Sex-linked traits do not follow the usual Punnett square, because sons and daughters inherit different chromosomes. This calculator crosses a mother’s and a father’s genotypes for a gene on the X chromosome, draws a sex-coded square and lists the chances of a normal, carrier or affected son or daughter. It covers red-green color blindness, hemophilia and any other X-linked recessive trait, and it has a Y-linked mode for genes that only fathers pass to sons.
A daughter receives one X from each parent; a son receives his mother’s X and his father’s Y. A recessive allele on the X, written Xᵃ here, is enough to affect a son, because the Y carries no matching gene to mask it. A daughter needs Xᵃ from both parents to be affected, so she is much more often a carrier: one Xᵃ, one normal X⁺, and no symptoms. This single fact explains why about 8% of men but under 1% of women have red-green color blindness.
A carrier mother (X⁺Xᵃ) and a normal father (X⁺Y) fill the square with X⁺X⁺, X⁺Xᵃ, X⁺Y and XᵃY. Each is one cell in four: a normal daughter, a carrier daughter, a normal son and an affected son, so half the sons are affected and half the daughters are carriers. An affected father (XᵃY) with a normal mother (X⁺X⁺) gives the opposite picture: every daughter is X⁺Xᵃ, a carrier, and every son is X⁺Y, unaffected. That is why hemophilia in Queen Victoria’s descendants moved from carrier daughters to affected grandsons and never from father to son.
The calculator gives the odds for one couple’s next child. To follow a trait through a whole family and test whether X-linked inheritance fits the pattern you see, draw the family in the pedigree chart maker; for autosomal genes with ordinary dominance use the main Punnett square calculator.
A man has one X chromosome, so a single recessive allele on it causes the condition. A woman has two X chromosomes and needs the allele on both, which is far less likely; with one she is an unaffected carrier.
No. A father gives his son a Y chromosome, not an X, so an X-linked allele cannot travel from father to son. It passes to all of his daughters instead, who become carriers.
Each son has a 50% chance of being affected and each daughter a 50% chance of being a carrier. Over all children that is one affected son, one normal son, one carrier daughter and one normal daughter in four.
The allele travels silently through carrier daughters. An affected man's daughters are all carriers but not affected; their sons then have a 50% chance of hemophilia, so the condition seems to jump from grandfather to grandson.
Yes, if she inherits the allele from both parents, an affected father and at least a carrier mother. The XᵃXᵃ option in the calculator models that case, and all of her sons will be affected.