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.

Probability in genetic crosses

A genetic cross predicts how likely different offspring outcomes are. It does not guarantee the outcome for an individual offspring or the exact numbers in a family.

When every cell in a Punnett square represents an equally likely combination, each cell counts equally towards the prediction. The probability of an outcome is:

For example, if three of the four cells show offspring with a particular characteristic, the predicted probability of that characteristic is $$\frac{3}{4}$$, or $$75%$$. The probability of an offspring without it is $$\frac{1}{4}$$, or $$25%$$.

Ratios and expected numbers

A ratio compares the predicted numbers of different outcomes. In the example above, the ratio with the characteristic to without it is $$3:1$$. For any two-category ratio $$a:b$$, the corresponding proportions are $$\frac{a}{a+b}$$ and $$\frac{b}{a+b}$$.

To find an expected number, multiply the predicted proportion by the total number of offspring:

For 12 offspring with a predicted ratio of $$3:1$$, the expected numbers are $$\frac{3}{4} \times 12 = 9$$ with the characteristic and $$\frac{1}{4} \times 12 = 3$$ without it.

These are predictions, not guaranteed counts: the actual numbers can differ by chance. The outcome for one offspring does not determine the outcome for another. For example, an offspring with the characteristic does not make the next offspring more or less likely to have it.

Gametes

Gametes are reproductive sex cells. Sperm cells and egg cells are examples of gametes.

Most body cells contain two sets of chromosomes. A gamete contains only one set, so it is described as haploid. Chromosomes carry genes, and a gamete normally carries one allele of each gene present on its chromosomes. An allele is a version of a gene.

At fertilisation, two gametes fuse and their chromosome sets combine. The resulting cell therefore has two sets of chromosomes, with one set from each parent. This means the offspring receives genetic information from both parents.

Chromosomes

A chromosome is a structure made of one long DNA molecule associated with proteins. Along the DNA are many genes: sections of DNA at particular positions on the chromosome.

Homologous chromosome pairs

In a cell with two sets of chromosomes, the chromosomes form pairs. One chromosome in each pair is inherited from each parent. The two chromosomes in a pair are called homologous chromosomes.

Homologous chromosomes carry the same genes at corresponding positions. However, they do not necessarily carry identical versions of those genes. Different versions of a gene are called alleles, so the two chromosomes may carry different alleles of the same gene.

Genes and alleles

A gene is a section of DNA containing information used to make a functional product, such as a protein. Different versions of the same gene are called alleles. Alleles differ in their DNA sequence, although they contain information relating to the same gene.

An inherited allele can affect the protein made from a gene or the amount of that protein produced. Proteins influence cell structures and processes, so these effects can contribute to an organism’s phenotype: its observable characteristics.

Different alleles do not necessarily produce different observable phenotypes. A difference in DNA sequence may have no effect on the protein, or its effect may not lead to an observable difference in the organism.

Polygenic inheritance

A characteristic can be influenced by several genes working together, rather than by a single gene. This is called polygenic inheritance.

Each gene can have different versions, called alleles. In polygenic inheritance, alleles at the different genes contribute to the characteristic together. The resulting phenotype—the characteristic expressed by the individual—depends on their combined effects.

Most characteristics are influenced by multiple genes. A genetic cross that follows only one gene may show how alleles of that gene are inherited, but it cannot generally represent the full inheritance of a polygenic characteristic because it leaves out the effects of the other genes.

Interpreting genetic family trees

A genetic family tree, or pedigree, shows how members of a family are related across generations and which individuals express a particular characteristic. By comparing relatives, we can consider how the characteristic may be inherited.

Reading a genetic family tree

Each symbol represents one person. A square represents a male and a circle represents a female. A shaded symbol identifies a person who expresses the characteristic stated in the diagram’s key; an unshaded symbol identifies a person who does not.

A line connecting two individuals indicates a parental relationship. Lines leading down from them connect them to their offspring. Individuals at the same level usually belong to the same generation, with earlier generations shown above later ones.

Inferring inheritance and genotypes

The pattern of shaded and unshaded relatives can help test a proposed inheritance pattern. Any proposed genotype must fit both the person’s characteristic and the alleles they could have inherited from their parents.

For example, suppose a characteristic is caused by a recessive allele $$a$$, while $$A$$ is dominant. A person who expresses the recessive characteristic must have genotype $$aa$$. If two parents who do not express it have an affected child, each parent must have passed on an $$a$$ allele. Under this proposed pattern, both parents must therefore be $$Aa$$.

A pedigree does not always identify every genotype uniquely. In the same recessive example, an unaffected person could be either $$AA$$ or $$Aa$$. Other family relationships may narrow down the possibilities, but if the information shown does not distinguish them, both must remain possible.

A pattern in a family tree can support a proposed inheritance pattern without proving it is the only possible one. Check that the proposed pattern and genotypes are consistent with all the relatives shown, rather than drawing a conclusion from a single individual.