Autosomal dominant type of inheritance with incomplete penetrance of traits. Autosomal dominant diseases The trait is inherited in an autosomal dominant manner if

If the disease is caused by a rare autosomal dominant gene, then the vast majority of patients in the population are born in marriages between an affected and a healthy spouse. In this case, one of the parents is heterozygous for the autosomal dominant gene (Aa), and the other is homozygous for the normal allele (aa).

In such a marriage, the following variants of genotypes in the offspring are possible (Fig. IX.5).

Every future child, regardless of gender, has a 50% chance of receiving both the A gene (and therefore being affected) and the “normal” A gene and being healthy. Thus, the ratio of the number of healthy children in the offspring to the number of affected ones is 1:1 and does not depend on the gender of the child.

To date, more than 2,500 autosomal dominant human traits have been described. The most common in clinical practice are the following monogenic diseases with an autosomal dominant type of inheritance: familial hypercholesterolemia, hemochromatosis, Marfan syndrome, neurofibromatosis type 1 (Recklinghausen disease), Ehlers-Danlos syndrome, myotonic dystrophy, achondroplasia, osteogenesis imperfecta and others. In Fig. IX.6 depicts a pedigree characteristic of an autosomal dominant type of inheritance. A typical example of an autosomal dominant disease is Marfan syndrome, a generalized connective tissue disorder. Patients with Marfan syndrome are tall, have long limbs and fingers, and characteristic skeletal changes in the form of scoliosis, kyphosis, and curvature of the limbs. The heart is often affected; a characteristic sign is subluxation of the lens of the eye. The intelligence of such patients is usually preserved.

In some autosomal dominant diseases, there are cases of generation skipping, or “skipping,” i.e. the individual has an affected parent and an affected offspring, but remains healthy (Fig. IX.7).

Dominantly inherited diseases are characterized by wide clinical polymorphism, even among relatives of the same family.

For example, with Marfan syndrome, one patient may experience minor musculoskeletal disorders and mild myopia, while another may have severe chest deformities, joint damage, retinal detachment and aortic aneurysm.

Patients with autosomal dominant forms of pathology are often socially adapted and can have children, but in the future each of their children has a 50% risk of having a similar disease.

However, there are also autosomal dominant diseases in which reproductive function is either reduced or completely disrupted. A significant proportion of patients with such diseases are new mutants, i.e. they received (a pathological gene from one of the phenotypically normal parents, in whose germ cells a mutation occurred. A new mutation is a fairly common phenomenon for autosomal dominant, severe diseases (Table 1X.1). An example is achondroplasia - a severe skeletal lesion with pronounced shortening of the limbs and increased head size (pseudohydrocephalus).In 80% of patients, the disease is registered as a sporadic case, resulting from a mutation that arose in the germ cells of one

from parents. It is very important to identify such cases (of a new mutation), since the risk of having the next sick child in a given family does not exceed the population one.

In general, the main signs that allow one to suspect an autosomal dominant type of inheritance of the disease are the following:

1) the disease manifests itself in every generation without gaps. Exceptions are cases of a new mutation or incomplete penetrance (manifestation) of the gene;

2) each child of a parent with an autosomal dominant disease has a 50% risk of inheriting this disease;

3) males and females are affected equally often and to the same extent;

4) there is a “vertical” nature of transmission of the disease in the pedigree, i.e. a sick child has a sick parent;

5) unaffected family members are free from the mutant gene, and in this regard, the risk of having an affected child is comparable to the frequency of the mutation.

Autosomal dominant pathologies

The most famous autosomal dominant diseases are Huntington's chorea, Marfan syndrome, Holt-Oram syndrome, neurofibromatosis, sickle cell anemia, and periodic paralysis. A characteristic sign of these pathologies is a violation of the synthesis of structural or specific proteins (for example, hemoglobin).

The effect of the mutant gene is almost always manifested. Affected boys and girls are born with equal frequency.

Huntington's chorea.

