An affected male infant born to normal parents could be an example of all of the following, except:
**Question:** An affected male infant born to normal parents could be an example of all of the following, except:
A. Autosomal Recessive (AR) inheritance
B. X-linked Recessive (XR) inheritance
C. X-linked Dominant (XD) inheritance
D. Autosomal Dominant (AD) inheritance
**Correct Answer:** C. X-linked Dominant (XD) inheritance
**Core Concept:**
Inheritance patterns are essential to understand in genetics, as they dictate how genetic traits are passed on from parents to offspring. They are classified based on the sex chromosome involved: Autosomal (A), X-linked (X), and Mitochondrial (M). Four types of inheritance patterns are recognized: Autosomal Recessive (AR), X-linked Recessive (XR), X-linked Dominant (XD), and Autosomal Dominant (AD).
**Why the Correct Answer is Right:**
XD inheritance refers to a situation where the affected individual has one normal X chromosome and one mutated X chromosome. The mutated gene is expressed in males (XY) due to the single X chromosome, while females (XX) have two X chromosomes and one is usually masked by the other. In an XD pattern, the carrier female (heterozygous) does not exhibit the trait or disease, while the affected male (homozygous) does.
**Why Each Wrong Option is Incorrect:**
A. Autosomal Recessive (AR) inheritance: In an AR pattern, both the parents are carriers, and the affected individual is homozygous. Neither parent shows the trait.
B. X-linked Recessive (XR) inheritance: In an XR pattern, both parents are carriers, and the affected individual is homozygous. Neither parent shows the trait.
D. Autosomal Dominant (AD) inheritance: In an AD pattern, one affected individual is homozygous, and the other is a carrier (heterozygous). The trait is expressed in both genders.
**Clinical Pearl:**
Understanding inheritance patterns helps in predicting the likelihood of an affected offspring and guiding genetic counseling for couples planning a family. Familiarizing yourself with these patterns can assist in diagnosing genetic disorders and making informed decisions about family planning.