What percentage of offspring will have the disorder from heterozygous parents?

A trait that is inherited in a recessive fashion only manifests phenotypically in homozygous individuals, i.e., when the individual has two copies of the same recessive allele. Humans have two versions of all autosomal genes, called alleles, one from each parent. The recessive trait is hidden in the heterozygous individual (Dd) if the other allele is inherited in a dominant fashion, and so this person is a called a “carrier” of the recessive allele, but does not manifest the disease or trait. A recessive trait can only be passed to the offspring if both parents carry (Dd or dd) and transmit the recessive allele to their offspring. In the scenario where both parents are heterozygous carriers of the recessive trait, the children have 25% chance of inheriting two copies (dd) of the recessive allele and exhibiting the recessive trait (see pedigree figure, Fig. 1). An example of a disease with a recessive inheritance is Tay-Sachs disease that occurs when a child has two...

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Recessive Inheritance, Fig. 1

What percentage of offspring will have the disorder from heterozygous parents?

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Genetic heredity is inherently probabilistic – sexual reproduction ensures that even when we know everything about the parents’ genomes, we don’t know what assortment of their genes will end up in each of their offspring. It can be fun to wonder if a new baby will look more like their mum or dad, but when a genetic condition runs in the family, the unpredictability can be worrying.

We can, however, predict the possible outcomes based on chance. For example, for a couple who are both carriers of the gene variant for a recessive condition, the chance that their child will be affected is 25%. But it does not follow that if they have three healthy children, then the fourth will have the condition. They could have four healthy children, or four who are all affected. The 1-in-4 chance is the same each time, for each child.

Penetrance and probability

Often, the fact that a person carries a gene variant associated with a particular disease does not guarantee that they will be affected.

For example, there is a wealth of evidence linking the BRCA genes to breast and ovarian cancers, but not every woman who carries a pathogenic variant on one of these genes will get cancer in her lifetime. Such genes are said to have incomplete penetrance.

Around 12% of women in the general population will develop breast cancer at some point during their lives, and this goes up to 72% of women with a pathogenic variant in BRCA1 and about 69% of women with a pathogenic BRCA2 variant. So, while the risk is much greater for women with these gene variants, it is by no means certain.

Conversely, many women develop breast cancers every year who do not carry these BRCA variants. In fact, because BRCA and other gene variants associated with breast cancer are comparatively rare, they only account for 5%-10% of all breast cancer diagnoses.

Researchers have identified more than 100 other genes linked to increased risk of breast cancer, but none have effects as significant as the BRCA genes. We also know that environmental and lifestyle factors may affect the risk of developing breast cancer. But even if we had all this information, we still cannot predict whether an individual will develop cancer or not. There will always be rare individuals at high risk who remain unaffected, and individuals at low risk who develop the condition against the odds.

Novel variants

Sometimes a genetic condition can arise with absolutely no warning, when a de novo variant occurs in a gene.

Achondroplasia is the most common form of dwarfism, affecting around one in 25,000 people. It is a genetic condition, resulting from a variant in a gene called FGFR3, and is inherited in an autosomal dominant pattern. It is 100% penetrant, so everyone who has the variant has achondroplasia.

However, around 80% of people with achondroplasia do not inherit the condition from their parents; it is the result of a new variant that arises when the egg or sperm (or their precursor cells) were made. Because this is a random event, there is no way to predict when this will happen.

Supporting the family

Where a genetic condition appears to run in a family, a useful first step is to take a genetic family history (you can learn more about this in our short online course). A referral to clinical genetics may be appropriate, along with access to genetic counselling.

For families who know they carry the gene variant for a genetic disease, options are available when it comes to family planning. Some couples opt for pre-implantation genetic testing (PGD) to select an unaffected embryo. Another option is prenatal testing during pregnancy to find out if their child will be affected, such as amniocentesis or CVS (chorionic villus sampling). Non-invasive tests are also being developed for some single-gene disorders.

Please note: This article is for informational or educational purposes, and does not substitute professional medical advice.

Heterozygous is a term used in genetics to describe when two variations of a gene, known as alleles, are paired at the same location (locus) on a chromosome. By contrast, homozygous is when there are two copies of the same allele at the same locus.

The term heterozygous is derived from "hetero," meaning different, and "zygous," meaning related to a fertilized egg, or zygote.

Humans are called diploid organisms because they have two alleles at each locus, with one allele inherited from each parent. The specific pairing of alleles translates to variations in an individual’s genetic traits.

An allele can either be dominant or recessive. Dominant alleles are those that express a trait even if there is only one copy. Recessive alleles can only express themselves if there are two copies.

One such example is the genetic inheritance of brown eyes, which is dominant, and blue eyes, which is recessive. If the alleles are heterozygous, the dominant allele would express itself over the recessive allele, resulting in brown eyes.

At the same time, the person would be considered a "carrier" of the recessive allele, meaning that the blue-eye allele could be passed to offspring even if that person has brown eyes.

Alleles can also be incompletely dominant, an intermediate form of inheritance where neither allele is expressed completely over the other.

