Understand the relationship
The reasoning behind the result
The XY parent's X and Y follow different paths
In this limited model, the XX parent contributes an X bearing A or a. The XY parent contributes its X to the XX chromosome class or its Y to the XY class, each with probability one half. The Y carries no modeled allele at this locus.
An XY offspring's modeled X allele therefore comes from the XX parent. An XX offspring receives one X allele from each parent. The labels describe the simplified chromosome model; they are not a complete account of sex determination or chromosome variation.
Within a class and across all offspring are different probabilities
If half of the XY class expresses a, that is 50% conditional on XY. With the model's one-half XY share, the joint probability of XY and a expression is 25% of all offspring. The result table provides both denominators so they cannot be silently substituted.
With recessive a expression, an XX genotype must be aa while an XY genotype needs its single X to carry a. For dominant A expression, XX needs at least one A and XY needs its single X to carry A. No penetrance, dosage compensation or clinical meaning is inferred.
Follow the numbers
XᴬXᵃ crossed with XᴬY
- The XX parent supplies Xᴬ or Xᵃ at 50% each; the XY parent supplies Xᴬ or Y at 50% each.
- The four combinations XᴬXᴬ, XᴬXᵃ, XᴬY and XᵃY each have probability 25%.
- For recessive a expression, only XᵃY matches in this cross.
- Within XY, its probability is 25% ÷ 50% = 50%; within XX it is zero.
The overall a-expression probability is 25%, with different conditional probabilities in the XX and XY classes.
Quick guide
How to use this calculator
- Select the known genotype of the XX parent and the X allele of the XY parent.
- Choose which generic expression class to summarize under complete dominance in XX.
- Read conditional XX/XY percentages separately from percentages of all offspring.
Calculation method
Calculation and interpretation
Show which X-linked allele comes from each parent and how chromosome-class conditioning changes the denominator.
P(XX genotype)=½ × P(XX-parent allele); P(XY genotype)=½ × P(XX-parent allele); P(expression | chromosome class)=class expression probability ÷ ½.
Worked example
XᴬXᵃ crossed with XᴬY
The overall a-expression probability is 25%, with different conditional probabilities in the XX and XY classes.
P(XX genotype)=½ × P(XX-parent allele); P(XY genotype)=½ × P(XX-parent allele); P(expression | chromosome class)=class expression probability ÷ ½.
Supported inputs
Precision and limits
Educational inheritance model
Uses a diploid, two-allele model with equal segregation. It does not interpret a person's genetic test, assess disease or reproductive risk, establish parentage, or predict a real offspring's traits. Linkage, selection, incomplete penetrance, mutation, viability differences and environmental effects are excluded.
One X-linked locus only
Assumes equal X/Y transmission and a fully specified XX × XY model. Y-linked inheritance, pseudoautosomal loci, aneuploidy, mosaicism, X-inactivation effects and sex-specific viability are not modeled.
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