Genetics still rely on Mendel's model to predict the likely outcome of genetic crosses. Capital letters refer to dominant alleles, and lowercase letters refer to recessive alleles. Capital and lowercase forms of the same letter must be used to designate the two forms of one gene. The allele for a dominant trait of tallness in pea plants is represented by T, and the allele for the recessive trait of shortness by t. Since there are two alleles for each trait, the genotype of a pea plant that is homozygous dominant for tallness is TT. A pea plant that is homozygous recessive for shortness has the genotype tt. Probability is the likelihood that something will occur.
A cross that provides data about one pair of contrasting traits is called a monohybrid cross. A cross between a pea plant that is true-breeding for tallness and one that is true-breeding for shortness is an example of a monohybrid cross.
Biologists can also predict the probable outcome of a cross by using a diagram called a Punnett square, named for its inventor, Reginald Punnett. A dihybrid cross is a cross that involves two pairs of contrasting traits. Predicting the results of a dihybrid cross is more complicated than predicting the results of a monohybrid cross because you have to consider how the two alleles of each of the two traits from each parent can combine.
In some organisms, an individual displays a trait that is intermediate between the two parents, a phenomenon known as incomplete dominance.
In some cases, two dominant alleles are expressed at the same time, a phenomenon called codominance. Codominance is different from incomplete dominance because both traits are displayed. An example of codominance is the roan coat in horses. A cross between a homozygous red horse and a homozygous white...