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.

Gametes

A gamete is a reproductive cell that carries genetic information from a parent to offspring. Sperm cells and egg cells are gametes.

Genetic information is carried on chromosomes. A gene is a section of DNA on a chromosome, and an allele is a version of a gene. Each gamete normally contains one set of chromosomes. It therefore carries one allele of a gene on a chromosome it contains.

At fertilisation, two gametes fuse. Their chromosomes come together, bringing alleles from both parents into the new cell. The offspring therefore receives genetic information from both parents.

Chromosomes

A chromosome is a long DNA molecule associated with proteins. It carries many genes, which are sections of DNA. A gene can exist in different versions, called alleles.

In body cells, chromosomes generally occur in pairs. One chromosome of each pair is inherited from each parent. The two chromosomes in a pair carry the same genes at corresponding positions, but they do not necessarily carry the same alleles. For example, they may carry different alleles of the same gene.

Gametes, such as egg and sperm cells, normally contain one chromosome from each pair. When an egg and sperm join, the new cell receives one chromosome of each pair from each parent.

Genes and alleles

A gene is a section of DNA that provides information for a functional product. Many genes carry instructions for making a protein. The order of DNA bases in a gene forms the information used to make that product.

An allele is a version of a gene. Different alleles of the same gene have different DNA base sequences. A difference in sequence can affect the protein produced or the amount of that protein made.

From alleles to phenotype

Proteins act in cells and can influence how an organism’s characteristics develop. An allele can therefore affect a characteristic by changing a protein or how much of it is made. The observable characteristics of an organism make up its phenotype.

Different alleles do not necessarily produce an observable difference. A change in DNA sequence may have no noticeable effect on the protein or on the characteristic it influences.

Dominance and zygosity

An allele is a version of a gene. For a gene with two copies, an individual has two alleles, which may be the same or different. The pair of alleles is the individual’s genotype for that gene.

• Homozygous means that the two alleles are the same.
• Heterozygous means that the two alleles are different.

The observable form of a characteristic is its phenotype. When two different alleles are present, their relationship determines how they affect the phenotype. A dominant allele is expressed when at least one copy is present, including in a heterozygous individual. In this two-copy model, a recessive allele is expressed when both alleles are recessive.

Dominance and zygosity in an allele pair

Suppose $$A$$ represents a dominant allele and $$a$$ represents a recessive allele. The possible genotypes are:

• $$AA$$: homozygous, with the dominant phenotype.
• $$Aa$$: heterozygous, with the dominant phenotype.
• $$aa$$: homozygous, with the recessive phenotype.

In $$Aa$$, the recessive allele is still present; the dominant allele does not remove it. Dominance describes the effect of the alleles on the characteristic, not how common either allele is in a population.

Single-gene and polygenic inheritance

A characteristic can be influenced by one gene or by several genes acting together. Genes are sections of DNA, and different versions of a gene are called alleles. The alleles an individual has contribute to their phenotype: the characteristics they express.

Single-gene inheritance

Some characteristics can be explained by the alleles of a single gene. For example, some variation in mouse fur colour can be influenced by one gene. Different alleles of that gene can contribute to different fur colours.

Red-green colour blindness can be a single-gene, X-linked recessive characteristic. The gene is on the X chromosome. A male usually has one X and one Y chromosome, so he has only one copy of this gene. If his X chromosome carries the allele for red-green colour blindness, he can express the characteristic: there is no corresponding allele on his Y chromosome to mask it.

Polygenic inheritance

Many characteristics are influenced by several genes acting together. This is called polygenic inheritance. Alleles at the different genes have combined effects that contribute to the phenotype.

A cross that follows alleles of only one gene therefore cannot represent the whole pattern of inheritance for a polygenic characteristic. It shows only one part of the genetic contribution.

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.

Interpreting genetic family trees

A genetic family tree, or pedigree, shows relationships between family members and which individuals have a particular characteristic. By following the characteristic through generations, it is possible to infer how it might be inherited and which alleles individuals might carry.

In a conventional pedigree:

• A square represents a male and a circle represents a female.
• A shaded symbol identifies an individual with the characteristic; an unshaded symbol identifies someone without it.
• A line connecting two individuals shows a partnership. Lines from the partnership lead to their offspring, shown in the next generation.

Inferring a recessive characteristic

For a simple recessive characteristic, an individual must inherit two recessive alleles to show the characteristic. An individual with one recessive allele and one dominant allele does not show it, but is a carrier.

Suppose $$a$$ is the recessive allele for a characteristic and $$A$$ is the dominant allele. An affected child has genotype $$aa$$, so the child must have received an $$a$$ allele from each parent. If both parents are unaffected, neither can be $$aa$$. Each must therefore be $$Aa$$: both parents are carriers.

Limits of family-tree evidence

A pedigree does not always reveal an individual's exact genotype. For the recessive characteristic above, an unaffected person could be $$AA$$ or $$Aa$$ unless further evidence distinguishes them. A family tree may also be consistent with more than one pattern of inheritance. Conclusions should therefore be stated as possibilities unless the evidence rules out the alternatives.