Homozygous and heterozygous genotypes

For a particular gene, an individual usually has two copies: one inherited from each parent. Different versions of the gene are called alleles. The pair of alleles an individual has is its genotype for that gene.

An individual is homozygous for a gene if its two alleles are identical, such as $$AA$$ or $$aa$$. It is heterozygous if its two alleles are different, such as $$Aa$$.

Alleles passed to offspring

Each parent passes one allele of the gene to an offspring. A homozygous individual has two identical alleles, so it can pass on only that allele. For example, an individual with genotype $$AA$$ can pass on only $$A$$.

When two individuals with the same homozygous genotype breed together, each passes on the same allele. Two $$AA$$ parents therefore produce offspring with genotype $$AA$$. They are pure-breeding for this gene: breeding individuals with that same genotype produces offspring with the same genotype.

A heterozygous individual has two different alleles, so it can pass on either one. An individual with genotype $$Aa$$ can pass on $$A$$ or $$a$$. Heterozygous individuals are not pure-breeding: two $$Aa$$ parents can produce offspring with different genotypes.

Dominant and recessive alleles

Alleles are different versions of the same gene. An individual usually has two alleles of a gene, one inherited from each parent. The allele combination is its genotype; the characteristic expressed is its phenotype.

A dominant allele is expressed in the phenotype whenever it is present. A recessive allele is expressed only when no dominant allele of that gene is present. Therefore, an individual with one dominant and one recessive allele shows the dominant phenotype. The recessive allele is still present in the genotype, but its effect is not expressed in the phenotype.

For example, if $$A$$ represents a dominant allele and $$a$$ represents a recessive allele:

• $$AA$$: both alleles are dominant, so the dominant phenotype is expressed.
• $$Aa$$: one allele is dominant, so the dominant phenotype is expressed.
• $$aa$$: no dominant allele is present, so the recessive phenotype is expressed.

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.

Test crosses

An individual showing a dominant phenotype may have either two dominant alleles or one dominant and one recessive allele. A test cross uses the phenotypes of its offspring to help distinguish between these two possible genotypes.

The individual is crossed with a homozygous recessive individual, which has two recessive alleles and can pass on only a recessive allele to each offspring.

Suppose the dominant allele is $$A$$ and the recessive allele is $$a$$. The individual being tested could be $$AA$$ or $$Aa$$; the homozygous recessive individual is $$aa$$.

• If the tested individual is $$AA$$: it can pass on only $$A$$. All offspring will be $$Aa$$ and will show the dominant phenotype.
• If the tested individual is $$Aa$$: it can pass on either $$A$$ or $$a$$. Each offspring has a one-half chance of being $$Aa$$, with the dominant phenotype, and a one-half chance of being $$aa$$, with the recessive phenotype.

A recessive offspring must have received an $$a$$ allele from each parent. Its appearance therefore shows that the tested individual is heterozygous.

If all offspring show the dominant phenotype, the result is consistent with the tested individual being homozygous dominant. It does not prove this with certainty: a heterozygous individual can produce only dominant-phenotype offspring by chance, especially if there are few offspring.

Codominance

A gene can have different versions, called alleles. An organism that has two different alleles of a gene is heterozygous for that gene. Its observable characteristics are its phenotype.

In codominance, both alleles contribute to the phenotype of a heterozygote. Neither allele completely masks the other, so the phenotype shows the effects of both.

For example, a person with blood-group alleles $$I^A$$ and $$I^B$$ has both A and B antigens on their red blood cells. Their blood group is AB: both alleles contribute to the phenotype.

Codominance differs from a dominant–recessive relationship. In a heterozygote with a dominant and a recessive allele, the dominant allele determines the phenotype, masking the contribution of the recessive allele.

Inheritance of ABO blood groups

A person’s ABO blood group is determined by the combination of two inherited alleles: one from each parent. There are three possible alleles in the population: $$I^{A}$$, $$I^{B}$$ and $$I^{O}$$. Each person inherits only two of them.

The alleles $$I^{A}$$ and $$I^{B}$$ are codominant: when both are present, both are expressed, producing blood group AB. Each is dominant over $$I^{O}$$, so a person with one $$I^{O}$$ allele has group A or B if the other allele is $$I^{A}$$ or $$I^{B}$$, respectively.

ABO genotypes and blood groups

A genotype is the pair of alleles a person has; the phenotype is the blood group produced by that pair.

