Understanding X-Linked Dominant Inheritance

Hypophosphatemia is caused by an X-linked dominant allele. This means the gene responsible is located on the X chromosome, and only one copy of the dominant allele is needed to express the trait.

Parental Genotypes

• Normal Male: Genotype is X^hY. (X^h represents the normal allele, Y is the male chromosome).
• Affected Heterozygous Female: Genotype is X^HX^h. (X^H represents the dominant disease allele, X^h represents the normal allele).

Punnett Square Analysis

We can predict the offspring genotypes using a Punnett square:

Offspring Phenotypes

Analyzing the Punnett square results:

• Daughters (XX): Inherit one X from the father (X^h) and one from the mother (X^H or X^h).
  • X^HX^h: Affected (1 out of 2 daughters)
  • X^hX^h: Normal (1 out of 2 daughters)
• Sons (XY): Inherit the Y from the father and one X from the mother (X^H or X^h).
  • X^HY: Affected (1 out of 2 sons)
  • X^hY: Normal (1 out of 2 sons)

Therefore, $\frac{1}{2}$ of the daughters and $\frac{1}{2}$ of the sons will manifest the disease.