It occurs with a frequency of approximately 1: 10,000 to 1: 20,000. mutant gene Nv, which causes this disease is localized in the short arm of the fourth chromosome (4-p16.3) (Fig. 5.2). The mutation consists of an increase in the number of triplet repeats (TSRs) of the gene region that encodes the final part of the huntingtin protein molecule, the function of which is not yet known. Normally, the number of repeats varies from 11 to 34 triplets. In patients it can be from 37 to 100 or more. The more repeats a mutant gene has, the earlier the disease begins. Men get sick more often than women. The basis of the pathology is progressive damage to brain cells, mainly the basal ganglia (striatum), in which the patient’s brain shrinks in size by about 20-30%.

Rice. 5.2.

Characteristic signs of the disease are chaotic involuntary contractions of the muscles of various parts of the body and behavioral disorder. The disease can begin with one of these symptoms or both at once at any age, but most often its first signs appear at 30-50 years of age.

Huntington's chorea develops gradually. The first symptoms may be restlessness, fussiness of movements, which neither the patient nor his relatives regard as a disease. However, over time, the abnormalities progress and can lead to disability. Characterized by frequent, sudden, irregular convulsive movements of the limbs or torso, spasms of the facial muscles, sobbing, and speech impairment are possible. Coordination of movements when walking worsens: it becomes dance-like (trochaic). Memory does not deteriorate until the later stages of the disease, but attention, thinking and executive functions are weakened early on. Depression, indifference, detachment, irritability, and loss of control over behavior are often observed. In some cases, delusions and obsessive states develop, and therefore schizophrenia is mistakenly diagnosed.

The duration of the disease varies, but the average is 15 years. In the case of early onset (before 20 years), the pathology is accompanied by a steady increase in muscle tone, impaired coordination of movements and progresses faster (average duration is eight years), frequent epileptic seizures are possible.

In most cases, Huntington's chorea occurs at the age of 40-50 with progressive involuntary movements accompanied by muscle cramps, as well as severe mental disorders (memory impairment, depression, suicide attempts, loss of emotional control with frequent outbursts of irritation and aggression).

Huntington's chorea is aggravated by the fact that signs of pathology usually appear in middle age, when many patients already have children. Once symptoms appear, life expectancy is up to 15 years. This slow decline is an additional source of distress for patients and their families. The gene encoding Huntington's chorea is dominant, it is always found, so if one of the parents is affected, the probability of having an affected child is 50%.

There is no specific treatment for the disease. Disorders of motor activity and behavior are reduced by the use of certain medications.

Marfan syndrome.

It consists of systemic damage to connective tissue and is characterized by high penetrance and variable expressivity. Its frequency is 1: 10,000-20,000. The disease is caused by a mutation of the I ^ BL / I gene, localized in the long arm of chromosome 15 (15 ^ 21.1) (Fig. 5.2). A large number of mutations of this gene have been discovered, which leads to significant clinical polymorphism of the disease. The ^MAGI gene encodes the synthesis of the fibrillin protein, which is a constituent of connective tissue and ensures its elasticity. Blocking the synthesis of this protein leads to increased stretching of the connective tissue.

Marfan syndrome affects the musculoskeletal, cardiovascular and visual systems. Patients have a characteristic appearance: tall stature, asthenic (weak, weak) physique (Fig. 5.3). Disorders of the musculoskeletal system are disproportionately long fingers (arachnodactyly - “spider” fingers), an elongated skull, deformation of the chest (funnel-shaped or keeled), curvature of the spine, excessive mobility of the joints, flat feet. Characteristic disorders of the cardiovascular system are protrusion of the mitral valve towards the left atrium, expansion of the aorta in the ascending or abdominal section with the development of an aneurysm (protrusion). The pathology of the visual organs consists of high myopia due to subluxation (or displacement) of the lens and different colors of the iris. Inguinal, femoral, and diaphragmatic hernias can also occur, and sometimes kidney prolapse, pulmonary emphysema, and hearing loss up to complete deafness. Despite all these disorders, the mental and mental development of patients is normal.

The life expectancy of a patient with Marfan syndrome is determined by the degree of damage to the cardiovascular system and reaches an average of 35 years.

