If a rare disease with an early onset of symptoms is inherited in a way where male and female are affected equally and only homozygous persons are affected, then the likely mode of inheritance is
**Question:** If a rare disease with an early onset of symptoms is inherited in a way where male and female are affected equally and only homozygous persons are affected, then the likely mode of inheritance is
A. Autosomal Recessive
B. X-linked Recessive
C. Autosomal Dominant
D. X-linked Recessive
**Correct Answer:** D. X-linked Recessive
**Core Concept:** Inheritance Modes
Inheritance modes are patterns by which genetic traits are passed from parents to offspring. These modes help in understanding the distribution of genetic diseases in families based on the sex of the parents and offspring. The following modes of inheritance are commonly discussed:
1. **Autosomal Dominant (Option C):** In this mode, an individual only needs to inherit one copy of the abnormal gene from a parent to develop the disease. Both male and female carriers are affected.
2. **Autosomal Recessive (Option A):** In this mode, an individual needs to inherit two copies of the abnormal gene (one from each parent) to develop the disease. Male and female carriers are affected equally.
3. **X-linked Recessive (Option D):** In this mode, an individual needs to inherit two copies of the abnormal gene (one from each parent), but the inheritance pattern is different for males and females. Males are affected when they inherit the abnormal gene, while females can be carriers with only one abnormal gene.
4. **X-linked Dominant (Option B):** In this mode, an individual needs to inherit one copy of the abnormal gene (from either parent) to develop the disease. Males are affected, while females can be carriers (with one abnormal gene).
**Why the Correct Answer is D:**
The correct answer is X-linked Recessive (Option D) because:
1. Male and female individuals are equally affected, which matches the equal occurrence of the disease in both sexes.
2. Only homozygous individuals (heterozygous females and homozygous males) are affected, as they have two copies of the gene.
**Why Each Wrong Option is Invalid:**
1. Autosomal Dominant (Option C) cannot be correct because females can be carriers without displaying the disease symptoms.
2. X-linked Dominant (Option B) is invalid because female carriers would also show symptoms, which is not the case in this scenario.
**Core Concept: Understanding X-linked Inheritance**
Understanding X-linked inheritance is crucial for understanding the mode of inheritance of sex-linked disorders. X-linked disorders can be further classified into two categories:
1. **X-linked Recessive (Option D):** In these disorders, affected individuals are homozygous (homozygous for the disease gene).
2. **X-linked Dominant (Option B):** In these disorders, carriers are heterozygous (heterozygous for the disease gene) and can present symptoms.
**Clinical Pearl:**
Recall that X-linked disorders are often characterized by a higher prevalence in affected males, as females are often carriers and do not typically display symptoms. However, if the disease is X-linked dominant, carriers will