An X-linked locus lies on the X chromosome. In an XY system, inheritance patterns differ from autosomal inheritance because XX individuals carry two X-linked allele copies while XY individuals are hemizygous for most X-linked loci.

X-linked recessive inheritance

Suppose $X^A$ is a dominant allele and $X^a$ produces a recessive phenotype.

An XX individual normally shows the recessive phenotype only with

$$X^aX^a,$$

whereas an XY individual shows it with the single genotype

$$X^aY,$$

because there is no second X-linked allele at that locus to mask $X^a$.

Consider a heterozygous XX parent crossed with an unaffected XY parent:

$$X^AX^a\times X^AY.$$

The first parent makes $X^A$ and $X^a$ gametes; the second makes $X^A$ and $Y$ gametes. The four equally likely zygotic combinations are

$$X^AX^A,\quad X^AX^a,\quad X^AY,\quad X^aY.$$

Thus half of the XY offspring are expected to show the recessive phenotype, while none of the XX offspring do in this cross.

Father-to-son transmission is absent for X-linked loci

In the simplified human XY system, a father gives a Y chromosome to a son. Therefore an X-linked allele carried by the father is not transmitted directly from father to son.

A father transmits his X chromosome to all daughters. This asymmetry is a powerful clue in pedigree analysis.

X-linked dominant inheritance

If an X-linked allele is dominant, one copy can produce the phenotype in an XX individual as well as in a hemizygous XY individual.

For an affected heterozygous mother,

$$X^AX^a,$$

each child has a $1/2$ probability of receiving $X^A$, independent of whether the other gamete contributes X or Y.

For an affected father with genotype

$$X^AY,$$

all daughters receive his $X^A$, while no sons receive it from him.

Pattern recognition follows chromosome transmission

The useful rules are not arbitrary pedigree mnemonics. They follow directly from which parent contributes X or Y chromosomes and from whether one or two allele copies are present.

The reasoning order is:

identify sex-chromosome genotype
        ↓
identify which X-linked allele each gamete can carry
        ↓
combine gametes
        ↓
apply dominant/recessive phenotype rule

This produces X-linked inheritance patterns without requiring a separate set of magical genetic laws.