Basic chart elements
A universal system of symbols is used to construct pedigrees. Generations are designated by Roman numerals (I, II, III), and each individual within a generation is assigned an Arabic numeral sequentially.
- A square indicates a male, and a circle indicates a female.
- A shaded (darkened) symbol indicates an affected person with phenotypic manifestation of the trait.
- A clear (unshaded) symbol corresponds to a phenotypically healthy individual.
- The arrow always points to the proband — the person who served as the starting point for constructing the chart.
X-linked recessive inheritance
This type of gene transmission has clear criteria that are easily identified during generational analysis:
- Predominance of affected males. Since males are hemizygous (they have only one X chromosome), the recessive gene manifests in them immediately.
- Female carriers. They are heterozygous and fall ill extremely rarely (only in cases of homozygosity).
- Absence of father-to-son transmission. An affected male transmits only the Y chromosome to his sons, so they remain healthy.
- Specific "knight's move". An affected father transmits the mutant gene to all his daughters, making them obligate carriers. These phenotypically healthy women pass the pathology on to their sons.
- Skipping generations. The disease may disappear in one generation and reappear in subsequent ones.
Y-linked and maternal inheritance
In addition to X-linked inheritance, other characteristic types of trait transmission are distinguished:
- Y-linked (holandric) type. Affects exclusively males. The pathology is transmitted strictly through the male line: from an affected father to all his sons. Daughters in such a family are always healthy and are not carriers.
- Maternal inheritance. The disease occurs in both sexes, but is transmitted to offspring only from an affected mother. She transmits the trait to absolutely all her children. If the father is affected, all his offspring will be healthy, as he does not transmit this gene.
Chromosomal and genomic diseases
Pathologies associated with changes in chromosome structure or number occur with a frequency of 6 per 1,000 newborns. Most major genomic mutations (polyploidies, autosomal monosomies) are lethal.
The severity rule applies in genetics: the more extensive the chromosomal imbalance, the earlier it manifests in ontogeny and the more severely physical and mental development are impaired.
Mutations are divided into:
- Complete forms: arise in gametes; the altered karyotype is present in all cells of the organism.
- Mosaic forms: arise in somatic cells during early stages of embryogenesis, forming clones of both normal and abnormal cells.
Sex chromosome abnormalities: Klinefelter syndrome
Non-disjunction of sex chromosomes leads to the formation of abnormal gametes (e.g., XX or 0). Their fusion results in quantitative disorders. Mosaicism is rarely observed in such pathologies.
A classic example is Klinefelter syndrome (most commonly the 47, XXY karyotype), occurring in 2–2.5 per 1,000 males.
Clinical manifestations:
- Tall stature and disproportionately long limbs.
- Eunuchoid body habitus and female-pattern fat distribution.
- Sparse body hair and gynecomastia.
- Hypogenitalism and infertility (due to impaired spermatogenesis and decreased testosterone).
A general pattern for such abnormalities: there is an increase in female sex hormone production and a decrease in intellect. Moreover, the degree of intellectual decline correlates directly with the number of extra chromosomes (e.g., in variants 48, XXXY or 49, XXXXXY).
Briefly: answers to topical questions
Which specific hereditary diseases are transmitted via mitochondrial (maternal) inheritance?
Mitochondrial inheritance transmits pathologies that frequently affect tissues with high energy demands, such as the brain and heart. Examples of such diseases include:
- Leber hereditary optic neuropathy — an example of a mitochondrial disease.
- Leigh syndrome — mitochondrial myoencephalopathy.
- Myoclonic epilepsy — transmitted maternally.
- Familial dilated cardiomyopathy — a mitochondrial disease.
What karyotype and clinical features are characteristic of Turner syndrome?
Turner syndrome is characterized by a 45,X0 karyotype; other variants and mosaicism also occur. Clinical features of the disease include:
- Short stature.
- Short neck with excess skin or webbed folds.
- Broad, often deformed chest.
- Deformity of the elbow joints.
- Underdevelopment of primary and secondary sex characteristics.
- Infertility.
For X-linked recessive inheritance, remember the "knight's move" rule: an affected grandfather transmits the gene through his healthy daughter to an affected grandson.
Class and exam questions
How to distinguish Y-linked inheritance from X-linked inheritance on a diagram?
In the Y-linked type, only males are affected, and an affected father transmits the trait to 100% of his sons. In X-linked recessive inheritance, a father never transmits the disease to his sons.
Who are obligate carriers?
These are phenotypically healthy females who are guaranteed to carry the mutant gene. For example, all daughters of an affected father in X-linked recessive inheritance.
Why does an affected father not transmit the disease in maternal inheritance?
In this type of inheritance, the trait is transmitted exclusively from the mother to all her children. The offspring of an affected father always have a healthy phenotype.
What is the difference between complete and mosaic forms of chromosomal disorders?
Complete forms arise due to mutations in germ cells (gametes), so the abnormality is present in all cells of the body. Mosaicism arises in the somatic cells of the embryo, causing the organism to consist of a mixture of healthy and mutant cells.
What else to study on this topic
- Mechanisms of sex chromosome non-disjunction in meiosis
- Cytogenetic variants of Klinefelter syndrome
- Pathogenesis of hypogenitalism and infertility in chromosomal abnormalities
- Effect of female sex hormone excess on phenotype in Klinefelter syndrome
- Differences between structural and numerical chromosomal mutations
Master the entire topic in the "Pathophysiology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on «Pedigree Analysis» and 4,913 more pages of subject material
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 135 video lessons on the subject
Other topics in chapter "Chapter 3. Forms of pathology caused by changes in the genome"
Free video lessons: pathophysiologyShort lessons with lecture notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.