Inheritance patterns

Definitions & Key takeaways

Inheritance patterns refer to the different ways in which traits are passed from one generation to another. There are three patterns of inheritance: autosomal dominant, autosomal recessive, and X-linked.

Autosomal dominant inheritance means that only one copy of the defective gene is necessary for the trait to be expressed. This pattern is seen in conditions such as Huntington's disease and Marfan syndrome.

Autosomal recessive inheritance means that two copies of the defective gene are necessary for the trait to be expressed. This pattern is seen in conditions such as cystic fibrosis and sickle cell disease.

X-linked inheritance means that the gene is located on the X chromosome. Males have only one X chromosome, so they will express a disease if it is carried on that chromosome. Females have two X chromosomes, so they will only express a disease if they inherit two defective copies of the gene (one from each parent). This pattern is seen in conditions such as hemophilia and color blindness.

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Inheritance patterns are the different ways in which traits are passed from one generation to another. Inheritance relies on homologous chromosomes, which come in pairs - one from mom and one from dad.
Each chromosome has genes, which are regions of DNA that carry information for a specific trait. And different versions of the same gene are called alleles.
As an example, brown eye color and blue eye color are both alleles for the eye color gene. Each parent offers one allele of a gene, which can be either dominant represented with a capital letter, like big A, or recessive, represented with a lowercase letter, like little a.
It only takes one dominant allele for its trait to be expressed, whereas it takes two recessive alleles for its trait to be expressed.
Human somatic cells - so, all of the cells aside from the gametes - have 23 pairs of chromosomes; 22 somatic pairs and one sexual pair - adding up to 46 chromosomes in total.
For the sex chromosomes, a genetic female has two X chromosomes, while a genetic male has an X and Y chromosome. All 46 of these chromosomes, along with the alleles they carry, segregate during meiosis - which is the process of making gametes.
Gametes only carry half the genetic information of the parent - so 23 chromosomes. Females require an egg and a sperm that are both “22, X”, whereas males require an egg that’s “22,X” and a sperm that’s “22,Y”.
Once the male and female gametes merge during fertilization, their alleles combine to give rise to one of three possible genotypes of the offspring, homozygous dominant - or AA -, heterozygous - or Aa - , and homozygous recessive - or aa.
This genotype determines a person’s features —or phenotype— such as hair color, or whether or not they have a genetic disease.
Genetic diseases develop when a gene doesn’t work well because of a mutation that affects one of the two alleles, and if the person has children these mutations can be inherited.
To get a quick picture of how different inheritance patterns work, we’ll use a pedigree - where we represent females with a circle and males with a square.
We’ll shade in the individuals with the disease - so it’s based on phenotype, not genotype. When a mutation affects a dominant allele, it only takes one mutant copy to cause a disease - this is a dominant inheritance pattern.
However, if a mutation affects a recessive allele, it takes two mutant copies to cause a disease - this is a recessive inheritance pattern.
Now, when the mutant allele is on a somatic chromosome, it’s called autosomal inheritance, and when it’s on a sex chromosome, it’s called sexual inheritance.
Sexual inheritance is divided into X-linked inheritance - where the mutant allele is on the X chromosome - and Y linked inheritance - where the mutant allele is on the Y chromosome.
Finally there’s mitochondrial inheritance. In addition to the 46 chromosomes in the nucleus, cells also have mitochondria which carry their own DNA.
When an egg and sperm come together, the cytoplasm and organelles like mitochondria come from the egg. So when looking at the pedigree of a family affected by mitochondrial disease, you can see that as a result, both males and females can develop mitochondrial diseases but only females can pass those diseases to their children.
So, let’s look at autosomal dominant inheritance, an example would be Huntington’s disease, where there’s a mutation resulting in a dominant allele on an autosome.
So both homozygous dominant people -(DD)- and heterozygotes - (Dd) - have the disease. But, homozygous dominant people rarely reproduce because the disease is too severe.
So, most of the individuals that reproduce are heterozygotes. So let’s say that we have an individual who’s heterozygous - or Dd and that the other parent doesn’t have the disease, so they’re homozygous - dd - .
Now, since the mutation is on an autosome, it doesn’t matter which parent has the disease. Now let’s create a Punnett square.
The affected parent can make either mutant gametes, which carry the D allele, or normal gametes, with the d allele. The healthy parent only makes normal gametes carrying the d allele.
When these gametes combine, the children have a 50% chance of being heterozygous Dd and having the disease and a 50% chance of being homozygous recessive - dd- and not having the disease.
In a family pedigree, the inheritance pattern would look like this, with affected members in every generation. Usually a person with the disease has one affected grandparent, affected and unaffected uncles and aunts, one affected parent, and even affected and unaffected siblings.
In addition, the mutation can also appear spontaneously in a person who’s family isn’t affected. Now, autosomal recessive diseases, like cystic fibrosis only occur when a person has two recessive alleles - or rr.
And heterozygotes with only one mutant allele - or Rr - are considered to be carriers of the disease, even though they don’t have the disease.
