For thousands of years, understanding what really happens in the passing of hereditary traits from a generation to the next had been the focus of curiosity. Because of his carefully conducted breeding experiments with pea plants, Gregor Mendel, who is known as the father of genetics, was the first person to discover the basic principles of heredity. One reason Mendel chose to work with peas is that they are available in many varieties with distinct heritable features, or characters such as seed color. Each alternative form of a character, such as green or yellow seeds, is called a trait. Mendel chose to follow characters that only occur in two distinct, alternative forms for example, his plants had either yellow seeds or green seeds with no colors intermediate between these two varieties. From Mendel's experimental results, it was concluded that alternative versions of genes account for variations in inherited characters. Second, an organism inherits two copies of a gene, one from each parent. (Variable copies of genes are called alleles). Third, if the two alleles are different, then one is the dominant allele and the other is the recessive allele. The dominant allele determines the organism's appearance. The recessive allele has no effect on the organism's appearance. Furthermore, an organism's traits do not always reveal its genetic makeup. Therefore, phenotype, an organism's outer appearance, is distinguishable from genotype, the genetic makeup of an organism. To further clarify, following the character of seed color for example, the gene (P/p) specifies seed color (allele Y, for yellow, is the dominant allele and allele y, for green, is the recessive allele. Also, an organism that has such gene is said to be heterozygote for that gene. Mendel also came up with a fourth and a fifth concepts: the law of segregation and the law of independent assortment. The law of segregation states that alleles for a character segregate during gamete formation and end up in different gametes, and then randomly reform as pairs during gamete fusion at fertilization.