In an autosomal Recessive (AR) disorder, one parent is normal and the other is carrier and the child is also affected. What is the reason?
**Core Concept:** Autosomal Recessive (AR) disorders are genetic diseases that require two mutated alleles (one from each parent) for an individual to be affected. In this scenario, one parent is a normal homozygote (both alleles are wild type), while the other is a carrier (one allele is mutated, but the individual appears healthy). The child inherits one mutated allele from the carrier parent and one wild-type allele from the normal parent. As a result, the child becomes a heterozygote and develops the disease.
**Why the Correct Answer is Right:** In autosomal recessive disorders, only homozygous genotype (having two mutated alleles) leads to a clinical phenotype (visible symptoms). In this case, the child inherits one mutated allele from the carrier parent and one wild-type allele from the normal parent. The combination of two mutated alleles (one from each parent) is necessary for the child to develop the disease.
**Why Each Wrong Option is Incorrect:**
A. This option is incorrect because it suggests that only a homozygous genotype (two mutated alleles) is required for the child to develop the disease. In reality, the child needs both mutated alleles from both parents.
B. This option is incorrect because it implies that only one mutated allele is needed for the child to develop the disease. In reality, two mutated alleles from both parents are necessary.
C. This option is incorrect as it implies that the child would have a normal phenotype due to inheriting one mutated allele from the carrier parent and one wild-type allele from the normal parent. However, this combination does not result in a clinical phenotype.
D. This option is incorrect as it suggests that the child would develop a disease due to inheriting two wild-type alleles from both parents. In reality, only two mutated alleles can lead to the disease phenotype.
**Clinical Pearl:** A key aspect to remember is that autosomal recessive disorders require two mutated alleles for the phenotype to manifest. This is useful when evaluating family pedigrees and predicting the likelihood of inheritance in future generations. By understanding this principle, you can accurately assess the risk of passing the trait to offspring and provide informed genetic counseling to patients and families.