous tall plant (Tt) is crossed with a dwarf plant (tt). Find the phenotypic ratio.

Answer: Tt × tt produces 50% Tt (tall) and 50% tt (dwarf). Ratio = 1 tall : 1 dwarf.

4. Two heterozygous parents for a monohybrid trait are crossed (Aa × Aa). What is the probability of an offspring being homozygous recessive?

Answer: The genotypic ratio is 1 AA : 2 Aa : 1 aa. Probability of aa = 1/4 or 25%.

5. A dihybrid cross AaBb × AaBb follows independent assortment and complete dominance. What is the expected phenotypic ratio?

Answer: 9:3:3:1.

6. A child has blood group O. What must be true of the child's ABO genotype?

Answer: The genotype must be IᴼIᴼ, so each parent must contribute an Iᴼ allele.

7. What is the maximum possible recombination frequency between two genes?

Answer: 50%. This is consistent with independent assortment or very distant genes on the same chromosome.

8. Which amino acid substitution occurs in sickle-cell anaemia at the sixth position of the beta-globin chain?

Answer: Glutamic acid is replaced by valine.

9. What chromosomal abnormality causes Down syndrome?

Answer: Trisomy 21, meaning there are three copies of chromosome 21.

10. Which histone is not part of the nucleosome core octamer?

Answer: H1, which acts as a linker histone.

11. What percentage of adenine is present in double-stranded DNA containing 24% guanine?

Answer: G = C = 24%. A + T = 52%, so adenine = 26%.

12. In which direction are DNA and RNA strands synthesised?

Answer: Both are synthesised in the 5′ to 3′ direction.

13. Which enzyme joins Okazaki fragments during DNA replication?

Answer: DNA ligase, which joins the fragments through phosphodiester bonds.

14. Which RNA-processing steps convert eukaryotic hnRNA into mature mRNA?

Answer: Splicing, 5′ capping and 3′ tailing.

15. Identify the start codon and the three stop codons.

Answer: Start codon: AUG. Stop codons: UAA, UAG and UGA.

16. What happens to the lac operon when lactose is absent?

Answer: The active repressor binds to the operator and prevents transcription of the structural genes.

17. Which lac operon gene codes for beta-galactosidase, and what does the enzyme do?

Answer: The z gene codes for beta-galactosidase, which breaks lactose into glucose and galactose.

18. Which DNA regions are commonly analysed in DNA fingerprinting?

Answer: VNTRs (Variable Number of Tandem Repeats), which show variation among individuals.

19. In the Hershey–Chase experiment, which radioactive isotopes were used to label DNA and protein?

Answer: Phosphorus-32 labelled DNA, while sulfur-35 labelled protein.

20. What is the difference between pleiotropy and polygenic inheritance?

Answer: Pleiotropy occurs when one gene affects multiple traits. Polygenic inheritance occurs when multiple genes contribute to one trait.

Want to practise more questions from Principles of Inheritance and Variation and Molecular Basis of Inheritance? Watch PW’s Principle of Inheritance & Variation + Molecular Basis of Inheritance FULL CHAPTER video for concept-building and question-answer practice across different topics. Use the notes and worked-out questions for revision, then attempt additional questions to identify concepts that need more practice.

Inheritance and molecular biology become easier to revise when you connect genetic principles with their molecular mechanisms and practise applying them to questions. Revise the key ratios, DNA processes, gene regulation and genetic disorders, then use the 20 worked-out questions to check your understanding. PW’s full-chapter video offers additional concept-building and question-answer practice to support your NEET Biology preparation.

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