Gregor Mendel's Law of segregation states that an organism must have 2 genetic factors called alleles for each trait. Furthermore, these alleles must separate during gamete formation in meiosis I, and then recombine at fertilization in a new genetic combination in the offspring. There are two types of alleles: dominant and recessive. In gamete formation, there is a possibility of three gene combinations depending of the genotypes (genetic makeup) of the parents. These combinations can consist of DD (homozygous dominant), Dd (heterozygous dominant), and dd (homozygous recessive). Each parent during gamete formation will donate one allele, which will form these examples of unlinked monohybrid genes. However offspring also inherit dihybrid genes, which are genes that contain four alleles that code for more than one trait (ex. TI, ti). Here too there are both dominant and recessive alleles. After Mendel performed dihybrid test crosses, he formulated the Law of Independent Assortment. This law states that factors for one trait are inherited independently of factors for another trait and that all genetic combinations are possible in offspring. However, the Law of Independent Assortment and Segregation only pertain to unlinked genes. This is because while unlinked genes are located on separate chromosomes, linked genes are located on the same chromosome. So for example, the genes that code for red hair and freckles could be located on the same gene, and would therefore not segregate independently as predicted by Mendel's laws. Linked genes can be undone by the process of crossing-over, which produces recombinant gametes. Crossing-over occurs during prophase I of meiosis and is the exchange of alleles between non-sister chromatids of a bivalent. This process has hence unlinked certain genes, therefore reapplying the validity of the Laws of Independent Assortment and Segregation.