Mitochondrial Inheritance Pattern

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Mitochondrial inheritance is different from the other genetic inheritance patterns in that it has nothing to do with the chromosomes of the father or the mother. Most of the cell DNA is in the nucleus, in the form of chromosomes or chromatin (depending on the level of DNA packing), but a small amount of DNA is inside the mitochondria (figure 1).

The mitochondria (singular: mitochondrion) are organelles inside the cell (figure 2). They carry on the oxidative metabolism of the nutrients and provide the cell with most of the energy that it needs. Due to their specialized function, defects in the mitochondria could produce severe diseases. The origin of a mitochondrial disease could originate from a mutation in the mitochondrial DNA, but could also originate from a mutation in the nuclear DNA since there are very few genes in the mitochondrial DNA and part of the proteins and enzymes of the mitochondria are encoded in nuclear genes, synthesized in the cytoplasm and then imported by the mitochondria. 

When the mitochondrial disease is due to a mutation in a nuclear gene, it will be transmitted according to the other genetic inheritance patterns. However, if the mutation is in the mitochondrial DNA, it will be transmitted with a specific pattern that it is very simple: the mitochondria, and their DNA, are only inherited from the mother. So all the children of a normal (gray color in the figures) father and an affected (orange color in the figures) mother will be affected (figure 3), while none of the children of an affected father and a normal mother will be affected (figure 4).

Figure 1. Structure of a mitochondrion. See the small circular fragments of mitochondrial DNA.

Figure 2. Diagram of a cell. See the nucleus and the mitochondria among other organelles.

Figure 3. Mitochondrial inheritance pattern when the mother is affected.

Figure 4. Mitochondrial inheritance pattern when the father is affected.

If you want to read about mitochondrial inheritance pattern in Spanish, you can follow the link herencia mitocondrial.

X-Linked Recessive Genetic Inheritance Pattern

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X-linked recessive is one of the possible ways that genetic traits can be inherited.  This pattern is similar to autosomal recessive genetic inheritance in that one copy of the normal allele is enough to hide the phenotype of the recessive trait. However, since in this case the gene is located in the X chromosome (men only have one X chromosome, while women have two X chromosomes), the gender of the affected parent play a significant role in the inheritance pattern among sons and daughters.

If the altered gene version is responsible for a disease, a man with one altered allele in his only one X chromosome or a woman with two recessive altered alleles will be affected and develop the altered phenotype associated to this disorder (orange color in the figures) instead of the normal one (grey color in the figures). An heterozygous woman with one altered and one normal copies of the gene would not be affected by the disease, but she is a carrier and could pass the altered gene on to her children.

If the mother is affected by a X-linked recessive disease, she will transmit the disorder to all her sons but to none of her daughters (although they will inherit a copy of the altered allele and be carriers of the disease) as can be seen in figure 1:

Figure 1. X-linked recessive genetic inheritance pattern when the mother is affected.

However, if the father is the one affected by the X-linked recessive disorder, none of the children will be affected, although the daughters will be carriers (figure 2). When a X-linked recessive carrier woman has children, daughters have a 50% chance of being carriers and sons a 50% chance of being affected by the disease (figure 3).

Figure 2. X-linked recessive genetic inheritance pattern when the father is affected.

Figure 3. X-linked recessive genetic inheritance pattern when the mother is a carrier.

Examples of genetic conditions that are inherited following an X-linked recessive pattern:

• Duchenne and Becker muscular dystrophy.
• Hemophilia.
• Red-green color vision defects.
• X-linked juvenile retinoschisis.
• X-linked lymphoproliferative disease (XLP).

If you want to read about X-linked recessive genetic inheritance in Spanish, you can follow the link herencia ligada al cromosoma X recesiva.

X-Linked Dominant Inheritance Pattern

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X-linked dominant is one of the possible ways that genetic traits can be inherited.  This pattern is similar to autosomal dominant inheritance in that one copy of the altered allele is enough to develop the altered phenotype (orange color in the figures) instead of the normal one (grey color in the figures). However, since in this case the gene is located in the X chromosome (men only have one X chromosome, while women have two X chromosomes), the gender of the affected parent play a significant role in the inheritance pattern among sons and daughters.

If the mother is affected by a X-linked dominant disease, all the children will have a 50% chance of inheriting the disorder, being it equally transmitted to sons and daughters (figure 1). However, if the father is the one affected, he will transmit the disorder to all his daughters but to none of his sons, this is explained in figure 2.

Figure 1. X-linked dominant inheritance pattern when the mother is affected.

Figure 2. X-linked inheritance pattern when the father is affected.

If you want to read about the X-linked dominant inheritance pattern in Spanish, you can follow the link herencia ligada al cromosoma X dominante.

Autosomal Recessive Inheritance Pattern

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Autosomal recessive is one of the possible ways that genetic traits can be inherited. In this case, the gene is located in one of the 22 autosomal chromosomes and two copies of the altered allele are needed to develop the altered phenotype (orange color in the figures) instead of the normal one (grey color in the figures). One copy of the normal allele is enough to hide the phenotype of the recessive trait. You can see in figure 1 how both the genotypes AA and Aa yield the same phenotype and all the children of an AA parent will have the normal phenotype.

Figure 1. Autosomal recessive aa x AA

If the altered gene version is responsible for a disease, the affected people have two altered alleles (aa). Heterozygous Aa are not affected by the disease, but they are carriers and could pass the altered gene on to their children. Affected children usually are born to couples where both parents are carriers but not affected by the disease. As can be seen in figure 2, their children have a 25% chance of being affected, 50% chance of being a carrier and 25% chance of not being affected and not being a carrier. It is transmitted equally to sons and daughters and the disease is usually not present in all the generations of the family. If one parent is affected (aa) and the other is a carrier (Aa) the likelihood of having an affected child goes up to 50% (figure 3).

