Monohybrid Crosses

Monohybrid crosses deal with a single trait. When studying monohybrid crosses it is useful to make a Punnett square. In a Punnett square, the genotype of one parent is written across the top while the genotype of the other parent is written vertically to the left. The cross is made and the possible genotypes of the offspring show up in the four boxes below.

Example:
In humans, weirdness (W) is dominant over normal (w). A man who is heterozygous for weirdness marries a woman who is heterozygous for weirdness. What are the possible genotypes of their offspring?

Image by Chernobyl Frog Dissections

Sometimes you are information about the offspring and are asked to obtain information about the parents.

Example:
In pea plants, yellow seeds (Y) are dominant and green seeds (y) are recessive. A pea plant with yellow seeds is crossed with a pea plant with green seeds. The resulting offspring have about equal numbers of yellow and green seeded plants. What are the genotypes of the parents?

Image by Chernobyl Frog Dissections