Sex-linked and cytoplasmic inheritance
For an X-linked recessive allele, an XY individual with that allele on the X expresses the modeled trait because there is no corresponding normal allele on a second X. A heterozygous XX parent can transmit either X allele. Analyze sons and daughters separately, then combine probabilities only if needed. Genes in organelles can follow cytoplasmic inheritance patterns; in many animals, mitochondrial DNA is inherited from the egg-producing parent. State the model rather than assuming every organism uses the same sex-determination or organelle-inheritance system.
Under an X-linked recessive model, X^A X^a × X^A Y produces an affected probability of 1/2 among sons and 1/4 among all offspring, assuming equal probabilities of XX and XY.
A father does not pass an X-linked allele to a son in the standard XX/XY model. The son gets the father’s Y.