Unaffected parents but affected son in a pedigree, autosomal recessive or X-linked recessive? How do you decide?
Nisha Verma12th Sample
20 Sept 2026 · 1 min read
Pedigree analysis is eating my marks in mock tests. The usual one: two unaffected parents, a shaded (affected) son, and the other kids (a daughter and a son) unshaded. The question wants "mode of inheritance".
I know both autosomal recessive and X-linked recessive can give an affected child from two normal-looking parents, because the parents would be carriers (or the mother would be a carrier in X-linked). So how do I choose? I tried counting affected males vs females, but in a small family the numbers don't say anything.
I also mix up the disorders themselves. From NCERT my list is: haemophilia, colour blindness (X-linked recessive); sickle-cell anaemia, thalassemia, cystic fibrosis, phenylketonuria (autosomal recessive); and Down's, Klinefelter's, Turner's as chromosomal disorders. Is that right? And what exactly is the cause of sickle-cell, one amino acid change? Which way round was it, I always lose a mark here.
8 answers & replies
Sign in to join the conversation.
[+][–]Ishita GoyalDropperSample· 1mo ago
With only that one family you genuinely can't tell, both fit. Pedigree questions give you something extra to break the tie, so look for these:
- an affected daughter whose father is unaffected, then X-linked recessive is impossible (she'd have got his X and he'd be affected too)
- an affected father with an affected son doesn't rule out X-linked by itself (sons get the Y from dad, so in X-linked the son's allele came from a carrier mum)
- parents who are related (double line in the pedigree), that leans to autosomal recessive
In your exact example, with just one affected son, either works unless the paper says otherwise.
[+][–]Nisha Verma12thSample· 29d agoAuthor
the daughter-with-normal-father check is the one I was missing, thanks everyone. Doing 10 pedigrees tonight with this.
[+][–]Rahul DubeyDropperSample· 1mo ago
What I tell juniors: X-linked recessive has a "criss-cross" pattern. Affected father passes the allele to all daughters (they become carriers) and to none of his sons. Carrier daughters then pass it to half their sons. So you see it skipping generations through females and showing mostly in males.
NCERT's haemophilia example: carrier mother x normal father gives half the sons haemophilic, half the daughters carriers. And a haemophilic daughter needs a carrier or haemophilic mother and a haemophilic father, which is why it's extremely rare.
[+][–]Mehak AroraDropperSample· 1mo ago
your list is right nisha! just to add, phenylketonuria is where the enzyme that converts phenylalanine to tyrosine is lacking, autosomal recessive. And sickle-cell is a single base change in the gene for beta globin chain, so the 6th amino acid is glutamic acid instead of valine. I remember it as "Valine is the Villain" lol
[+][–]Tanvi Deshmukh12thSample· 29d ago
one fix mehak! In sickle-cell it's the other way, glutamic acid is replaced by valine at the 6th position of the beta globin chain (GAG changes to GTG in the DNA, so Glu becomes Val). Normal has glutamic acid, sickle has valine. "Valine is the Villain" still works, just remember the villain replaces the glutamic acid.
[+][–]Mehak AroraDropperSample· 29d ago
oops yes, I wrote it backwards in my head and typed it that way. Glu goes to Val, thank you!!
[+][–]Riya Singh12thSample· 29d ago
Small thing about reading the diagram itself because I used to rush it: square = male, circle = female, filled = affected, half-filled/dot is usually carrier if they mark it, a horizontal line between them is the mating, a double horizontal line means consanguineous (related) marriage, and numbering of generations is I, II, III. Examiners love the double line because rare recessive conditions show up more in children of related parents. Autosomal recessive screams out of that.
[+][–]Farhan ShaikhDropperSample· 29d ago
Exam shortcut I use in the last 10 mins: first ask is it dominant or recessive (affected child from unaffected parents = recessive). Then ask "any affected daughter with normal father?" If yes, autosomal. If every affected person is male and mother's side carries it, X-linked. If it's still ambiguous, I pick autosomal recessive since it's the safer bet, and honestly that has worked ok.
