Monohybrid crosses using genetic diagrams

A monohybrid cross predicts the inheritance of one characteristic. It shows which allele each parent can pass on, then combines the parental alleles to find the possible offspring genotypes.

An allele is a version of a gene. An individual has two alleles for the gene, but each gamete carries only one. The combination of alleles is the genotype; the observable form of the characteristic is the phenotype.

For the crosses below, suppose flower colour is controlled by a dominant allele $$P$$ for purple and a recessive allele $$p$$ for white. A plant with $$PP$$ or $$Pp$$ has purple flowers because one dominant allele is enough to produce the purple phenotype. A plant must have $$pp$$ to have white flowers.

A genotype with two identical alleles, such as $$PP$$ or $$pp$$, is homozygous. A genotype with two different alleles, such as $$Pp$$, is heterozygous.

Punnett square: two heterozygous parents

In a Punnett square, the possible gamete alleles from one parent go across the top and those from the other parent go down the side. Each cell combines one allele from each parent to show a possible offspring genotype.

For a cross between two heterozygous parents, $$Pp \times Pp$$, each parent can pass on either $$P$$ or $$p$$:

The four cells give a predicted genotype ratio of 1 $$PP$$ : 2 $$Pp$$ : 1 $$pp$$. Three cells have at least one dominant allele and therefore predict purple flowers; one has two recessive alleles and predicts white flowers. The predicted phenotypic ratio is 3 purple : 1 white, or 3 dominant : 1 recessive.

Punnett square: heterozygous and homozygous recessive parents

For a cross between a heterozygous parent and a homozygous recessive parent, $$Pp \times pp$$, the first parent can pass on $$P$$ or $$p$$. The recessive parent can pass on only $$p$$.

Two cells show $$Pp$$ and two show $$pp$$. The predicted phenotypic ratio is therefore 1 purple : 1 white, or 1 dominant : 1 recessive.

These ratios are predictions of the relative likelihood of each phenotype. The actual numbers in a particular group of offspring may not match the predicted ratio exactly.

Interpreting inheritance pedigrees

A pedigree diagram shows a characteristic across generations of a family. Each person is represented by a symbol, and lines connect family members. The symbols show which individuals have the characteristic and which do not.

Reading pedigree symbols and relationships

A square usually represents a male and a circle a female. A shaded symbol usually means the person shows the characteristic; an unshaded symbol means they do not. Always check the diagram’s key, as shading conventions may vary.

A horizontal line joining two individuals represents a parental relationship. A line descending from it leads to their offspring; offspring connected along the same horizontal line are siblings. Generations are usually arranged in rows, with older generations above younger ones.

Following a characteristic through generations

To interpret a pedigree, start by identifying the individuals who show the characteristic. Then follow the connecting lines from parents to offspring and compare the generations. Look for whether the characteristic appears in every generation shown or is absent from one generation and appears again later.

For example, if one parent has a shaded symbol and the other has an unshaded symbol, follow their offspring’s symbols to see which children show the characteristic. The pedigree records the pattern in that family; a shaded or unshaded symbol alone does not necessarily reveal all the genetic information an individual carries.