Sex-Linked Disorders

Carriers and Genotypes

• Some people can carry a genetic mutation their entire lives without ever showing a single symptom.
• A carrier is an individual who possesses one dominant functioning allele and one recessive disorder allele. Their genotype is heterozygous (e.g., $X^R X^r$).
• The dominant allele completely masks the recessive one, meaning a carrier has a completely normal phenotype.
• Crucially, males cannot be carriers for X-linked recessive traits. Because they only have one X chromosome, they are either entirely affected ($X^r Y$) or completely unaffected ($X^R Y$).

Worked Example: Sex-Linked Inheritance

A female carrier for haemophilia has a child with a father who does not have the condition. Calculate the probability of their next child being a son with haemophilia.

Step 1: Identify the parent genotypes.

• Let $X^H$ be the normal dominant allele and $X^h$ be the recessive haemophilia allele.
• Mother's genotype: $X^H X^h$ (Carrier)
• Father's genotype: $X^H Y$ (Unaffected)

Step 2: Construct the Punnett square.

Step 3: Analyze the specific outcomes.

• Offspring genotypes: 1 $X^H X^H$ : 1 $X^H X^h$ : 1 $X^H Y$ : 1 $X^h Y$
• The only affected son is the $X^h Y$ genotype.

Step 4: Calculate the final probability.

• 1 out of 4 possible offspring is an affected male.
• Probability = $25\%$

Tracking Inheritance Patterns

• Tracking sex-linked traits through family trees requires careful attention to who passes which chromosome to whom.
• A father will pass his Y chromosome to all his sons, meaning it is biologically impossible for a male to pass an X-linked disorder down to his son.
• A son will always inherit his X chromosome from his mother.
• Therefore, if a son has an X-linked disorder, he must have inherited the recessive allele from his mother (who is either a carrier or affected herself).