Genetic shuffling

Two children of the same couple never look quite alike. Three independent mechanisms produce that diversity, and their effects multiply.

First shuffle: interchromosomal

During meiosis I, each pair of homologues sorts independently of the others. For each pair, the paternal copy may go to either daughter cell.

   pair 1 :  P or M        2 possibilities
   pair 2 :  P or M        2 possibilities
   ...
   pair 23:  P or M        2 possibilities

   total :  2²³ = 8,388,608 different combinations

More than eight million genetically distinct gametes, before any other mechanism.

Second shuffle: intrachromosomal

In prophase of meiosis I, homologous chromosomes pair up closely and exchange segments: this is crossing over.

   BEFORE                        AFTER

   ██████  ░░░░░░              ██░░░░  ░░████
   ██████  ░░░░░░              ██░░░░  ░░████
     P        M                RECOMBINED chromosomes
                               carrying alleles from BOTH parents

A transmitted chromosome is therefore no longer "the father's" or "the mother's": it is a mosaic of both. In humans there are on average one to three crossovers per chromosome pair, which makes the number of combinations practically infinite.

Third shuffle: fertilisation

The meeting of a sperm cell and an egg is random:

8,388,608 possible gametes (father)
        ×
8,388,608 possible gametes (mother)
        =
about 70 trillion combinations

And this still ignores crossing over. That is why two siblings — barring identical twins from a single fertilised egg — are never genetically identical.

Why sexual reproduction exists

It is costly: a partner must be found, and each parent passes on only half their genes. Against asexual reproduction, far more economical, it has persisted only for one decisive advantage: diversity.

population of CLONES                DIVERSE population
--------------------------------    --------------------------------
all vulnerable to the same parasite some resist, the population survives
no adaptation possible              natural selection has material to sort

This is the Red Queen hypothesis: in an environment where parasites and hosts continually evolve against each other, you must keep changing just to stay in place. A genetically uniform population is always caught in the end.

The raw material of evolution

These shuffles recombine existing alleles — they create none. The source of novelty remains mutation, an accidental change in the DNA sequence.

MUTATION    : creates new alleles              (rare)
MEIOSIS     : recombines existing alleles      (every generation)
SELECTION   : sorts the combinations

Together, these three mechanisms give evolution its raw material: rare novelty, massive shuffling, and sorting by the environment.

Summary

• Interchromosomal shuffling: independent sorting of the pairs → 2²³ combinations.
• Intrachromosomal shuffling: crossing over recombines the chromosomes themselves.
• Random fertilisation: about 70 trillion possible combinations.
• These mechanisms recombine alleles; only mutation creates new ones.
• Diversity protects a population from parasites and gives selection material to sort.