Treatment is mainly symptomatic: medications to slow down the destruction of the aorta, hormonal drugs to stimulate proportional puberty in girls. Massage, therapeutic exercises, and sometimes reconstructive cardiovascular surgery have a positive effect.

Rice, 5.3.

Holt-Oram syndrome (hand-heart syndrome).

It is accompanied by multiple congenital malformations. The frequency of the disease has not yet been determined. Gene mutations TVHH, located in the long arm of chromosome 12 (12 ^ 24.1), lead to the absence of its product, resulting in the development of the disease (Figure 5.2).

The clinical picture of Holt-Oram syndrome is characterized by abnormalities of the upper extremities and congenital heart defects. Defects in hand development vary from underdevelopment or absence of the first finger of the hand or its triphalanx to underdevelopment or complete absence of the radius bone with the formation of radial clubhand. The left hand is most often affected. Other skeletal changes are also observed: underdevelopment of the shoulder blades and collarbones, scoliosis (lateral curvature of the spine), funnel-shaped deformation of the sternum, curvature of the little finger, fusion of the fingers, underdevelopment of other fingers. In 50% of patients, the first finger is not opposed to the other fingers of the hand (Fig. 5.4).

The majority of patients (up to 85%) exhibit various forms of congenital heart defects: atrial and ventricular septal defects, patent ductus arteriosus (normally present in the fetal circulatory system), narrowing of the aorta and pulmonary artery, protrusion of the mitral valve towards the left atrium, etc. . The intelligence of patients with Holt-Oram syndrome is usually preserved. Life prognosis depends on the severity of heart damage.

Rice. 5.4.

Treatment of Holt-Oram syndrome consists of medication to prevent the development of infectious heart diseases (for example, endocarditis) and reconstructive surgery of the heart septum or valves.

Examples: Marfan syndrome, hemoglobinosis M, Huntington's chorea, colon polyposis, familial hypercholesterolemia, neurofibromatosis, polydactyly.

Peculiarities of inheritance: ❖ one of the patient’s parents is usually sick; ❖ the severity and number of manifestations depend on the action of environmental factors; ❖ the frequency of pathology in males and females is the same; ❖ there are patients in each generation (the so-called vertical distribution of the disease); ❖ the probability of having a sick child is 50% (regardless of the gender of the child and the number of births); ❖ unaffected family members, as a rule, have healthy offspring (since they do not have the mutant gene).

Autosomal recessive mode of inheritance

❖ monotonous manifestations of the disease (due to high penetrance); ❖ symptoms of the disease are usually detected in childhood; ❖ the frequency of pathology in males and females is equal; ❖ in a pedigree, pathology manifests itself horizontally, often in siblings; ❖ the disease is absent in half-blooded (children of the same father from different mothers) and half-brothers and sisters (children of the same mother from different fathers); ❖ the appearance of an autosomal recessive pathology is more likely in consanguineous marriages due to the greater likelihood of meeting two spouses who are heterozygous for the same pathological allele received from their common ancestor.

X-linked dominant inheritance

Examples: one of the forms of hypophosphatemia is vitamin D-resistant rickets, Charcot-Marie-Tooth disease X-linked dominant, orofacial-digital syndrome type I. Features of inheritance: ❖ affecting males and females;

❖ men have a more severe course of the disease; ❖ transmission of a pathological allele by a sick man only to daughters, but not to sons (sons receive the Y chromosome from their father); ❖ transmission of the disease to both sons and daughters by a sick woman is equally likely.

X-linked recessive inheritance

Examples of diseases: hemophilia A, hemophilia B, color blindness, Duchenne-Becker muscular dystrophy, Hunter disease (mucopo-

❖ a carrier of a mutant gene has a 25% chance of having a sick child (50% of boys born are sick).

Hollandric, or Y-linked, type of inheritance

Examples: hypertrichosis of the ears, excess hair growth on the middle phalanges of the fingers, azoospermia.

Peculiarities of inheritance: ❖ transfer of a trait from the father to all sons (only sons, daughters never inherit a trait from their father);

❖ “vertical” nature of inheritance of a trait; ❖ the probability of inheritance for males is 100%;

Mitochondrial inheritance

Examples of diseases (“mitochondrial diseases”): Leber optic atrophy, Leigh syndrome (mitochondrial myoencephalopathy), MERRF (myoclonic epilepsy), familial dilated cardiomyopathy.