Figure 2. Autosomal recessive Aa x Aa.

Figure 3. Autosomal recessive Aa x aa.

If you want to read about autosomal recessive inheritance pattern in Spanish, you can follow the link herencia autosomica recesiva.

Autosomal Dominant Inheritance Pattern

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Autosomal dominant is one of the possible ways that genetic traits can be inherited. In this case, the gene is located in one of the 22 autosomal chromosomes and one copy of the altered allele is enough to develop the altered phenotype (orange color in the figures) instead of the normal one (grey color in the figures). You can see in figure 1 how both the genotypes AA and Aa yield the same phenotype and all the children of an AA parent will have the altered phenotype.

Figure 1. Autosomal dominant AA x aa.

If the altered gene version is responsible for a disease, the affected people usually have only one altered allele (Aa), unless both his parents were affected and he inherited two altered alleles, but this is quite infrequent. The children of an affected parent usually have a 50% chance of inheriting the disorder. It is transmitted equally to sons and daughters and the disease is usually present in all the generations of the family as can be seen in figure 2:

Figure 2. Autosomal dominant Aa x aa.

If you want to read about autosomal dominant inheritance pattern in Spanish, you can follow the link herencia autosomica dominante.

How genetic traits are inherited?

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My father has brown eyes and my mother green eyes, what eye color would their children have?

Well, my eyes are brown, my brother's are blue and my sister's are green!

To explain how this happens first we need to introduce some definitions:

ALLELE: For a gene in a specific position there could be more than one variant. These variants are called alleles. For example for the gene of the eye color, the version of the gene that encodes for brown eyes is one allele and the version that encodes for blue is another allele of this gene.

GENOTYPE / PHENOTYPE: Each one of us has two complete set of genes, one inherited from the mother and the other from the father. The specific alleles of your genes are the genotype. For example you have inherited the brown (B) allele from your father and the blue (b) allele from you mother, so your genotype is Bb. The phenotype is the expression in your body of your genotype combined with the environment. In the previous example if the genotype for eye color is Bb and you have the brown and the blue alleles the phenotype would be brown eyes because this allele is dominant. An example to illustrate the effect of the environment is height: you may have inherited from your parents a genotype that predisposes you to be taller than the average, but if during your childhood yo do not have access to enough food and nutrients (environment), the resulting phenotype is that you will not be as tall as you could have been just according to your genotype.

HOMOZYGOUS / HETEROZYGOUS: Homozygous for one gene is when you have both alleles (the one from the father and the one from the mother) identical. For example regarding to eye color being BB (two brown alleles) or bb (two blue alleles). Likewise, heterozygous is when you have two different alleles. For example Bb (one brown allele and one blue allele). The phenotype of an heterozygous depends on the dominance.

DOMINANT / RECESSIVE: A dominant allele masks the expression of a recessive allele. For example the phenotypic eye color of the heterozygous Bb is brown like the phenotype of the homozygous BB. In this case we can say that the brown allele B is dominant over the blue allele b (usually the dominant allele is typed in uppercase and the recessive allele in lowercase). The phenotype of the recessive allele will only be observed as a phenotype in the homozygous recessive bb.

AUTOSOMAL / X-LINKED: Humans have 23 pairs of chromosomes: 22 pairs of autosomes and a pair of sex chromosomes (women have XX and men XY). The two chromosomes of each pair of autosomes and each X in the XX pair are equivalent, however the pair XY in the men is not equivalent since the Y chromosome is significantly smaller that the X chromosome. As a consequence, some of the genes in the X chromosome of a man are in single copy as they are not present in the Y chromosome. This makes the inheritance partters of autosomal and x-linked genes different.

Coming back to the eye color inheritance, I have to say that eye color is a polygenic trait, so it is the interaction of several genes that originates the phenotypic eye color that we can see, but it can be roughly simplified to a gene with three alleles for the shake of making an easily understandable example to explain the concepts involved in genetic inheritance. We can assume that there are three different alleles: brown (B), green (g) and blue (b); being brown dominant over green and blue, green dominant over blue, and blue always recessive.

In the following figure you can see the phenotype and the genotype of the eye color in my family:

The phenotype is obvious, is the eye colour that you can see, but how is it possible to know the genotype?

Well, it is just logic deduction. For every person there are two alleles and we know at least one that it is the same as the phenotype. For example, since my father has brown eyes I know he has one brown allele B, but the second allele could be B, g or b since the 3 genotypes BB, Bg and Bb would express the same phenotype of brown eyes. Likewise, I know that my mother has one green allele g because her eyes are green, but the second allele could be g or b (it could not be B or she would have brown instead of green eyes). The key is my brother. He has blue eyes and b is recessive compared to brown and green so his genotype has to be bb. The only way he can be bb is that he has inherited one b allele from each parent, so now I know that my father is Bb and my mother gb. My sister is gb since she has inherited the green allele from my mother and the blue one from my father (it is the only combination compatible with her green eyes phenotype) and I am the only one that I can not tell the genotype for sure. I have a brown allele from my father for sure, since my eyes are brown, but I can not know if I have inherited the blue or the green allele from my mother. I will have to wait to have children to have more data. :)

After this introduction to how the genetic traits are inherited, in future posts we will see in detail the different genetic inheritance patterns:

• Autosomal dominant inheritance.
• Autosomal recessive inheritance.
• X-linked dominant inheritance.
• X-linked recessive inheritance.
• Mitochondrial inheritance.

If you want to read about genetic inheritance patterns in Spanish, you can follow the link patrones de herencia genetica and mutacion de novo.