Final answer:
The probability of an Rh-negative mother reacting to her second baby's Rh-positive blood, when the father is heterozygous for the Rh-factor allele, is 50%. This is due to the Mendelian inheritance of the Rh factor, which leads to a 50% chance of the child having Rh+ blood and causing HDN if the mother has been sensitized.
Step-by-step explanation:
The probability that an Rh-negative mother will react to her second baby's blood if the father is heterozygous for the Rh-factor allele depends on the genetics of blood type inheritance. The Rh factor follows a simple Mendelian inheritance pattern, where the Rh+ allele is dominant and the Rh- allele is recessive. If the father is heterozygous (Rh+/-) and the mother is Rh- (-/-), each child has a 50% chance of inheriting the Rh+ allele from the father and a 50% chance of being Rh- like the mother.
Therefore, there is a 50% probability (answer B) that a second baby will have Rh+ blood and cause the mother, who is already sensitized from her first pregnancy with an Rh+ child, to produce anti-Rh antibodies and potentially develop an immune response known as hemolytic disease of the newborn (HDN).