Peculiarities of inheritance: ❖ the presence of pathology in all children of a sick mother; ❖ the birth of healthy children from a sick father and a healthy mother (explained by the fact that mitochondrial genes are inherited from the mother).

CHROMOSOMAL DISEASES

The severity of the disorders usually directly correlates with the degree of chromosomal imbalance: the more chromosomal material is involved in the aberration, the earlier the chromosomal imbalance manifests itself in ontogenesis and the more significant the disturbances in the physical and mental development of the individual.

cystism: some of the body's cells have a normal karyotype, and the other part has an abnormal karyotype.

Abnormalities of sex chromosomes. Violation of the divergence of sex chromosomes leads to the formation of abnormal gametes: in women - XX and 0 (in the latter case, the gamete does not contain sex chromosomes); in men - XY and 0. When germ cells merge in such cases, quantitative disturbances of the sex chromosomes occur. In diseases caused by a deficiency or excess of X chromosomes, mosaicism is often observed.

Klinefelter's syndrome: ❖ Frequency: 2-2.5 per 1000 newborn boys. ❖ Karyotype: various cytogenetic variants (47,XXY; 48,XXXY; 49,XXXXY, etc.), but the 47,XXY variant is more common. ❖ Manifestations: high stature, disproportionately long limbs, female-type fat deposition, eunuchoid physique, scanty hair growth, gynecomastia, hypogenitalism, infertility (as a result of impaired spermatogenesis, decreased testosterone production and increased production of female sex hormones), decreased intelligence (the more in karyotype of additional chromosomes, the more pronounced). ❖ Treatment with male sex hormones is aimed at correcting secondary sexual characteristics, but even after therapy, patients remain infertile.

Trisomy X - Jacob's disease - the most common syndrome from the group of polysomy X; frequency 1:1000 newborn girls, karyotype 47.XXX; gender - female, female phenotype; As a rule, the physical and mental development of women with this syndrome does not deviate from the norm.

Shereshevsky-Turner syndrome. ❖ Frequency of the syndrome: 1:3000 newborn girls ❖ Karyotype: 45.X0, but other variants are also found. ❖ Manifestations: short stature, short neck with excess skin or pterygoid fold, wide, often deformed chest, deformation of the elbow joints, underdevelopment of primary and secondary sexual characteristics, infertility. ❖ Early treatment with female sex hormones may be effective.

DISEASES WITH HEREDITARY PREDISPOSITION

Diseases with a hereditary predisposition are also called multifactorial (multifactorial), since their occurrence is determined by the interaction of hereditary factors and environmental factors. Diseases with a hereditary predisposition include coronary heart disease (CHD), hypertension, bronchial asthma, mental illness, diabetes, rheumatic diseases, gastric ulcer, congenital malformations (CD) and many others. Diseases with a hereditary predisposition are classified, depending on the number of genes that determine the predisposition, into monogenic and polygenic.

Monogenic diseases with a hereditary predisposition are determined by one mutant gene and arise under the influence of a specific and obligatory environmental factor. An example is lactose intolerance: with a mutant form of the lactase gene, drinking milk leads to the development of intestinal discomfort and diarrhea.

Polygenic diseases. Predisposition to the development of polygenic diseases is determined by the interaction of normal and altered (mutated) genes, although each of them individually does not lead to the development of the disease. An individual with such a combination of genes, under the influence of a certain environmental factor, reaches the “threshold of occurrence” of the disease and becomes ill.

Characteristics of multifactorial diseases: ❖ inheritance does not correspond to Mendelian laws; ❖ pathogenesis depends on the “specific contribution” of genetic and environmental factors; this dependence is different both for different diseases and for each person; ❖ characterized by the presence of a large number of clinical variants; ❖ There is a higher concordance for the disease in monozygotic twins compared to dizygotic twins.

Physical mutagens. In first place among physical mutagens are ionizing radiation and UV radiation. The peculiarity of ionizing radiation is that it can induce mutations in low doses that do not cause radiation damage.

