explain the meanings of the terms haploid (n) and diploid (2n)

Passage of Information from Parents to Offspring

Learning Objectives (AO1)

• Define haploid (n) and diploid (2n) and explain their significance in sexual reproduction.
• Describe the flow of genetic information: DNA → RNA → protein.
• Apply Mendelian principles (monohybrid, dihybrid, test‑crosses) and explain how meiosis generates genetic variation.
• Interpret pedigrees, recognise sex‑linked inheritance and understand genetic linkage.
• Explain how genetic variation provides material for natural selection and evolution.

Key Points – Mapping to the Cambridge Syllabus

Syllabus Topic	What you must be able to do (AO1)
6.1 – Structure of nucleotides & DNA	Describe the structure of a nucleotide and the double‑helix model of DNA.
6.2 – DNA replication	Explain semi‑conservative replication and name the main enzymes (DNA polymerase, helicase, ligase).
6.3 – Gene expression	Outline transcription and translation, naming RNA polymerase and the role of ribosomes.
17.1 – Mendelian inheritance	Predict genotype and phenotype ratios for monohybrid and dihybrid crosses; use test‑crosses.
17.2 – Sex‑linked traits & pedigrees	Analyse X‑linked inheritance patterns and construct simple pedigrees.
17.3 – Linkage & recombination	Explain how linked genes affect ratios and calculate map distance.
17.4 – Polygenic & quantitative traits	Describe traits controlled by many genes and how they appear in pedigrees.
18 – Natural selection & evolution	Discuss how variation generated by meiosis and mutation provides raw material for selection.

1. Chromosome Numbers and Ploidy

Cell / Organism	Chromosome Number	Notation	Ploidy
Human somatic cell (e.g., skin fibroblast)	46	2n	Diploid
Human sperm (male gamete)	23	n	Haploid
Human egg (female gamete)	23	n	Haploid
Human zygote (fertilised egg)	46	2n	Diploid

Haploid (n): a cell that contains one complete set of chromosomes (e.g., human gametes, n = 23).

Diploid (2n): a cell that contains two complete sets of chromosomes, one inherited from each parent (e.g., human somatic cells, 2n = 46).

2. Meiosis – Production of Haploid Gametes

Meiosis reduces the chromosome number from 2n to n and introduces genetic diversity through crossing‑over and independent assortment.

2.1 Overview of the Two Divisions

1. Meiosis I – Reductional Division
   • Prophase I – Homologous chromosomes pair (synapsis) and exchange non‑sister chromatids (crossing‑over).
   • Metaphase I – Paired homologues line up on the metaphase plate; orientation is random (independent assortment).
   • Anaphase I – Homologous chromosomes separate to opposite poles.
   • Telophase I & Cytokinesis – Two haploid cells form, each still containing sister chromatids.
2. Meiosis II – Equational Division
   • Resembles mitosis: sister chromatids separate, giving four genetically distinct haploid gametes.

2.2 Key Outcomes of Meiosis

• Halving of chromosome number (2n → n).
• Genetic recombination via crossing‑over.
• Independent assortment of maternal and paternal chromosomes.
• Production of four non‑identical haploid gametes.

2.3 Genetic Linkage & Recombination (Syllabus Topic 17.3)

• Genes that are close together on the same chromosome tend to be inherited together – linked genes.
• Crossing‑over can separate linked genes; the frequency of recombination (cM) reflects the physical distance:
  Map distance (cM) = (Number of recombinant offspring ÷ Total offspring) × 100.
• In a test‑cross (linked heterozygote × recessive homozygote) the observed ratios deviate from the 9:3:3:1 pattern and allow calculation of map distance.

3. Fertilisation – Restoration of Diploidy

Fusion of a haploid sperm (n) with a haploid egg (n) restores the diploid chromosome complement:

2n = n_sperm + n_egg

The resulting zygote contains one set of chromosomes from each parent and will undergo mitotic divisions to form the embryo.

4. Flow of Genetic Information (DNA → RNA → Protein)

4.1 DNA Structure

• Double helix of two antiparallel strands.
• Each strand is a polymer of nucleotides (sugar‑phosphate backbone + nitrogenous base).
• Base‑pairing: A↔T (2 H‑bonds), G↔C (3 H‑bonds).

4.2 DNA Replication (Semi‑conservative)

1. Helicase unwinds the double helix.
2. RNA primase synthesises short RNA primers (often omitted in syllabus – simply “RNA primer”).
3. DNA polymerase adds nucleotides to the 3’ end (continuous synthesis on the leading strand, discontinuous on the lagging strand).
4. DNA ligase joins Okazaki fragments on the lagging strand.

