The yllis only has three pairs of chromosomes but can produce many different possible gamete combinations. Think about humans, who have 23 pairs of chromosomes. How do you think the number of possible gamete combinations of a human compares to that of the yllis?

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

 drawing of 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.

Explanation: