A mother is diagnosed to have sickle cell disease, her husband is normal. What is the chances of having children with sickle cell disease and sickle cell trait respectively?
**Core Concept:**
Sickle cell disease is an autosomal recessive genetic disorder caused by the inheritance of two mutated HbS genes (HBB: c.6T>G) from parents. In heterozygous state (one normal allele and one mutated allele), a person is considered a carrier for sickle cell trait.
**Why the Correct Answer is Right:**
The correct answer is derived from the principles of inheritance of genetic disorders, specifically autosomal recessive traits. In this case, the mother is a homozygous sickle cell carrier (HbSS), meaning she has two mutated HBB genes, while the father is a heterozygous carrier (HbAS), carrying one mutated and one normal HBB gene.
When the mother and father have children, the possible combinations of HbS gene inheritance are:
1. Homozygous normal (HbAA): Both parents are carriers and have healthy children (Option C)
2. Homozygous sickle cell (HbSS): Only one child is affected with sickle cell disease (Option A)
3. Heterozygous (HbAS): Both parents are carriers, and the child can be a carrier (HbAS) or affected with sickle cell disease (HbSS)
4. Heterozygous normal (HbAC): Only one child is affected with sickle cell trait (HbAS) and the other is a carrier (HbA)
5. Homozygous sickle cell trait (HbSS): Only one child is affected with sickle cell trait (HbAS) and the other is a carrier (HbA)
**Why Each Wrong Option is Incorrect:**
A. Homozygous normal (HbAA): The mother is homozygous sickle cell (HbSS), which means she cannot pass on a homozygous normal (HbAA) genotype to her child.
B. Homozygous sickle cell (HbSS): The father is heterozygous (HbAS), meaning he cannot pass on a homozygous sickle cell (HbSS) genotype to his children.
C. Heterozygous (HbAS): This option is correct, as both parents are carriers, and there is a 25% chance of each child inheriting the sickle cell trait (HbAS) from both parents.
D. Homozygous sickle cell trait (HbSS): This option is incorrect because only one of the parents is a carrier (HbAS), and there is a 50% chance of inheritance, making this option less likely than heterozygous inheritance (HbAS).
**Clinical Pearl:**
Inheritance of sickle cell disease depends on the genotypes of the parents. If one parent is a carrier (HbAS) and the other is normal (HbAA), there is a 25% chance of each child inheriting the sickle cell trait (HbAS) and a 50% chance of each child inheriting the normal genotype (HbAA). The remaining 25% of children will inherit the sickle cell trait (HbAS) from both parents.
In this case, the correct answer is **C** (HbAS), as both parents are carriers of sickle cell