Autosomal inheritance patterns
In autosomal dominant inheritance, the mutant gene is located on an autosome and makes itself known even in the heterozygous state. This includes pathologies such as polydactyly, Marfan syndrome, familial hypercholesterolemia, neurofibromatosis, hemoglobin M disease, Huntington's chorea, and colonic polyposis.
In turn, autosomal recessive diseases require the presence of the mutant gene only in the homozygous state. This group includes galactosemia, phenylketonuria, hemoglobin S disease, albinism, glycogen storage diseases, cystic fibrosis, adrenogenital syndrome, and hyperlipoproteinemia.
X-linked disorders
Pathologies associated with sex chromosomes are divided into dominant and recessive variants.
- X-linked dominant inheritance is characterized by the manifestation of the trait in heterozygotes. Examples include vitamin D-resistant rickets, oral-facial-digital syndrome, frontonasal dysplasia, and cataracts.
- X-linked recessive inheritance manifests predominantly in hemizygotes (males) or homozygotes (females). This list includes hemophilia A and B, color blindness, hypogammaglobulinemia, and Duchenne muscular dystrophy.
Holandric and mitochondrial inheritance
The holandric type represents inheritance strictly linked to the Y chromosome. Such a mutation is transmitted exclusively through the male line—from father to son. Typical examples include hypertrichosis of the ear rims and azoospermia.
Mitochondrial diseases are associated with alterations in mitochondrial DNA. The main feature of this type is maternal transmission exclusively. These include Leber's hereditary optic neuropathy, mitochondrial encephalopathy, myoclonic epilepsy, and cardiomyopathy.
Features of manifestation in homozygotes and heterozygotes
The clinical picture of autosomal dominant diseases in heterozygous patients is often practically identical to that in homozygous patients. However, the severity of the course differs: symptoms are much more severe in homozygotes than in heterozygotes.
Because of this variability, clinical practice and scientific literature sometimes use the working terms "semidominant" or "partially dominant" inheritance.
Briefly: answers to topical questions
Specifically, which clotting factors or genes are impaired in hemophilia A and B?
In hemophilia A and B, the genes encoding plasma blood clotting factors VIII and IX, respectively, are impaired.
- Hemophilia A is caused by a mutation in the gene encoding factor VIII (localization of this gene: Xq28). The disease results from a deficiency of plasma factor VIII components.
- Hemophilia B is caused by a mutation in the gene encoding factor IX (localization: Xq27). It leads to a deficiency of factor IX.
Both diseases have an X-linked recessive inheritance pattern, in which gene mutations disrupt the normal synthesis of the specified blood clotting factors.
What are the molecular mechanisms underlying mitochondrial encephalopathy?
The mitochondrial diseases described in sources, including MELAS syndrome with encephalopathy, are based on the following molecular mechanisms:
- Mitochondrial DNA mutations; in MELAS syndrome, a mutation in the gene encoding tRNA-Leu.
- Impaired synthesis of mitochondrial enzymes: translation is affected, with possible disrupted stages being elongation or recognition.
- Respiratory complex defect: mutations lead to structural defects in complexes I, III, IV, and ATP synthase.
- Decreased ATP synthesis: the efficiency of electron transfer and proton gradient generation is reduced.
- Impaired NADH oxidation: the blockade of the respiratory chain prevents the oxidation of NADH to NAD+, the NADH/NAD+ ratio increases, and excess NADH inhibits isocitrate dehydrogenase and α-ketoglutarate dehydrogenase.
- Tissue vulnerability: nervous and muscle tissues are the first to suffer from oxidative phosphorylation defects due to high ATP demand.
For X-linked recessive diseases, remember: a male is always affected if he receives the defective X chromosome from his mother, since he lacks a second X chromosome for compensation (he is hemizygous).
Class and exam questions
In what state does the mutant gene manifest in an autosomal dominant disorder?
The mutant gene in an autosomal dominant type manifests in the heterozygous state.
How are mitochondrial diseases transmitted?
Mitochondrial diseases are transmitted via mitochondrial DNA exclusively through the maternal line.
Who predominantly inherits X-linked recessive diseases?
They manifest predominantly in hemizygotes (males) or homozygotes (females).
Why are some autosomal dominant diseases called semidominant?
The term is used due to the varying severity of the course: disease manifestations are significantly more severe in homozygotes than in heterozygotes.
What else to study on this topic
- Gene mosaicism in pathologies
- Features of the hemizygous state in males
- Clinical manifestations of Huntington's chorea
- Differences in the course of Marfan syndrome in homozygotes and heterozygotes
- Molecular mechanisms of mitochondrial DNA transmission
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