Chemical mutagens. This group includes acids, alcohols, salts, heavy metals, etc. Chemical mutagens are found in the air (hydrogen sulfide, arsenic, mercaptan, chromium, fluorine, lead, etc.), soil (pesticides and other chemicals), water and food products , in medicines. The strongest mutagen is cigarette smoke condensate, which contains benzopyrene. Smoke condensation and surface crusts formed when frying fish and beef contain tryptophan pyrolysates, which are chemical mutagens. The peculiarity of chemical mutagens is that their effect depends on the dose and stage of the cell cycle. The higher the dose of mutagen, the stronger the mutagenic effect. In this case, the stage of DNA synthesis (S-phase) is most sensitive to the action of mutagens.

Biological mutagens. Bacterial toxins, viruses (herpes viruses, hepatitis, mumps, etc.). In pregnant women, viral infections can provoke the occurrence of mutations in the fetus, which leads to spontaneous abortions.

10. Chromosomal diseases, their mechanisms, methods of study, types of inheritance. Manifestations of major chromosomal diseases and syndromes (47: 21,21,21; 46:1521,21,21; 45:2121; 45:XO; 47:XXX; 47:XXY). The etiological factors of chromosomal pathology are all types of chromosomal mutations and some genomic mutations. : tetraploidy, triploidy, aneuploidy. Moreover, of all the variants of aneuploidy, only trisomies on autosomes, polysomies on sex chromosomes (tri-, tetra- and pentasomies) are found, and among monosomies, only monosomy X is found. From a clinical-cytogenetic point of view, a deletion in one of the homologous chromosomes means a lack of a section or partial monosomy in this area, and duplication - excess or partial trisomy. Syndromes associated with numerical abnormalities of sex chromosomes include: 1. Klinefelter syndrome (47,XXY; 48,XXYY; 48,XXXY; 49,XXXXY). The frequency of occurrence is 1:1000 boys. The number of X chromosomes correlates with the degree of mental retardation. The syndrome was described in 1942. Manifestations of the syndrome: tall stature with disproportionately long limbs, in childhood - a fragile physique, in adults - obesity, hypogenitalism (hypoplasia of the testicles and penis), underdevelopment of secondary sexual characteristics, sometimes female-type hair growth, in 50% cases - gynecomastia. Histological examination revealed hyalinosis and fibrosis of the seminiferous tubules, aspermia. Characterized by decreased sexual desire, impotence, infertility, there is a tendency to alcoholism, homosexuality, and antisocial behavior.2. Shereshevsky-Turner syndrome (45,XO). The frequency of occurrence is 1:3000 newborns. Manifestations of the syndrome: swelling of the hands and feet at birth, skin folds on the neck, short stature (up to 140 cm), congenital heart defects, amenorrhea, infertility, and sometimes decreased mental development. Basically socially adapted, they can get a specialty and work.3. Trisomy X and polysomy X. Frequency of occurrence - 1:1000 girls. It manifests itself as hypoplasia of the ovaries and uterus, infertility, and sometimes mental retardation. As the number of X chromosomes increases, deviations from the norm increase.4. Polysomy Y. Population frequency - 1:1000 boys. Characterized by a tendency towards antisocial behavior and homosexuality.

Examples of syndromes associated with numerical abnormalities of autosomes:

1. Patau syndrome(trisomy 13, 47,XX,+13 or 47,XY,+13). Population frequency is 1:7800 newborns. First described in 1960. It is characterized by microcephaly, polydactyly, cleft lip and palate, low-set ears, microphthalmia, congenital heart defects, ventricular septal defect, kidney anomaly, and malformations of the digestive organs. Cryptorchidism, hypoplasia of the external genitalia, duplication of the uterus and vagina, bicornuate uterus, and hypospadias are observed.

