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MCAT

Unit 2: Lesson 1

Foundation 1: Biomolecules

Mendelian inheritance of immunodeficiency disorders

Problem

Immunodeficiency disorders occur when the immune system’s ability to fight infectious disease is compromised. While immunodeficiency disorders vary widely, they are typically characterized by the person’s inability to make sufficient amounts of immune cells or by the production of immune cells that are mutated such that they cannot function properly.
Bruton syndrome, or X-linked agammaglobulinemia (XLA), is a rare genetic disorder caused by mutations in the gene encoding a cytoplasmic protein tyrosine kinase, Bruton’s tyrosine kinase (Btk), a signal transduction molecule downstream of pre-B-cell receptor (PreBCR). In a healthy person, the enzyme is activated by the pre-B-cell receptor and delivers biochemical signals that prompt the B cells to divide or mature.
To better understand the inheritance patterns of Bruton’s syndrome, a family was followed for three generations (Figure 1).
Figure 1 Pedigree of family with cases of Bruton’s syndrome
In another study, researchers followed a family with a different immunodeficiency disorder called hyper-IgE syndrome (Figure 2). There are several forms of hyper-IgE syndrome, which is characterized by mutations that prevent proper neutrophil chemotaxis.
Figure 2 Pedigree of family with cases of hyper-IgE syndrome
According to the passage, mutations in Btk most likely result in:
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