The origin of diploidy lies in sexual reproduction where two chromosomes (one from each parent) come together in the egg nucleus during fertilization. One chromosome is from the maternal parent the other from the paternal parent. In the description of the alleles, different letter styles were used to depict different allelic expressions. Some allelic expressions can overpower other allelic expressions so when present in a heterozygous condition, their effects will be seen while the other allele's effects will go unnoticed. Such alleles are said to be DOMINANT while the unexpressed allele is said to be RECESSIVE. Note that not all genes express themselves in such a simple manner. Many genes interact with each other to produce effects unlike those of the homozygous genes. If two genes produce a product intermediate between their individual expressions (such as the pink snapdragons produced when the parents are red and white), the genes are said to be co-dominant or to express incomplete dominance. Dominant genes are written in capital letters, recessive genes in lower case letters. Flip to the next card for more details concerning these relationships.
