Gregor Mendel found the patterns of simple inheritance in 1860 which are the basic principles scientists still use today. He identified the hereditary unit, genes. The principle of segregation states that during the formation of gametes, the two traits carried by each parent separate. This can be shown in the following figure:
Mendel's principle of independent assortment states that during gamete formation, the alleles of a gene for one trait (Tt), segregate independently from the alleles of a gene from another trait (Yy). This principle applies when a cross takes place between two individuals hybrid for two or more traits that do not exist on the same chromosome. However, if two genes are linked on the same chromosome, they will not sort independently. For example, if the gene for tall is linked to the gene for yellow seed color. If the plant is tall it will have yellow seeds, if it is short it will have green seeds.
Many traits are expressed as dominant- recessive traits. If a gene pair consists of two alleles, and one is expressed and the other is not they are dominant and recessive. The expressed allele is dominant, and the unexpressed allele is recessive. If a flowers genotype consists of a dominant allele, it will be expressed. For example, a flower with a homozygous dominant genotype (FF), and another with a heterozygous genotype (Ff), will both express the dominant trait. The gametes from the genotype Ff are F and f. For the recessive trait to be expressed the genotype must be homozygous recessive (ff). The gametes from this genotype are simply f and f. This relationship occurs with all dominant and recessive alleles.
In codominance, both traits are shown. For example, when a homozygous red-flowering snapdragon (FF) is crossed with a...