An example of this might include an allele corresponding to dark skin, in which a person has more melanin, when paired with an allele corresponding to light skin, in which there is less melanin. This might create a skin tone somewhere in between.

The heterozygous Punnett square is a basic mathematical grid used to plot inherited traits, such as eye color or the likelihood of sickle cell disease. As the study of molecular genetics advances, the Punnett square remains a useful tool but within much broader thinking about gene expression and heterozygous genotype and phenotype.

Beyond the physical characteristics of an individual, the pairing of heterozygous alleles can sometimes translate into a higher risk of certain conditions such as birth defects or autosomal disorders (diseases inherited through genetics).

If an allele is mutated, a disease can be passed to offspring even if the parent experiences no signs of the disorder. With respect to heterozygosity, this could take one of several forms:

  • If the alleles are heterozygous recessive, the faulty mutated allele would be recessive and not express itself. Instead, the person would be a carrier.
  • If the alleles are heterozygous dominant, the faulty allele would be dominant. In such a case, the person may or may not be affected (compared to homozygous dominance where the person would be affected).

Other heterozygous pairings would simply predispose a person to a health condition such as celiac disease and certain types of cancer. This doesn’t mean that a person will get the disease; it simply suggests that the individual is at higher risk.

Other factors, such as lifestyle and environment, would also play a part.

People often wonder who has stronger genes, the mother or father in a pair. The answers are complex and depend on how you define stronger; for example, some genes are only inherited from one parent or the other. Much research is focused on "parent of origin" genetic contributions.

Single gene disorders are those that are caused by a single mutated allele rather than two. If the mutated allele is recessive, the person will usually not be affected.

However, if the mutated allele is dominant, the mutated copy can override the recessive copy and cause either a less severe form of a disease or a fully symptomatic disease.

Single gene disorders are relatively rare. Among some of the more common heterozygous dominant disorders are:

  • Huntington’s disease, an inherited disorder that results in the death of brain cells. The disease is caused by a dominant mutation in either one or both alleles of a gene called Huntingtin.
  • Neurofibromatosis type-1 is an inherited disorder in which nerve tissue tumors develop on the skin, spine, skeleton, eyes, and brain. Only one dominant mutation is needed to trigger this effect.
  • Familial hypercholesterolemia (FH) is an inherited disorder characterized by high cholesterol levels, specifically "bad" low-density lipoproteins (LDLs). It is by far the most common of these disorders, affecting around one of every 500 people.

A person with a single gene disorder has a 50/50 chance of passing the mutated allele to a child who will become a carrier.

If both parents have a heterozygous recessive mutation, their children will have a one-in-four chance of developing the disorder. The risk will be the same for every birth.

If both parents have a heterozygous dominant mutation, their children have a 50% chance of getting the dominant allele (partial or complete symptoms), a 25% chance of getting both dominant alleles (symptoms), and a 25% of getting both recessive alleles (no symptoms).

Compound heterozygosity is the state where there are two different recessive alleles at the same locus that, together, can cause disease. These are typically rare disorders that are often linked to race or ethnicity.

Among these conditions are:

Tay-Sachs disease

Tay-Sachs disease is a rare, inherited disorder that causes the destruction of nerve cells in the brain and spinal cord. It is a highly variable disorder that can cause disease during infancy, adolescence or later adulthood.

While Tay-Sachs is caused by genetic mutations of the HEXA gene, it is the specific pairing of the alleles that ultimately determines which form the disease takes. Some combinations translate to childhood disease, while others translate to later onset disease.

Phenylketonuria

Phenylketonuria (PKU) is a genetic disorder primarily affecting children in which a substance known as phenylalanine accumulates in the brain. This causes seizures, brain disorders, and intellectual disability. There is a vast diversity of genetic mutations associated with PKU, the pairings of which can lead to milder or more severe forms of the disease.

Other diseases in which compound heterozygotes can play a part are cystic fibrosis, sickle cell anemia, and hemochromatosis (excessive iron in the blood).

While a single copy of a disease allele usually doesn’t result in illness, there are cases where it can provide protection against other diseases. This is a phenomenon referred to as heterozygote advantage.

In some cases, a single allele can alter the physiological function of an individual in such a way as to make that person resistant to certain infections. Among the examples:

Sickle cell anemia

Sickle cell anemia is a genetic disorder caused by two recessive alleles. Having both alleles causes the malformation and rapid self-destruction of red blood cells. Having only one allele can cause a less severe condition called sickle cell trait in which only some cells are malformed.

These milder changes are enough to provide a natural defense against malaria by killing off the infected blood cells faster than the parasite can reproduce.

Cystic fibrosis

Cystic fibrosis (CF) is a recessive genetic disorder that can cause a severe impairment of the lungs and digestive tract. In persons with homozygous alleles, CF causes a thick, sticky buildup of mucus in the lungs and gastrointestinal tract.

In persons with heterozygous alleles, the same effect, albeit reduced, can lower a person’s vulnerability to cholera and typhoid fever. By increasing mucus production, a person is less susceptible to the damaging effect of infectious diarrhea.

The same effect may explain why people with heterozygous alleles for certain autoimmune disorders appear to have a lower risk of later-stage hepatitis C symptoms.