X-Linked Inheritance Simulator

Parents

Father

Mother

Child outcomes (each 25%)

Child	Genotype	Phenotype	Prob.

By sex

Outcome		Prob.

Chromosome key

Normal X allele (Xᴬ) Disease-allele X (Xᵃ) Y chromosome

How it works

Each parent hands one sex chromosome to every child: the father passes either his X or his Y (50/50, which decides the child's sex), the mother always passes one X but which of her two copies is random (also 50/50). Sons therefore have only a single X — whatever allele sits on it, normal or disease-causing, is expressed directly, since there is no second X to mask it. Daughters have two X's, so a daughter is only affected if both the X she gets from her father and the X she gets from her mother carry the disease allele; one copy makes her an unaffected carrier.

Because a son's fate depends only on his mother, and a daughter's fate depends on both parents, the two rows of the family tree below are built from independent 25%-probability branches: father's gamete (X → daughter, Y → son) crossed with mother's gamete (her first or second X).

Father unaffected, mother carrier — classic case: sons have 50% risk (from mother alone), daughters have 0% risk of being affected but 50% chance of being carriers.
Father affected, mother non-carrier — every daughter receives his disease-allele X and becomes at least a carrier; every son receives only his Y, so 0% of sons are affected.
Father affected, mother carrier — daughters now have a 50% chance of being affected themselves (his X plus her disease-allele X), sons still can never inherit the condition from him.

Unaffected Carrier Affected

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X-Linked Recessive Inheritance Simulator