Week 1 - L7 - X-Linked Recessive Inheritance and Genetic Disorders

Genetic Notation and Principles of X-Linked Inheritance

• General Representation of X-Linked Alleles:

  • When documenting X-linked traits, alleles are written in superscript to the right of the $X$ chromosome.

  • It is considered best practice to always include the $Y$ chromosome in the genotype to provide the full context of the individual's sex, even though the $Y$ chromosome does not carry the allele for the specific X-linked disease.

• X-Linked Dominant Conditions:

  • In a dominant model (using the letter $D$ as an example), the presence of the capital allele ($X^D$) renders the person affected.

  • Male Genotypes:

    • Affected: $X^D Y$

    • Healthy: $X^d Y$

  • Female Genotypes:

    • Affected: $X^D X^D$ (homozygous dominant) or $X^D X^d$ (heterozygous).

    • Healthy: $X^d X^d$ (homozygous recessive).

• X-Linked Recessive Conditions:

  • In a recessive model, only the lowercase allele ($X^d$) causes the disease phenotype in the absence of a dominant allele.

  • Male Genotypes:

    • Healthy: $X^D Y$ (possessing the dominant healthy allele).

    • Affected: $X^d Y$ (possessing the recessive disease allele).

  • Female Genotypes:

    • Healthy: $X^D X^D$ (homozygous dominant) or $X^D X^d$ (heterozygous/carrier).

    • Affected: $X^d X^d$ (homozygous recessive).

Biological Basis and Mechanisms of X-Linked Recessive Inheritance

• Hemizygosity in Males:

  • Males are described as hemizygous because they possess only one $X$ chromosome.

  • Because they lack a second $X$ chromosome to potentially carry a compensatory dominant healthy allele, a single recessive allele on their lone $X$ is sufficient to express the disease phenotype.

  • Consequently, X-linked recessive conditions disproportionately affect males.

• Transmission Patterns:

  • Absence of Male-to-Male Transmission: An affected male ($X^d Y$) cannot pass an X-linked trait to his sons. This is because he contributes the $Y$ chromosome to his male offspring, while the son's $X$ chromosome must come from the mother.

  • Daughter Obligate Carriers: An affected male will pass his affected $X^d$ chromosome to all of his daughters, making them at least carriers (if the mother is unaffected).

  • Carrier Mothers: Affected males are typically born to unaffected mothers who are asymptomatic carriers ($X^D X^d$). These mothers may have affected male relatives (e.g., brothers or maternal uncles).

• Affected Females:

  • Females being affected by X-linked recessive conditions is rare but possible under specific circumstances:

    1. The father is affected ($X^d Y$) and the mother is a carrier ($X^D X^d$) or also affected ($X^d X^d$).

    2. Occurs occasionally due to non-random $X$ inactivation (a process where the healthy $X$ chromosome is silenced in a high proportion of cells).

Pedigree Analysis and Probability Outcomes

• Identifying X-Linked Recessive Traits in Pedigrees:

  • The trait often appears to skip generations.

  • Look for "obligate carriers"—individuals who must carry the allele based on the phenotypes of their parents or offspring.

  • If a healthy father and carrier mother have children, the probability breakdown is as follows:

    • Daughters: $50\%$ chance of being homozygous normal ($X^D X^D$) and $50\%$ chance of being an asymptomatic carrier ($X^D X^d$). None will be affected.

    • Sons: $50\%$ chance of being healthy ($X^D Y$) and $50\%$ chance of being affected ($X^d Y$).

Example 1: Color Blindness (Daltonism)

• Historical Context:

  • Named "Daltonism" after John Dalton, an English chemist.

  • Dalton published the first scientific paper on the subject in $1798$.

• Characteristics:

  • An example stimulus for testing shows that color-blind individuals may only see a yellow circle, while those with normal vision can see a faint brown (or reddish-brown) square.

• Prevalence in Australia:

  • Males: Approximately $8\%$

  • Females: Approximately $0.4\%$

  • The high disparity between male and female prevalence confirms its status as an X-linked recessive condition.

• Reciprocal Cross Experiments:

  • Experiment A: Normal female ($X^D X^D$) × color-blind male ($X^d Y$) results in $100\%$ of offspring (both male and female) having normal vision in the $F_1$ generation.

  • Experiment B (Reciprocal): Color-blind female ($X^d X^d$) × normal male ($X^D Y$) results in all female offspring being carriers with normal vision, but all male offspring being color-blind. This discrepancy between the two crosses is a hallmark of X-linked inheritance.

Example 2: Hemophilia A and B

• Pathophysiology:

  • A disorder of blood coagulation (clotting) caused by variants in specific genes.

  • Hemophilia A: Deficiency in clotting Factor VIII ($F8$).

  • Hemophilia B: Deficiency in clotting Factor IX ($F9$).

• Prevalence (Male Births):

  • Hemophilia A: $1$ in $5,000$ to $10,000$ newborns.

  • Hemophilia B: Significantly rarer at $1$ in $100,000$ newborns.

• Clinical Presentation:

  • Failure of blood to clot normally.

  • Symptoms include external bleeding and dangerous internal bleeding into soft tissues and muscles.

  • Bleeding can start hours or days after physical trauma and persist for weeks.

• Treatment and Outlook:

  • Current treatment involves regular intravenous (IV) replacement of the missing clotting factors.

  • Life expectancy has improved drastically due to medical intervention: from an average of only $1.5$ years in the early $1900\text{s}$ to approximately $65$ years today.

Case Study: The "Royal Disease"

• History in the British Royal Family:

  • Queen Victoria was a spontaneous carrier of the hemophilia allele.

  • Offspring Impact:

    • Her daughter Alice was a carrier.

    • Her son Leopold was affected and ultimately died in his thirties following a fall.

  • Spread through Europe: Because the British royals married into other European dynasties, the allele was introduced into the Prussian, Russian, and Spanish royal families, leading to the nickname "The Royal Disease."