You have identified a novel DNA variant in the FGFR3 gene located on chromosome 4 on the 4p arm in an individual with a neurogenetic disorder. The variant is a missense mutation (c.1138GA) resulting in an amino acid substitution of glycine to arginine at position 380 (p.Gly380Arg) in the FGFR3 protein. Functional studies show that this substitution causes a gain of function by constitutively activating the receptor, and the allele is also known to be heterosufficient for the diseased phenotype.

Which of the following Mendelian inheritance patterns is this variant most likely to follow?

Question

You have identified a novel DNA variant in the FGFR3 gene located on chromosome 4 on the 4p arm in an individual with a neurogenetic disorder. The variant is a missense mutation (c.1138G>A) resulting in an amino acid substitution of glycine to arginine at position 380 (p.Gly380Arg) in the FGFR3 protein. Functional studies show that this substitution causes a gain of function by constitutively activating the receptor, and the allele is also known to be heterosufficient for the diseased phenotype.

Which of the following Mendelian inheritance patterns is this variant most likely to follow?

BlackTom AI · Accepted Answer

The scenario describes a missense variant in FGFR3 (a gene on an autosome) that causes a gain-of-function with constitutive receptor activation. In addition, the variant is described as heterosufficient for the disease phenotype, meaning a single pathogenic allele can produce the disease. These factors together point toward an autosomal dominant pattern, where one mutated copy is enough to manifest the condition.

Option 1: Autosomal recessive. This would require two pathogenic copies (one from each parent) for the phenotype to appear. The described gain-of-function effect from a single allele and the notion of heterosufficiency argue against recessive inheritance, so this choice is inconsistent with the data.

Option 2: X-linked dominant. The FGFR3 gene is not located on a sex chromosome; it's on chromosome 4, an autosome. Therefore, an X-linked pattern is unlikely and does not fit the autosomal context of the variant described.

Option 3: X-linked recessive. Similar to the previous, X-linked patterns involve genes on the X chromosome and typically affect males more severely when the variant is recessive. This contradicts the autosomal location and the heterosufficient dominant effect described.

Option 4: Mitochondrial inheritance. Mitochondrial conditions are inherited maternally and involve mitochondrial DNA, not a nuclear autosomal gene like FGFR3. This option does not fit the chromosomal location or inheritance suggested.

Option 5: Autosomal dominant. Given that a single pathogenic allele suffices (heterosufficient) and the variant yields a gain-of-function with constitutive FGFR3 activation, this aligns with a classic autosomal dominant pattern where one altered copy drives disease.

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