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

Passage of Information from Parents to Offspring

Learning Objective

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

Key Concepts

• Gametes are haploid cells (n) that contain one set of chromosomes.
• Fertilisation restores the diploid number (2n) by the union of a sperm and an ovum.
• The combination of chromosomes from each parent is random, leading to genetic variation.
• Two main mechanisms generate this randomness:

  1. Independent assortment of homologous chromosome pairs during meiosis.
  2. Cross‑over (recombination) between homologous chromosomes.

Independent Assortment

During metaphase I of meiosis, each homologous pair aligns independently of the others. The number of possible gamete chromosome combinations can be expressed as:

\$\text{Number of combinations} = 2^{n}\$

where n is the haploid chromosome number. For humans (n = 23):

\$2^{23} \approx 8.4 \times 10^{6} \text{ different gametes per individual}\$

Cross‑Over (Recombination)

During prophase I, homologous chromosomes exchange segments, creating new allele combinations on each chromosome. This further increases genetic diversity beyond that predicted by independent assortment alone.

Random Fusion of Gametes

When a sperm and an ovum fuse, the resulting zygote inherits a random combination of the parental chromosomes. The total number of possible zygotic genotypes is the product of the number of possible sperm and ovum genotypes.

\$\text{Possible zygotes} = (2^{n}){\text{sperm}} \times (2^{n}){\text{ovum}} = 2^{2n}\$

For humans this yields:

\$2^{46} \approx 7.0 \times 10^{13} \text{ possible genotypes}\$

Illustrative Table: Gamete Combinations in a Simple Model

Consider a species with a haploid number of 2 (chromosome pairs A/a and B/b). The possible gametes and resulting zygotes are shown below.

Consequences of Genetic \cdot ariation

• Provides raw material for natural selection.
• Increases population resilience to environmental changes.
• Explains why siblings (except identical twins) are genetically distinct.