Question
A student is given a pedigree chart showing a trait that appears in some family members. How do you determine whether the trait is X-linked (dominant or recessive) rather than autosomal?
Solution — Step by Step
Count how many males vs females are affected. If far more males show the trait than females, that’s a strong signal of X-linked recessive inheritance. Males have only one X chromosome, so a single recessive allele on it is enough to express the trait — no second X to “cover” it.
If any affected father passes the trait to his son, the trait CANNOT be X-linked. Why? A father gives his Y chromosome to his son, not his X. So if the trait jumps from father to son, it must travel on an autosome. If you see NO father-to-son transmission, X-linkage stays on the table.
For X-linked recessive, females are often carriers — they carry one affected X but one normal X, so they don’t show the trait. Look for unaffected mothers whose sons are affected. That pattern (normal mother, affected sons) is a textbook X-linked recessive flag.
For X-linked dominant, an affected father passes the trait to ALL his daughters but NONE of his sons. This is the key test. Also, affected mothers (who are heterozygous) pass to roughly half their children of both sexes. If you see this criss-cross dominant pattern, suspect X-linked dominant.
Write your answer stating: (a) the evidence you found in the pedigree, and (b) which type of X-linked inheritance it matches. Examiners want your reasoning, not just the label.
Why This Works
X-linked traits show these patterns because of how X chromosomes are distributed during meiosis. Males inherit their X from their mother and their Y from their father — so males always express whatever allele is on their single X, whether dominant or recessive.
Females inherit one X from each parent, making them capable of being carriers. This asymmetry between sexes is the diagnostic fingerprint of X-linked inheritance.
The father-to-son test is the single most powerful rule. No X passes from father to son — ever. So if you confirm transmission from an affected father to an affected son, you can cross X-linkage off your list immediately.
Alternative Method
Sometimes pedigrees are tricky because few individuals are shown. Use elimination:
- Assume autosomal first and see if it fits — skip-generations and carrier patterns can look similar.
- Then test X-linked recessive: if all affected individuals are male (or mostly male), and their mothers are unaffected, it fits.
- If an affected female appears, she must have TWO affected X chromosomes (homozygous for X-linked recessive) — her father must be affected too. If that condition isn’t met, X-linked recessive is ruled out.
The “no father-to-son” rule is your quickest shortcut. Check that first. If father-to-son transmission exists anywhere in the pedigree, cross out all X-linked possibilities immediately.
Common Mistake
Students often confuse “X-linked recessive” with “only males get it.” Females CAN be affected if they are homozygous (both X chromosomes carry the recessive allele). This happens when an affected male marries a carrier female. Check whether any affected female’s father is also affected — if yes, X-linked recessive is still valid.