2. Down syndrome(trisomy 21). Population frequency - 1:600-700. Manifestations of the syndrome - flat face, Mongoloid eye shape, epicanthus (skin fold at the inner corner of the eye), open mouth, short nose, flat bridge of the nose, strabismus (strabismus), pigment spots along the edge of the iris (Brushfield spots), flat back of the head, dysplastic ears, arched hard palate, dental anomalies, grooved tongue, joint hypermobility, muscle hypotonia, congenital heart defects, transverse palmar fold, mental retardation, sometimes combined with epilepsy (40%), leukemia (8%). The development of the syndrome is associated with the age of the mother.

3. Edwards syndrome(trisomy 18) - manifestations are similar to Patau syndrome. Population frequency - 1:6500.

With an autosomal dominant type of inheritance, the absolute majority of patients in the population are born in marriages between an affected spouse (heterozygous for the autosomal dominant gene Aa) and a healthy spouse (homozygous for the normal allele Aa), when the following variants of genotypes in the offspring are possible (Fig.).

Thus, the probability of receiving an affected gene A is 50%; the ratio of the number of healthy children in the offspring to the number of affected ones is 1:1 and does not depend on the gender of the child.

Among monogenic diseases with an autosomal dominant type of inheritance, the most common are: familial hypercholesterolemia, hemochromatosis, Marfan syndrome, neurofibromatosis type 1 (Recklinghausen disease), Ehlers-Danlos syndrome, myotonic dystrophy, achondroplasia, osteogenesis imperfecta and others. A typical example of an autosomal dominant disease is Marfan syndrome (Fig.) - a hereditary disease that is a generalized lesion of connective tissue with high penetrance and variable expressivity. Frequency – 1:10,000. The disease is based on a mutation in the gene for fibrillin, a protein that is part of connective tissue and ensures its elasticity. The gene is localized on chromosome 15 in regions 15q21.1. The clinical picture of the syndrome includes damage to three body systems: musculoskeletal, cardiovascular and visual organs. Patients are characterized by tall stature, asthenic physique, disproportionately long fingers (arachnodactyly, or “spider” fingers), dolichocephalic skull, chest deformity (funnel-shaped or keeled), spinal curvature (scoliosis, kyphosis), joint hypermobility, flat feet. From the cardiovascular system, the most characteristic are mitral valve prolapse, dilatation of the aorta in the ascending or abdominal section with the development of an aneurysm. Pathology of the organs of vision in the form of high myopia is associated with subluxation (or displacement) of the lens, heterochrony (different color) of the iris. Inguinal, femoral, and diaphragmatic hernias are often observed. In rare cases, kidney prolapse, emphysema, hearing loss and deafness have been described. The mental and mental development of patients corresponds to the norm. The prognosis for life and life expectancy are determined by the degree of damage to the cardiovascular system.

Rice. Pedigree with an autosomal dominant type of inheritance of the disease (Marfan syndrome)

Rice.Pedigree with autosomal dominant type

inheritance with incomplete penetrance

(Waardenburg syndrome)

In some cases, with autosomal dominant diseases, generational “skipping” is observed (Fig.).

Dominantly inherited diseases are characterized by wide clinical polymorphism. Patients with autosomal dominant forms of pathology are often socially adapted and can have children. However, with certain autosomal dominant diseases, a decrease or complete disruption of reproductive function is noted.

Thus, the main signs of an autosomal dominant type of inheritance of the disease are:

1) the disease manifests itself in each generation without gaps (exceptions are cases of a new mutation or incomplete penetrance (manifestation) of the gene);

2) each child of a parent with an autosomal dominant disease has a 50% risk of inheriting this disease;

3) males and females are affected equally often and to the same extent;

4) there is a “vertical” nature of transmission of the disease in the pedigree, i.e. a sick child has a sick parent;

5) unaffected family members are free from the mutant gene, and in this regard, the risk of having an affected child is comparable to the frequency of the mutation.

Most often, the pathology is transmitted by the autosomal dominant type of inheritance. This is monogenic inheritance of one of the traits. In addition, diseases can be transmitted to children by autosomal recessive and autosomal dominant inheritance, as well as by mitochondrial inheritance.

Types of inheritance

Monogenic inheritance of a gene can be recessive or dominant, mitochondrial, autosomal or linked to sex chromosomes. When crossed, offspring can be obtained with a variety of types of traits:

autosomal recessive;
autosomal dominant;
mitochondrial;
X-dominant linkage;
X-recessive linkage;
Y-clutch.

