Discrete units called genes control the appearance of inherited traits.
Genes come in alternative forms called alleles that are responsible for the expression of different forms of a trait.
Body cells of sexually reproducing organisms carry two copies of each gene. When the two copies of a gene are the same allele, the individual is homozygous for that gene. When the two copies of a gene are different alleles, the individual is heterozygous for that gene.
The genotype is a description of the allelic combination of the two copies of a gene present in an individual. The phenotype is the observable form of the trait that the individual expresses.
A cross between two parental lines (P) that are pure-breeding for alternative alleles of a gene will produce a first filial (F1) generation of hybrids that are heterozygous. The phenotype expressed by these hybrids is determined by the dominant allele of the pair, and this phenotype is the same as that expressed by individuals homozygous for the dominant allele. The phenotype associated with the recessive allele will reappear only in the F2 generation in individuals homozygous for this allele. In crosses between F1 heterozygotes, the dominant and recessive phenotypes will appear in the F2 generation in a ratio of 3:1.
The two copies of each gene segregate during the formation of gametes. As a result, each egg and each sperm or pollen grain contains only one copy, and thus, only one allele, of each gene. Male and female gametes unite at random at fertilization. Mendel described this process as the law of segregation.
The segregation of alleles of any one gene is independent of the segregation of the alleles of other genes. Mendel described this process as the law of independent assortment. According to this law, crosses between Aa Bb F1 dihybrids will generate F2 progeny with a phenotypic ratio of 9(A- B-):3(A- bb):3(aa B-):1(aa bb)
