6. The following pedigree represents the inheritance of a rare disorder.
Based on the above pedigree. what is the most likely mode of inheritance?
(1) Autosomal dominant
(2) X-linked recessive
(3) X-linked dominant
(4) Y-linked dominant
The most likely mode of inheritance in this pedigree is X‑linked dominant.
Introduction
Pedigree analysis questions on rare genetic disorders often test the ability to distinguish autosomal from sex‑linked and dominant from recessive inheritance using a small family tree. In this CSIR‑NET style problem, only females are affected across generations and there is no male‑to‑male transmission, which are classic clues for X‑linked dominant inheritance.
Reading the given pedigree
In the pedigree, affected individuals are shaded and unaffected are unshaded.
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An affected female in the first generation has unaffected children, one of whom later produces affected daughters but unaffected sons.
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The trait appears in every generation, does not skip, and all affected individuals are female, while males are consistently unaffected despite being present in multiple matings.
These observations immediately suggest:
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A dominant trait (no skipping of generations).
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A sex‑linked, most likely X‑linked, pattern because only one sex (female) is affected and there is no father‑to‑son transmission.
Why option (3) X‑linked dominant is correct
Key pedigree rules for X‑linked dominant traits include:
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Trait appears in every generation (dominant).
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Often more females than males are affected because females have two X chromosomes.
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There is no male‑to‑male transmission, since fathers pass the Y chromosome to sons, not the X.
In the given chart:
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The disorder affects only females across generations and never any male, which fits a dominant X‑linked condition that is either lethal or highly rare in males.
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The presence of unaffected males born to affected or carrier females with no affected sons strongly argues against an autosomal pattern and supports an X‑linked dominant mode.
Therefore, X‑linked dominant (option 3) best explains the pattern.
Why option (1) Autosomal dominant is incorrect
Autosomal dominant pedigrees typically show:
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Both males and females affected in roughly equal numbers because the gene is on a non‑sex chromosome.
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Vertical transmission with affected individuals in each generation, and affected individuals must have at least one affected parent.
If the disorder were autosomal dominant:
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At least some males should be affected, particularly when affected females have sons, because each child, regardless of sex, would have a 50% chance of inheriting the dominant allele.
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The complete absence of affected males despite multiple male offspring makes an autosomal dominant explanation very unlikely.
Thus, autosomal dominant does not fit this pedigree.
Why option (2) X‑linked recessive is incorrect
Typical X‑linked recessive features are:
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Males are affected much more frequently than females, since one mutant allele on the single X chromosome is enough for expression in males.
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Affected males are usually born to carrier females, and there is no father‑to‑son transmission.
In this pedigree:
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No male is affected; all affected individuals are female, which is the opposite of the expected X‑linked recessive pattern.
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For a female to be affected by an X‑linked recessive trait, she would generally need two mutant alleles (father affected and mother carrier), which is not compatible with the unaffected males and the pattern shown.
Therefore, X‑linked recessive cannot explain the observed inheritance.
Why option (4) Y‑linked dominant is incorrect
Y‑linked (holandric) traits show:
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Only males are affected, because only males carry the Y chromosome.
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Every son of an affected father is affected, giving a strict father‑to‑son transmission pattern.
In the given pedigree:
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No male is affected, and all affected individuals are female, which directly contradicts any Y‑linked pattern.
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There is also no father‑to‑son transmission of the trait.
Thus, Y‑linked dominant is clearly not compatible with the pedigree.
Summary of options
| Option | Mode of inheritance | Key expected pedigree pattern | Match with given pedigree |
|---|---|---|---|
| (1) | Autosomal dominant | Both sexes affected, vertical transmission, no sex bias | Fails: no affected males |
| (2) | X‑linked recessive | Mostly affected males, carrier females, no father‑to‑son | Fails: only females affected |
| (3) | X‑linked dominant | Often more affected females, no male‑to‑male transmission, dominant | Fits: only females affected, no skipping |
| (4) | Y‑linked dominant | Only males affected, every affected father has affected sons | Fails: no affected males |
Therefore, the correct answer is (3) X‑linked dominant.