For example, if two parents are carriers for cystic fibrosis - they are both heterozygous Rr. Let’s use a Punnett square again.
They make gametes that have the normal R allele, or the recessive mutant r allele. So, their children have 25% chance of ending up homozygous dominant - RR - which means they don’t have cystic fibrosis, aren’t carriers so are healthy.
They have 25% chance of ending up homozygous recessive - rr - which means they do have cystic fibrosis. And finally, they have 50% chance of ending up being heterozygous carriers - Rr - just like their parents.
However if one of the parents is affected - rr - and the other one is not - RR -, then all the children would end up heterozygous -Rr- carriers.
So in a pedigree, the pattern is that the disease tends to skip generations, so a person with the disease usually has unaffected parents, or even grandparents, but they do have affected great-grandparents who passed on the recessive mutant allele.
So, members of the same family have a higher chance of having heterozygous genotypes compared to the general population.
That’s why children of consanguineous unions - where two related individuals have children - are more likely to have autosomal recessive diseases.
Now let's switch gears and look at sex-linked inheritance patterns. Males have only one copy of each sex chromosome meaning that they have only one allele for the genes on the X and Y chromosome - these are called hemizygous genotypes.
Females, on the other hand, have two alleles for the genes on the X chromosome - so they can have either homozygous or heterozygous genotypes.
This causes X-linked inheritance patterns to behave in slightly different ways between men and women. X-linked dominant diseases, like fragile X syndrome, are caused by a mutation in the dominant allele of the X chromosome.
Both heterozygous - XDXd - and homozygous -XDXD- females have the disease; and males - XDY - also have the disease, since you only need one mutant X chromosome to be present.
But the chance of passing on the disease differs in these three situations. If a male has fragile X, his genotype is XDY, and his partner is a female without the disease, then her genotype is XdXd.
Using a Punnett square, the father can make gametes which carry the mutant XD allele, or gametes which carry the Y chromosome.
And mom only makes gametes carrying a normal Xd allele. Daughters of this couple have 100% chance of ending up heterozygous - XDXd - - so they’d all have the disease.
Sons, however, are in luck - because they get a normal Xd chromosome from mom, and a Y from dad - and as a result won’t have the disease.
On the other hand, if mom is heterozygous - XDXd - and therefore has fragile X, then she can make either normal - Xd - or mutant - XD - gametes.
And if her partner is a male without the disease, then his genotype is - XdY and he can make normal - Xd - gametes, or normal - Y - gametes.
In this case, the sons have 50% chance of ending up XDY - and the daughters have 50% chance of ending up - XDXd - , so both sons and daughters have the same 50% chance of having the disease.
In a pedigree, the pattern would show affected members in every generation - with females affected twice as often as males.
Also, males only pass on the mutant allele to their daughters - which differentiates this pattern from autosomal dominant inheritance, while females pass on the mutant allele to both sons and daughters equally.
Now, there’s also X-linked recessive diseases, like hemophilia, which develops when there are two recessive mutations on the X chromosomes.
In this case, only homozygous recessive females - XrXr- are affected, while heterozygous females - XRXr - are carriers, and homozygous dominant females - XRXR - are healthy.
Males, however, can only be healthy - XRY - or affected - XrY -, in other words they can’t be carriers. The chance of passing on the disease is also different for both males and females.
Let’s look at an affected male - XrY - and a healthy female - XRXR -. In a Punnett square, the father can make mutant - Xr - gametes, or normal - Y - gametes, while the mother can only makes - XR gametes.
So the daughters have 100% chance of being heterozygous - XRXr - carriers, and the sons have 100% chance of being hemizygous dominant - XRY - so they would neither develop nor be carriers of hemophilia.
Now let’s say that the mother is a carrier - XRXr - and the father is healthy - XRY. The mother can make either normal -XR- or mutant -Xr- female gametes, while the father can only make normal - Y - gametes or dominant - XR - gametes.
So their sons would have 50% chance of ending up hemizygous dominant - XRY - and 50% chance of ending up hemizygous recessive and having the disease.
But the daughters have 50% chance of ending up heterozygous- XRXr - carriers and 50% chance of being XRXR homozygous dominant which means they’re healthy.
In a pedigree there would be twice as many affected males than females, the disease wouldn’t transmit from father to son, and both mothers and daughters of an affected male would be definite carriers.
Finally, let’s look at Y-linked diseases, like some cases of baldness which only affect males. Because there’s only one Y chromosome, a mutation on it results in the disease.
The disease would always be passed on from father to son, and never from father to daughter. In a pedigree, there would be only males affected, and the disease would be present in every generation.
So, to recap… inheritance patterns can be visualized in pedigrees. With dominant inheritance, you only need one mutant allele to develop the disease, wheres in recessive inheritance you need two mutant alleles to develop the disease.
Now, autosomal inheritance means the mutant allele is located in somatic chromosomes, whilst sex-linked inheritance - x-linked and y-linked - means the mutant allele is located in sex chromosomes.
Finally, there are other more rare patterns of inheritance such as mitochondrial inheritance, where females passes on mutations in the mitochondrial DNA to both sons and daughters, but males do not pass on the mutation.