egg
Together with random fertilization, more possibilities for genetic variation exist between any two people than the number of individuals alive today. Sexual reproduction is the random fertilization of a gamete from the female using a gamete from the male. In humans, over 8 million 2 23 chromosome combinations exist in the production of gametes in both the male and female. Essentially, when the homologous pairs of chromosomes line up during metaphase I and then are separated at anaphase I, there are 2 23 possible combinations of maternal and paternal chromosomes. During random fertilization, a sperm cell, with over 8 million possible chromosome combinations, fertilizes an egg cell, which also has over 8 million possible chromosome combinations. Together, there are over 64 trillion unique combinations, not counting the additional variation produced by crossing-over during prophase I. In other words, each human couple could produce a child with over 64 trillion unique chromosome combinations!.