Different types of inheritance of traits - autosomal dominant, autosomal recessive and others - are capable of transmitting mutant genes to different generations.

Features of autosomal dominant inheritance

The autosomal dominant type of inheritance of the disease is characterized by the transmission of the mutant gene in a heterozygous state. The offspring that receive the mutant allele may develop a gene disorder. At the same time, the probability of manifestation of the altered gene in men and women is the same.

When manifested in heterozygotes, the inheritance trait does not have a serious impact on health and reproductive function. Homozygotes with a mutant gene that conveys an autosomal dominant type of inheritance are, as a rule, not viable.

In parents, the mutant gene is located in the reproductive gamete together with healthy cells, and the probability of receiving it in children will be 50%. If the dominant allele is not completely changed, then the children of such parents will be completely healthy at the gene level. At a low level of penentrance, the mutant gene may not appear in every generation.

Most often, the type of inheritance is autosomal dominant, which transmits diseases from generation to generation. With this type of inheritance in a sick child, one of the parents suffers from the same disease. However, if only one parent in a family is sick, and the other has healthy genes, then the children may not inherit the mutant gene.

An example of autosomal dominant inheritance

The autosomal dominant type of inheritance can transmit more than 500 different pathologies, among them: Marfan syndrome, Ehlers-Danlos syndrome, dystrophy, Recklinghuysen's disease, Huntington's disease.

When studying the pedigree, one can trace the autosomal dominant type of inheritance. There may be different examples of this, but the most striking is Huntington's disease. It is characterized by pathological changes in nerve cells in the structures of the forebrain. The disease manifests itself as forgetfulness, dementia, and involuntary body movements. Most often, this disease manifests itself after 50 years.

When tracing the pedigree, you can find out that at least one of the parents suffered from the same pathology and passed it on in an autosomal dominant manner. If the patient has a half-brother or sister, but they do not show any manifestation of the disease, it means that the parents passed on the pathology for the heterozygous trait Aa, in which gene disorders occur in 50% of children. Consequently, the patient’s offspring may also give birth to 50% of children with the modified Aa gene.

Autosomal recessive type

In autosomal recessive inheritance, the father and mother are carriers of the pathogen. To such parents, 50% of children are born carriers, 25% are born healthy and the same number are born sick. The probability of transmitting a pathological trait to girls and boys is the same. However, diseases of an autosomal recessive nature may not be transmitted to every generation, but may appear after one or two generations of offspring.

An example of diseases transmitted by an autosomal recessive type can be:

Toy-Sachs disease;
metabolic disorders;
cystic fibrosis, etc.

When children with an autosomal recessive type of gene pathology are detected, it turns out that the parents are related. This is often observed in gated communities, as well as in places where consanguineous marriages are allowed.

X chromosome inheritance

The X-chromosomal type of inheritance manifests itself differently in girls and boys. This is due to the presence of two X chromosomes in a woman and one in a man. Females receive their chromosomes one at a time from each parent, while boys receive their chromosomes only from their mother.

According to this type of inheritance, pathogenic material is most often transmitted to women, since they are more likely to receive pathogens from their father or mother. If the father is the carrier of the dominant gene in the family, then all boys will be healthy, but girls will show pathology.

With the recessive type of X-linkage of chromosomes, diseases appear in boys with the hemizygous type. Women will always be carriers of the diseased gene, since they are heterozygous (in most cases), but if a female has a homozygous trait, then she can get the disease.

Examples of pathologies with a recessive X chromosome can be: color blindness, dystrophy, Hunter's disease, hemophilia.

Mitochondrial type

This type of inheritance is relatively new. Mitochondria are transferred with the cytoplasm of the egg, which contains more than 20,000 mitochondria. Each of them contains a chromosome. With this type of inheritance, pathologies are transmitted only through the maternal line. From such mothers all children are born sick.

When the mitochondrial trait of heredity manifests itself, healthy children are born to men, since this gene cannot be transmitted from father to child, since there are no mitochondria in sperm.