explain that the random fusion of gametes at fertilisation produces genetically different individuals

Passage of Information from Parents to Offspring

Learning Objective

Explain how the random fusion of gametes at fertilisation produces genetically different individuals, and describe the related genetic mechanisms required by the Cambridge IGCSE/A‑Level Biology syllabus.

Key Genetic Terminology (AO1)

• Allele – alternative form of a gene.
• Genotype – genetic constitution of an individual (e.g. AA, Aa, aa).
• Phenotype – observable traits produced by a genotype.
• Homozygous – two identical alleles at a locus (e.g. AA or aa).
• Heterozygous – two different alleles at a locus (e.g. Aa).
• Dominant / Recessive – relationship where the dominant allele masks the recessive allele in a heterozygote.
• Sex‑linked – genes located on the sex chromosomes (usually the X chromosome).
• Linkage – genes close together on the same chromosome tend to be inherited together.
• Recombination frequency – proportion of recombinant offspring; used to map gene distance (1 % = 1 cM).
• Haploid (n) – a cell with one set of chromosomes.
• Diploid (2n) – a cell with two sets of chromosomes (one from each parent).
• Mutation – a change in DNA sequence (point, insertion, deletion, frameshift, or chromosomal).
• Non‑disjunction – failure of chromosome pairs to separate, leading to aneuploidy (e.g., Down syndrome).

Overview of Meiosis – Formation of Haploid Gametes

Meiosis reduces chromosome number from diploid (2n) to haploid (n) and creates genetic variation.

1. Meiosis I – reductional division

   • Prophase I – homologous chromosomes pair (synapsis) and undergo cross‑over (recombination).
   • Metaphase I – each homologous pair aligns independently of the others (independent assortment).
   • Anaphase I – homologues separate to opposite poles.
   • Telophase I & Cytokinesis – two haploid cells are formed, each still containing sister chromatids.

2. Meiosis II – equational division

   • Similar to mitosis: sister chromatids separate, giving four genetically distinct haploid gametes.

Mechanisms Generating Genetic Variation

1. Independent Assortment

During metaphase I each homologous pair lines up independently of the other pairs. The number of possible chromosome combinations in a gamete is:

Number of combinations = 2ⁿ where n = haploid chromosome number.

Example – human gametes (n = 23): 2²³ ≈ 8.4 × 10⁶ different sperm or ovum.

2. Crossing‑Over (Recombination)

Non‑sister chromatids exchange homologous segments in prophase I, producing new allele combinations on each chromosome. This adds variation beyond that predicted by independent assortment alone.

3. Random Fertilisation

The fusion of any one sperm with any one ovum is random, so the zygote receives a unique combination of the two gamete genotypes.

Possible zygotes = (2ⁿ)_sperm × (2ⁿ)_ovum = 2²ⁿ

For humans (2n = 46): 2⁴⁶ ≈ 7.0 × 10¹³ possible genotypes.

4. Mendelian Genetics – Monohybrid & Di‑hybrid Crosses

Classic Mendel experiments illustrate how alleles segregate and assort independently.

Monohybrid cross (Aa × Aa)

Phenotypic ratio 3 : 1 (dominant : recessive).

Di‑hybrid cross (AaBb × AaBb)

Phenotypic ratio 9 : 3 : 3 : 1, demonstrating independent assortment of two loci.

5. Sex‑Linked Inheritance

Traits whose genes are on the X chromosome show characteristic patterns because males (XY) have only one X.

Example – Red‑green colour blindness (X^c recessive)

Result: 50 % of sons are colour‑blind, 50 % of daughters are carriers.

6. Linkage & Recombination Frequency

Genes that are close together on the same chromosome tend to be inherited together. The proportion of recombinant offspring (R) gives the distance between genes:

Recombination frequency (%) = (Number of recombinant offspring ÷ Total offspring) × 100

1 % recombination ≈ 1 centiMorgan (cM). Example:

• Cross a plant heterozygous for two linked genes (A and B). Out of 1000 progeny, 120 are recombinant (Ab or aB).
• Recombination frequency = (120 ÷ 1000) × 100 = 12 % → genes are 12 cM apart.

7. Types of Mutation & Their Consequences

Mutations provide new alleles for evolution, but many are deleterious; a few can be advantageous.

8. Non‑Disjunction & Aneuploidy

Failure of homologues (Meiosis I) or sister chromatids (Meiosis II) to separate produces gametes with n ± 1 chromosomes.

• Trisomy 21 (Down syndrome) – an extra chromosome 21 (47 chromosomes total).
• Turner syndrome (XO) – missing one X chromosome in females (45 chromosomes).

These conditions illustrate how errors in meiosis affect genotype and phenotype.

Consequences of Genetic Variation

• Supplies raw material for natural selection and evolution.
• Increases a population’s capacity to adapt to changing environments.
• Explains why siblings (except identical twins) are genetically distinct.
• Forms the basis for inheritance patterns studied in genetics (Mendelian ratios, linkage, sex‑linked traits, mutation).