2.4.3 Inheritance Patterns

Understanding Inheritance Patterns 🧬

Inheritance patterns describe how genetic traits and diseases are passed from parents to children. For USMLE purposes, you must be able to recognize these patterns from pedigrees and from short clinical vignettes, and to predict risks for family members. In this chapter, the focus is on the classic Mendelian patterns and a few important non‑Mendelian features that often appear in questions.

Key Concepts for Reading Pedigrees 🧾

A pedigree is a family tree that shows who is affected or unaffected by a trait or disease. Squares represent males, circles represent females. A filled symbol means affected, an empty symbol means unaffected. Horizontal lines show mating and vertical lines show offspring.

You should immediately try to answer three questions when you see a pedigree. First, does the trait appear in every generation or does it skip generations. Second, are males and females affected in roughly equal numbers or is one sex predominantly affected. Third, do affected individuals usually have affected parents or can the disease appear in a child whose parents are both unaffected.

These questions guide you toward autosomal dominant, autosomal recessive, X linked, or mitochondrial patterns, which will be detailed below.

Autosomal Dominant Inheritance 💡

Autosomal dominant disorders occur when a single copy of a mutant allele on an autosome is sufficient to cause disease. An affected person usually has one affected parent. Each child of an affected heterozygous parent and an unaffected parent has a 50% chance of inheriting the mutation.

In pedigrees, autosomal dominant traits typically appear in every generation, which is called vertical transmission. Both males and females are affected and can transmit the disease to their children. Male to male transmission is possible, which helps distinguish autosomal dominant from X linked patterns.

If the affected parent is heterozygous $Aa$ and the unaffected parent is $aa$, then the possible genotypes for each child are $Aa$ or $aa$. The probability of an affected child is:

$$P(\text{affected}) = \frac{1}{2} = 50\%$$

Some autosomal dominant conditions show variable expressivity, which means individuals with the same mutation may show different severity, and reduced penetrance, which means not everyone with the mutation shows clinical signs. These features can make pedigrees look confusing, with apparently “skipped” generations even though the pattern is dominant.

A useful comparison for exam questions is the difference between a new mutation and an inherited mutation. A child may present with an autosomal dominant condition while both parents are clinically normal. This suggests a de novo mutation in the child, especially when the disorder is usually severe and reduces reproductive fitness.

Autosomal Recessive Inheritance 🧩

Autosomal recessive disorders occur when both alleles on an autosome are mutated. Most affected individuals are born to parents who are carriers. Carriers have one normal allele and one mutant allele but are usually clinically unaffected.

In pedigrees, autosomal recessive traits often skip generations which is called horizontal transmission. A classic pattern is several affected siblings within the same generation, while parents and offspring of an affected individual are unaffected. Both males and females are affected equally. Consanguinity, which means mating between related individuals such as cousins, increases the chance that both parents carry the same recessive mutation and is a common clue in exam questions.

If both parents are carriers, genotype $Aa$, the Punnett square gives the following probabilities for each child:

So there is a 25% chance of an affected child, 50% chance of a carrier, and 25% chance of a completely unaffected non carrier.

Autosomal recessive diseases often present in childhood and may be more severe, because both copies of the gene are defective. In populations with high carrier frequency, such as certain ethnic groups for specific diseases, the likelihood of autosomal recessive disease is higher.

X Linked Recessive Inheritance 👨‍👦

X linked recessive disorders involve mutations on the X chromosome. Because males have only one X chromosome, a single mutant allele in a male usually causes disease. Females have two X chromosomes, so a mutation on one X is usually masked by the normal allele on the other X, and they are typically carriers.

In pedigrees, X linked recessive conditions show affected males much more often than affected females. An affected male usually has carrier daughters and unaffected sons if the mother is not a carrier. There is no male to male transmission, because fathers pass their Y chromosome to sons, not their X.

For an affected male, genotype $X^{a}Y$, and a carrier female, genotype $X^{A}X^{a}$, the possible children are:

For a carrier female and a normal male, each son has a 50% risk of being affected and each daughter has a 50% risk of being a carrier. For an affected male and a normal female, all daughters are obligate carriers and all sons are unaffected.

X inactivation in females, also called lyonization, can lead to manifesting carriers. A female carrier may show mild symptoms if the X chromosome with the normal allele is inactivated in many cells. This can give slightly affected females in an otherwise typical X linked recessive pedigree.

