**Question:** In an Autosomal Recessive (AR) disorder, one parent is normal and the other is a carrier, and the child is also affected. What is the reason?

**Core Concept:** Autosomal Recessive (AR) disorders are genetic conditions where an individual needs to inherit two copies of the mutated gene (one from each parent) to develop the disease, due to the presence of a normal copy of the gene on one of the chromosomes. In such cases, one parent is a carrier, meaning they have one normal gene and one mutated gene.

**Why the Correct Answer is Right:**

In an Autosomal Recessive (AR) disorder, one parent is normal (homozygous for the wild-type gene) and the other is a carrier (heterozygous for the mutated gene). When these parents have a child, there are four possible combinations of genes that can be inherited:

1. Parent 1 (homozygous wild type) and Parent 2 (homozygous wild type): Neither parent has the disorder, and the child is also unaffected (homozygous wild type).
2. Parent 1 (homozygous wild type) and Parent 2 (heterozygous): The child inherits one wild type gene from each parent and remains unaffected (heterozygous).
3. Parent 1 (heterozygous) and Parent 2 (homozygous wild type): The child inherits one wild type gene from Parent 1 and one mutated gene from Parent 2 and develops the disorder (heterozygous).
4. Parent 1 (heterozygous) and Parent 2 (heterozygous): The child inherits one mutated gene from each parent and develops the disorder (homozygous heterozygous).

In this case, the child is affected because both parents are carriers (heterozygous) for the mutated gene, and the child inherited two mutated genes (homozygous) due to a combination of these two parents being carriers.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because the child would have inherited two wild type genes from a normal parent, resulting in no disease expression.
B. This option is incorrect because the child would require two mutated genes from carriers to develop the disease, as explained in the correct answer and shown in option D.
C. This option is incorrect for the same reason as Option A. The child would inherit two wild type genes from normal parents, preventing disease expression.

**Clinical Pearls:**

1. Autosomal Recessive disorders can be prevented when both parents are tested and do not carry the mutated gene, ensuring they have no affected children.
2. Genetic counseling is crucial for families with a history of Autosomal Recessive disorders to prevent the disease from being transmitted to future generations.

**Correct Answer Explanation:**

D. In this scenario, the child inherits two mutated genes from the heterozygous parents, leading to the expression of the Autosomal Recessive disorder in the offspring.