4.3 Transcription (DNA → mRNA)

• RNA polymerase binds to the promoter region of a gene.
• RNA strand is synthesised 5’→3’ using the antisense DNA strand as template.
• In eukaryotes the primary transcript is processed:
  • 5’ capping
  • Splicing to remove introns
  • Poly‑A tail addition

4.4 Translation (mRNA → Polypeptide)

1. Ribosome binds to the start codon (AUG) on mature mRNA.
2. tRNA molecules deliver specific amino acids; the anticodon pairs with the codon.
3. Peptide bonds form as the ribosome moves along the mRNA (elongation).
4. Translation stops at a stop codon (UAA, UAG, UGA); the polypeptide is released and folds into a functional protein.

5. Mendelian Inheritance (Topic 17.1)

5.1 Fundamental Concepts

• Gene – unit of inheritance located on a chromosome.
• Allele – alternative form of a gene.
• Genotype – genetic makeup (e.g., AA, Aa, aa).
• Phenotype – observable trait.
• Dominant allele masks the effect of a recessive allele in a heterozygote.
• Co‑dominance, incomplete dominance and polygenic inheritance extend the simple dominant‑recessive model (see Section 7).

5.2 Monohybrid Cross

Cross between individuals that differ in a single trait (e.g., Rr × Rr).

Parental Genotype	Gametes Produced
Rr	R, r

Resulting Punnett square gives a 3:1 phenotypic ratio (dominant : recessive) and a 1:2:1 genotypic ratio.

5.3 Dihybrid Cross

Two traits are considered simultaneously (e.g., RrYy × RrYy).

Assuming independent assortment, the expected phenotypic ratio is 9 : 3 : 3 : 1.

5.4 Test‑Cross

Cross a homozygous recessive individual (aa) with an individual of unknown genotype.

• If all offspring show the dominant phenotype → unknown parent is homozygous dominant (AA).
• If both phenotypes appear in a 1:1 ratio → unknown parent is heterozygous (Aa).

5.5 Sex‑Linked Inheritance (X‑linked)

• Genes located on the X chromosome (e.g., haemophilia, red‑green colour blindness).
• Male (XY) expresses a recessive X‑linked allele because there is no second X to mask it.
• Typical pedigree pattern: vertical transmission, affected males, carrier females (half‑filled circles).

5.6 Pedigree Analysis (Topic 17.2)

• Symbols: square = male, circle = female, filled = affected, half‑filled = carrier (for recessive X‑linked).
• Steps:
  1. Identify the mode of inheritance (autosomal dominant, autosomal recessive, X‑linked).
  2. Determine the genotype of each individual where possible.
  3. Calculate probabilities for future offspring.

6. Polygenic & Quantitative Traits (Topic 17.4)

• Traits controlled by many genes, each contributing a small effect (e.g., human height, skin colour, milk yield in cattle).
• Phenotypic distribution is continuous and approximates a normal curve.
• In pedigrees polygenic traits appear as a range of phenotypes rather than discrete categories; they are often analysed with statistical methods rather than simple Punnett squares.

7. Genetic Variation and Evolution (Topic 18)

• Sources of variation:
  • Meiotic crossing‑over and independent assortment.
  • Genetic linkage and recombination.
  • Mutations (point, insertion, deletion, chromosomal rearrangements).
• Natural selection acts on phenotypic variation; advantageous genotypes increase in frequency over generations.
• Example – Sickle‑cell allele (Hb^S): heterozygotes (Hb^A/Hb^S) are resistant to malaria, illustrating balanced polymorphism.

8. Abnormal Ploidy and Clinical Relevance

• Triploidy (3n) – usually lethal; embryos fail to develop.
• Down syndrome (Trisomy 21) – extra chromosome 21 (2n + 1); results in characteristic developmental features.
• Turner syndrome (45,X) – monosomy of a sex chromosome; short stature, infertility.
• Klinefelter syndrome (47,XXY) – extra X chromosome in males; reduced fertility, tall stature.

9. Summary

• Haploid (n) cells contain one complete set of chromosomes; diploid (2n) cells contain two sets.
• Meiosis creates haploid gametes and introduces variation through crossing‑over, independent assortment and recombination of linked genes.
• Fertilisation restores diploidy, producing a genetically unique zygote.
• DNA stores genetic information; it is replicated, transcribed into RNA, and translated into proteins.
• Mendelian principles (monohybrid, dihybrid, test‑cross) predict inheritance of single‑gene traits; sex‑linked patterns are traced with pedigrees.
• Linkage modifies expected ratios and allows calculation of map distances; polygenic traits produce continuous variation.
• Variation generated by meiosis and mutation provides the raw material for natural selection and evolution.

Suggested Diagrams for Classroom Use

1. Chromosome complement in haploid vs. diploid cells (human example).
2. Detailed stages of Meiosis I and Meiosis II, highlighting crossing‑over and independent assortment.
3. Linkage map showing recombination frequencies and a test‑cross analysis.
4. Flowchart of DNA replication, transcription and translation (with syllabus‑approved enzyme names).
5. Punnett squares for monohybrid and dihybrid crosses.
6. Pedigree chart illustrating an X‑linked recessive trait.
7. Histogram of a polygenic trait (e.g., human height) showing normal distribution.
8. Illustration of natural selection acting on a population with a sickle‑cell allele.