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Sex-linked Disorders

Sex-linked Disorders

Like autosomes, sex chromosomes contain a variety of genes necessary for normal body function. When a mutation in one of these genes results in biological deficits, the disorder is considered sex-linked.

X-linked Traits

X-linked Traits

In most mammalian species, females have two X sex chromosomes and males have an X and Y. As a result, mutations on the X chromosome in females may be masked by the presence of a normal allele on the second X. In contrast, a mutation on the X chromosome in males more often causes observable biological defects, as there is no normal X to compensate. Trait variations arising from mutations on the X chromosome are called “X-linked”.

X-linked Traits

X-linked Traits

In most mammalian species, females have two X sex chromosomes and males have an X and Y. As a result, mutations on the X chromosome in females may be masked by the presence of a normal allele on the second X. In contrast, a mutation on the X chromosome in males more often causes observable biological defects, as there is no normal X to compensate. Trait variations arising from mutations on the X chromosome are called “X-linked”.

Lethal Alleles

Lethal Alleles

Agouti: A Lethal Allele
Lucien Cuénot discovered lethal alleles in 1905 while studying the inheritance of coat color in mice. The agouti gene is responsible for the color of the coat in mice. This gene codes for an agouti-signaling protein, which is responsible for melanin distribution in mammals. The wild-type allele gives rise to gray-brown coat color in mice, while the mutant allele gives rise to yellow coat color. In addition to coat color, the agouti gene is associated with the yellow...

Nondisjunction

Nondisjunction

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate correctly and move to the opposite poles of the cells. This produces daughter cells with abnormal chromosome numbers. Nondisjunction is common during anaphase I or anaphase II of meiosis. Mutations in synaptonemal complex proteins that attach homologous chromosomes increase the chances of nondisjunction in anaphase I of meiosis I. In contrast, mutations in topoisomerases and condensins that hold sister...

Nondisjunction

Nondisjunction

During meiosis, chromosomes occasionally separate improperly. This occurs due to failure of homologous chromosome separation during meiosis I or failed sister chromatid separation during meiosis II. In some species, notably plants, nondisjunction can result in an organism with an entire additional set of chromosomes, which is called polyploidy. In humans, nondisjunction can occur during male or female gametogenesis and the resulting gametes possess one too many or one too few chromosomes.

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Related Experiment Video

Updated: May 14, 2026

Multi-exon Skipping Using Cocktail Antisense Oligonucleotides in the Canine X-linked Muscular Dystrophy

Multi-exon Skipping Using Cocktail Antisense Oligonucleotides in the Canine X-linked Muscular Dystrophy

Published on: May 24, 2016

Mutations in MED12 cause X-linked Ohdo syndrome.

Anneke T Vulto-van Silfhout¹, Bert B A de Vries, Bregje W M van Bon

¹Department of Human Genetics, Radboud University Nijmegen Medical Centre, Nijmegen, the Netherlands.

American Journal of Human Genetics

|February 12, 2013

Summary

This summary is machine-generated.

Mutations in the MED12 gene cause X-linked Ohdo syndrome Maat-Kievit-Brunner type, a disorder characterized by intellectual disability and distinctive facial features. This finding highlights MED12 as a key gene in Ohdo syndrome pathogenesis.

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Last Updated: May 14, 2026

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Published on: August 24, 2013

Area of Science:

Genetics
Molecular Biology
Developmental Biology

Background:

Ohdo syndrome is a group of disorders involving intellectual disability and facial anomalies, with distinct subtypes.
The Maat-Kievit-Brunner (MKB) type is characterized by X-linked inheritance and facial coarsening in older individuals.

Purpose of the Study:

To identify the genetic cause of Ohdo syndrome MKB type.
To investigate the role of MED12 mutations in this specific subtype of Ohdo syndrome.

Main Methods:

Whole exome sequencing was performed on affected males from two families with Ohdo syndrome MKB type.
Genetic analysis was extended to include nine additional male individuals with Ohdo syndrome.

Main Results:

Three distinct MED12 missense mutations (p.Arg1148His, p.Ser1165Pro, and p.His1729Asn) were identified in affected males across three unrelated families.
These mutations segregated with the Ohdo syndrome MKB phenotype, indicating MED12 as the causative gene for this X-linked disorder.

Conclusions:

Mutations in MED12 are the primary cause of X-linked Ohdo syndrome MKB type.
Aberrant chromatin modification, potentially involving MED12, is implicated in the pathogenesis of Ohdo syndrome, alongside previously identified KAT6B mutations.