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The disorder is inherited in an X-linked dominant manner. [ 1 ] [ 2 ] This means the defective gene responsible for the disorder (PHEX) is located on the X chromosome, and only one copy of the defective gene is sufficient to cause the disorder when inherited from a parent who has the disorder.
There are fewer X-linked dominant conditions than X-linked recessive, because dominance in X-linkage requires the condition to present in females with only a fraction of the reduction in gene expression of autosomal dominance, since roughly half (or as many as 90% in some cases) of a particular parent's X chromosomes are inactivated in females.
Sex-limited genes are responsible for sexual dimorphism, which is a phenotypic (directly observable) difference between males and females of the same species regardless of genotype. [3] These differences can be reflected in size, color, behavior (ex: levels of aggression), and morphology .
[24] [25] Some autosomal recessive disorders are common because, in the past, carrying one of the faulty genes led to a slight protection against an infectious disease or toxin such as tuberculosis or malaria. [26] Such disorders include cystic fibrosis, [27] sickle cell disease, [28] phenylketonuria [29] and thalassaemia. [30]
An example pedigree chart of an autosomal dominant disorder An example pedigree chart of an autosomal recessive disorder An example pedigree chart of a sex-linked disorder (The gene is on the X chromosome.) The description of a mode of biological inheritance consists of three main categories: 1. Number of involved loci
X-linked genetic disorders can arise when there is a spontaneous and permanent change in the DNA sequence of an X-linked gene, known as mutation. Traits or diseases caused by X chromosome genes follow X-linked inheritance, the difference between recessive and dominant inheritance affects the probability of an offspring acquiring it from the ...
The difference between dominant and recessive inheritance patterns also plays a role in determining the chances of a child inheriting an X-linked disorder from their parentage. [ citation needed ] X-linked dominant disorders tend to affect females more often because they tend to be developmentally fatal in males.
In a dominant-recessive inheritance, an average of 25% are homozygous with the dominant trait, 50% are heterozygous showing the dominant trait in the phenotype (genetic carriers), 25% are homozygous with the recessive trait and therefore express the recessive trait in the phenotype. The genotypic ratio is 1: 2 : 1, and the phenotypic ratio is 3: 1.