• Group A: $$I^{A}I^{A}$$ or $$I^{A}I^{O}$$
• Group B: $$I^{B}I^{B}$$ or $$I^{B}I^{O}$$
• Group AB: $$I^{A}I^{B}$$
• Group O: $$I^{O}I^{O}$$

A blood group therefore does not always identify a unique genotype. For example, a person with group A may have two $$I^{A}$$ alleles or one $$I^{A}$$ and one $$I^{O}$$ allele. In contrast, group AB and group O each identify one genotype.

Sex-linked inheritance

A sex-linked characteristic is determined by a gene on a sex chromosome. For a gene on the X chromosome, females usually have two copies of the gene because they have two X chromosomes. Males usually have one copy because they have one X and one Y chromosome.

A recessive X-linked allele can be expressed in a male even when he has only one copy of it: he has no second copy of the gene on another X chromosome to mask it. A female with one recessive allele and one dominant allele usually does not show the recessive characteristic because the dominant allele is expressed. She is a carrier of the recessive allele and can pass it to her children.

A child usually inherits an X chromosome from their mother. A female child also inherits an X from her father, while a male child inherits a Y from his father. A recessive X-linked characteristic can therefore be more common in males than in females.

Red-green colour blindness is an example of a recessive X-linked characteristic. A male with the recessive allele on his X chromosome usually has red-green colour blindness. A female usually needs the recessive allele on both X chromosomes to have the characteristic.

Predicting codominant crosses

In a monohybrid cross, one allele for a characteristic is passed on by each parent. To predict the offspring, combine the possible alleles from one parent with those from the other. Each combination gives an offspring genotype, which can then be used to determine its phenotype.

When two different alleles are codominant, both contribute to the heterozygous phenotype; neither masks the other. The two homozygous genotypes and the heterozygous genotype can therefore produce three different phenotypes.

Cross between two heterozygous parents

Consider a hypothetical characteristic with two codominant alleles: $$C^B$$ produces black patches and $$C^W$$ produces white patches. An offspring with $$C^B C^W$$ has both black and white patches.

If both parents are heterozygous, each has genotype $$C^B C^W$$. Each parent can pass on either $$C^B$$ or $$C^W$$, with equal probability. Combining their alleles gives four equally likely outcomes:

• $$C^B$$ from each parent gives $$C^B C^B$$: black patches.
• $$C^B$$ from the first parent and $$C^W$$ from the second gives $$C^B C^W$$: black and white patches.
• $$C^W$$ from the first parent and $$C^B$$ from the second gives $$C^B C^W$$: black and white patches.
• $$C^W$$ from each parent gives $$C^W C^W$$: white patches.

One of the four combinations produces black patches, two produce black and white patches, and one produces white patches. The predicted phenotypic ratio is therefore 1 black : 2 black and white : 1 white.

The ratio comes from counting the predicted offspring combinations and assigning a phenotype to each genotype. It describes probabilities, not a guarantee that every group of four offspring will contain exactly one of each homozygous phenotype and two with the heterozygous phenotype.

Predicting sex-linked crosses

To predict an X-linked cross, track the sex chromosome each parent passes to an offspring. Write each allele on the X chromosome that carries it. A mother passes one of her X chromosomes to every child. A father passes his X chromosome to a daughter or his Y chromosome to a son.

Combining one sex chromosome from each parent gives the offspring’s sex chromosomes and X-linked genotype. Interpret daughters and sons separately: a daughter has two X-linked alleles, while a son has only one. A recessive allele on a son’s X chromosome can therefore be expressed without a second copy of the allele.

An X-linked cross

Suppose $$X^N$$ carries a dominant allele for an unaffected phenotype and $$X^n$$ carries a recessive allele for an affected phenotype. A carrier mother has genotype $$X^N X^n$$; she is unaffected but carries the recessive allele. An unaffected father has genotype $$X^N Y$$.

The mother can pass on $$X^N$$ or $$X^n$$. The father can pass on $$X^N$$ or $$Y$$. Combining these possibilities gives four equally likely outcomes:

• $$X^N X^N$$: unaffected daughter.
• $$X^N X^n$$: unaffected carrier daughter; the dominant allele is expressed.
• $$X^N Y$$: unaffected son.
• $$X^n Y$$: affected son; his only X-linked allele is recessive.

Each outcome has a predicted probability of $$\frac{1}{4}$$. Both possible daughters are unaffected, although one is a carrier. Among the possible sons, half are unaffected and half are affected.

Counting all four outcomes gives an unaffected : affected phenotypic ratio of $$3:1$$. This overall ratio does not show the difference between the sexes: in this cross, the affected outcome is a son. These are predicted probabilities, not a guarantee of the exact numbers in a family.