Key pedigree terms and symbols
Chart construction always begins with the proband — the individual whose pedigree is being studied (indicated by an arrow on the chart). Their brothers and sisters are called sibs (siblings).
Designation of individuals and health status:
- Square — male, circle — female, diamond — sex unknown.
- White symbol — healthy individual, black — affected by the studied trait (patient).
- Half-shaded symbol (or with a dot inside) — heterozygous carrier of a recessive gene.
- Additional markers: ! — personally examined, + (strike-through) — deceased.
Designation of relationships:
- Horizontal line between symbols — marriage (double line indicates consanguinity, dashed line — non-marital union).
- Broken line indicates divorce, horizontally crossed-out line — infertile marriage.
Offspring and pregnancy outcomes:
- Children are depicted in a "cluster" extending from the marriage line, arranged strictly from left to right in birth order.
- Monozygotic twins branch out as a triangle from a single common point, dizygotic twins simply form an angle.
- Miscarriage is indicated by a triangle, medical abortion by a small black circle, stillbirth by a large black circle.
Autosomal inheritance patterns
This type of inheritance is associated with non-sex chromosomes (autosomes), so the ratio of affected males and females in the first and second generations is equal.
- Autosomal dominant inheritance. The trait is transmitted "vertically" and appears in every generation. Due to incomplete penetrance and variable expressivity, a rare trait is usually inherited by half of the offspring. Both parents transmit it to offspring with equal probability. A classic example is achondroplasia.
- Autosomal recessive inheritance. Transmission is "horizontal" (often seen among sibs, while their parents may be completely phenotypically healthy). The risk of trait manifestation in the grandchild generation is 25%. In a test cross, the probability of transmission is 50%. A typical example is albinism.
Sex-linked inheritance
Here, trait transmission depends on the sex-determining chromosomes (X or Y).
- X-linked dominant. Females are affected more frequently because they have two X chromosomes. If a homozygous mother is affected, 100% of the children will inherit the trait. If she is heterozygous, 50% will (regardless of sex). An affected father will transmit the gene to all his daughters, but all his sons will be healthy. Examples: amelogenesis imperfecta, X-linked hypophosphatemia.
- X-linked recessive. This type accounts for only 1–2% of mutations. Males are predominantly affected. The gene is transmitted to girls from their father (they become healthy carriers). If the mother is a carrier, 50% of her sons will inherit the trait. The pedigree typically shows a tendency toward generation skipping. An example is hemophilia.
- Holandric (Y-linked). The trait is transmitted strictly through the male line — exclusively from father to all sons.
A distinct category is sex-influenced autosomal inheritance, where the manifestation of the trait is dictated by sex (e.g., pattern baldness).
Genetic heterogeneity
Pedigree analysis in practice is often complicated by the fact that inheritance does not always follow a single classical pattern. This is due to the phenomenon of genetic heterogeneity — a situation where the exact same clinical trait can be caused by mutations in completely different genes.
For example, hereditary hearing loss is caused by defects in more than 100 genes located on almost all human chromosomes. Therefore, genetic counseling must always account for the possibility of such complex mechanisms.
Briefly: answers to topical questions
Which diseases are inherited in an X-linked recessive manner?
Hemophilia is inherited in an X-linked recessive manner. This mode of inheritance is characterized by the following features in the pedigree:
- Males inherit the trait more frequently than females.
- The trait is transmitted to girls from their father.
- If the parents are healthy, but the mother is a carrier, 50% of the sons will inherit the trait.
- There is a tendency toward generation skipping: a generation with a high number of affected males is followed by a generation with fewer affected males.
This variant may involve a criss-cross inheritance mechanism: transmission of the trait from mother to son.
What are the criteria for mitochondrial inheritance?
The criterion for mitochondrial inheritance is maternal transmission of the trait. The main characteristics of this mode of inheritance are:
- All mitochondria are of maternal origin, as they are derived from the egg cytoplasm.
- Paternal mitochondria from the sperm do not enter the zygote upon fertilization.
- Maternal transmission is characteristic of mutations in the mitochondrial genome, which consists of circular mitochondrial DNA molecules.
Examples of pathologies with this mode of inheritance include mitochondrial myopathy, mitochondrial cardiomyopathy, and Leber's hereditary optic neuropathy.
To avoid confusing X-chromosome transmission, remember: "From father to daughters only" (sons always receive the Y chromosome), "From mother to all" (a mother transmits her X chromosome with equal probability to both sons and daughters).
Class and exam questions
Who is a proband and how do they differ from sibs?
A proband is the individual with whom data collection and pedigree construction begin (indicated by an arrow on the chart). Sibs are brothers and sisters.
How to distinguish autosomal dominant from recessive inheritance on a chart?
In dominant inheritance, the trait is transmitted "vertically" and is present in every generation. In recessive inheritance, it appears "horizontally": it may suddenly appear in sibs even though their parents are completely healthy.
Can a father transmit an X-linked disease to his son?
No. A son always receives only a Y chromosome from his father, so X-linked traits (both dominant and recessive) are transmitted to sons exclusively from the mother.
What else to study on this topic
- The problem of incomplete penetrance and variable gene expressivity
- The principle and significance of test crossing
- Features of sex-influenced autosomal inheritance
- Genetic heterogeneity using hereditary hearing loss as an example
- Calculating the risk of having an affected child during genetic counseling
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