X Linked Dominant Inheritance 👩‍👦‍👦

X linked dominant disorders occur when one mutant allele on the X chromosome is sufficient to cause disease in both males and females. This pattern is less common than X linked recessive, but it is important for exams.

In pedigrees, both males and females can be affected, but affected females are often more frequent because they have two X chromosomes and can inherit the mutation from either parent. An affected male will transmit the mutant X chromosome to all of his daughters, who will all be affected, and to none of his sons. An affected heterozygous female has a 50% chance of passing the mutant X to each child, regardless of sex.

A characteristic feature is the absence of male to male transmission, like all X linked traits, but with affected females in multiple generations. Some X linked dominant conditions are lethal in males, so only affected females are seen in pedigrees, and affected males may die before birth or early in life. This can create a pattern where every affected individual is female and each has an affected mother.

Penetrance and variability can still play a role. Due to X inactivation, females with X linked dominant conditions can show a wide range of severity.

Y Linked (Holandric) Inheritance 👨‍👨‍👦

Y linked traits are carried on the Y chromosome and therefore occur only in males. An affected father passes the trait to all of his sons and to none of his daughters. These traits are rare and usually limited to features directly related to male sexual development.

In pedigrees, a Y linked trait shows only affected males, in every generation, with an affected father and all affected sons. The pattern is very simple, but Y linked disorders are not a common focus of USMLE questions, so the main task is to recognize that male to male exclusive transmission points to the Y chromosome.

Mitochondrial Inheritance 🔋

Mitochondrial DNA is inherited almost exclusively from the mother, because sperm contribute essentially no mitochondria to the zygote. Therefore mitochondrial disorders are transmitted through the maternal line.

In pedigrees, mitochondrial traits show that all children of an affected mother can be affected, but none of the children of an affected father are affected, unless the mother is also affected. Both males and females can be affected, but only females transmit the disease to the next generation.

Mitochondria exist in multiple copies within each cell. Heteroplasmy is the presence of a mixture of normal and mutant mitochondrial DNA within a cell or organism. The proportion of mutant mitochondria can vary between tissues and individuals, leading to variable severity of disease, even within the same family.

Complicating Features in Inheritance Patterns 🧠

Several genetic concepts can modify classic inheritance patterns and are frequently tested through short clinical scenarios.

Variable expressivity means individuals with the same genotype show different degrees of phenotypic severity. This does not change the inheritance pattern but makes prediction of clinical features harder. In contrast, penetrance describes the proportion of individuals with a mutation who actually show the phenotype. Reduced penetrance can create apparently “unaffected” carriers in dominant pedigrees.

Pleiotropy occurs when one gene mutation causes multiple different clinical effects in different organ systems. Locus heterogeneity means mutations in different genes at different loci can produce the same clinical phenotype, so the same disease in unrelated families might follow different inheritance patterns if different genes are involved.

Anticipation is a feature in some disorders where disease severity increases, or age of onset becomes earlier, in successive generations. This is often due to expansion of unstable DNA repeats. The pedigree may show increasingly severe disease or earlier onset from grandparents to parents to children, even though the formal mode of inheritance remains the same.

Genetic imprinting, mosaicism, and other advanced concepts have their own detailed patterns, but for basic inheritance analysis it is most important to recognize when clinical features do not perfectly match a simple Mendelian pattern and to think about these modifying concepts.

Using Probabilities in Inheritance Questions 📊

USMLE questions often ask you to calculate recurrence risk for future children. Once you have identified the inheritance pattern, combine that information with carrier probabilities to obtain the final risk.

For instance, if a woman has a brother with an autosomal recessive disease and both parents are carriers, but the woman is healthy, her probability of being a carrier is 2/3. This is because the three possible genotypes for an unaffected sibling of an affected individual are AA, Aa, and aA, so 2 of 3 are carriers. If her partner is from the general population with carrier frequency $q$, the probability of an affected child is:

$$P(\text{affected child}) = P(\text{mother is carrier}) \times P(\text{father is carrier}) \times \frac{1}{4}$$

In this example:

$$P(\text{affected child}) = \frac{2}{3} \times q \times \frac{1}{4} = \frac{q}{6}$$

Recognizing when to apply the 2/3 carrier risk for an unaffected sibling in autosomal recessive disease is particularly important for exam calculations.

Summary of Classic Mendelian Patterns 📚

The table below summarizes the main distinguishing features you should recall quickly for exam questions.

Understanding these patterns and the associated probability rules allows you to solve most inheritance questions on USMLE Step 1 and Step 2 CK efficiently.