A monohybrid cross is one of the most easily explained examples of genetic inheritance. It involves one trait that is coded for by two different alleles. When you breed two individuals, each contributing one of their two alleles to the possible offspring, the result is a monohybrid cross. As you add genes or alleles, the patterns of inheritance become more complex, so the monohybrid cross is a good place to start understanding genetics.
The Punnett Square as a Tool
The Punnett square is a visual way to conceptualize inhertiance patterns. It was invented by Reginald Punnett at the turn of the twentieth century. The Punnett square for a monohybrid cross is a box divided into four squares. The alleles are symbolized by letters, generally upper case and lower case. The genotypes of the parents are written along the outside of the square and the possible offspring are marked on the inside of the square. The Punnett square can be expanded to map more complicated inheritance as well.
Determine the Genotypes of the Parents
It can be difficult to determine the genotype of the parents from the phenotype, or how the individual expresses the trait. Recessive alleles can be masked or switched off by dominant alleles. The genotype can be determined by a genetic test or by examining the lineage of the individual. Once you know the genotypes of each parent, you write the genotype of the first parent along the top of the square, with one allele over each column. You then write the genotype of the second parent along the left side of the square, with one allele for each row.
Complete the Punnett Square
Each parent will donate one gamete, carrying one allele of the gene in question, to their offspring. The alleles written along the outside of the Punnett square represent this fact. To complete the square, start with the first parent, writing the allele at the head of the column in each box of that column. Repeat the process for the second parent, filling in the alleles along the rows. In the end you will have four squares with two alleles written into each, which represent the possible offspring of this cross.
Determine the Genotypes and Phenotypes of the Possible Offspring
There is a 25 percent probability that the offspring will have the genotype in any one box of the Punnett square. If two squares have matching genotypes, there is a 50 percent probability that the offspring will have that genotype. Once you determine how many genotypes are possible and the probability of each, you need to determine the possible phenotypes. The trait that is expressed for each genotype depends on the dominance pattern, for example, dominant alleles mask recessive alleles, so an individual with a dominant and recessive allele will express the same trait as one with two dominant alleles.