Recombination and Linkage

Each human somatic cell contains two of each type of chromosome.  One chromosome of each of the 23 pairs came from the mother and the other from the father.  When gametes are produced (by meiosis), the paired homologous chromosomes separate so that each gamete contains only one of the pair of alleles for each trait.

Homologous chromosomes separating in the production of sex cells

Which chromosome from each of the 23 homologous pairs of both parents is inherited is a matter of chance.  There are 8,324,608 possible combinations of 23 chromosome pairs.  As a result, two gametes virtually never have exactly the same combination of chromosomes.  Each chromosome contains dozens to thousands of different genes.  The total possible combination of alleles for those genes in humans is approximately 70,368,744,177,664.  This is trillions of times more combinations than the number of people who have ever lived.  This accounts for the fact that nearly everyone, except monozygotic twins, is genetically unique.

While homologous pairs of chromosomes are independently assorted in meiosis, the genes that they contain are also independently assorted only if they are part of different chromosomes.  Genes in the same chromosome are passed on together as a unit.  Such genes are said to be linked.  For example, the "A" and "B" alleles (in the illustration below) will both be passed on together if the lower chromosome is inherited.  "A" and "B" are linked due to their occurrence in the same chromosome.  Similarly, "a" and "b" are linked in the other chromosome.

Genetic linkage continues as homologous chromosomes separate in the formation of sex cells

Linked genes most likely account for such phenomena as red hair being strongly associated with light complexioned skin among humans.  If you inherit one of these traits, you will most likely inherit the other.

Genetic linkage of this sort can be naturally ended.  During the first division of meiosis, sections near the ends of chromosomes commonly intertwine and exchange parts of their chromatids with the other chromosome of their homologous pair.  This process of sections breaking and reconnecting onto a different chromosome is called crossing-over.  In the example shown below, "A" and "B" are unlinked by this process.

Crossing-over unlinks alleles of genes as homologous chromosomes separate in the formation of sex cells

Crossing-over usually results in a partial recombination, or creation of combinations of alleles in chromosomes not present in either parent.  For instance, the linkage between red hair and light complexion can be broken if the chromosome breakage occurs between the genes for these traits.  The further apart the genes are from each other in a chromosome, the greater the likelihood that they will be unlinked as a result of crossing-over.  Likewise, genes located closer to the ends, rather than the middle, of a chromosome are more likely to be recombined during meiosis.  Subsequently, they are more likely to vary from generation to generation.  As a consequence, it is probable that they provide more new genetic combinations that can affect the outcome of natural selection and the evolution of a population. 

Crossing-over does not produce new alleles.  Rather, it only exchanges existing alleles between homologous chromosomes.

Why Sex?

From an evolutionary perspective, the most important consequence of meiosis and crossing-over is the rearrangement of genetic information.  It constantly assures that each generation has significantly new genetic combinations from which nature can select for winners and losers in the competition for survival.  The more genetic variation existing in a population, the greater the chance it will survive when there are stressful changes in the environment.  In other words, there will more likely be some individuals who will have a genetic combination that will allow them to survive changes such as major climate shifts or new predators and diseases.  Those survivors will be the parents of future generations.  This is very likely the reason that sexual reproduction was so successful in the history of evolution on earth.  In contrast, organisms that reproduce asexually do not have the advantage of extensively new genetic combinations each generation.  They must rely on periodic mutations to provide their variation.  Subsequently, they usually are less responsive to rapid changes in their environments.