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Difficulty: Very hardApplications of Genetics in Medicine and Agriculture

A Rhesus-negative woman (rrrr) who did not receive anti-Rh immunoglobulin therapy after giving birth to her first Rhesus-positive child is expecting a second child. The father is Rhesus-positive (R−R-), but his mother was Rhesus-negative (rrrr). What is the probability that this second child will suffer from erythroblastosis fetalis, and what physiological mechanism triggers this medical condition?

  1. 50%, because maternal anti-Rh IgG antibodies produced after sensitization during the first delivery cross the placenta and destroy red blood cells if the second fetus inherits the Rhesus-positive allele (RrRr).Answer
  2. B
    100%, because all offspring of a Rhesus-positive father and Rhesus-negative mother inherit the dominant Rhesus antigen, guaranteeing erythroblastosis fetalis in all subsequent pregnancies.
  3. C
    25%, because Rhesus antigens exhibit codominance with ABO blood group alleles, requiring homozygous recessive inheritance of maternal antibody genes to trigger agglutination.
  4. D
    50%, because the mother's immune system acquired Rhesus-positive genes during her first pregnancy and directly passes these acquired genetic adaptations to her second offspring.

Answer

The probability is 50%, occurring because maternal anti-Rh IgG antibodies cross the placenta and agglutinate fetal red cells if the fetus inherits the Rhesus-positive allele.
The father is heterozygous (RrRr) because his mother was Rhesus-negative (rrrr). A cross between a Rhesus-negative mother (rrrr) and a heterozygous father (RrRr) gives a 50% chance of producing a Rhesus-positive (RrRr) child. Because the mother was sensitized during her first pregnancy with a Rhesus-positive child, her immune system contains anti-Rh IgG antibodies. In a second Rhesus-positive pregnancy, these IgG antibodies cross the placenta and lyse fetal red blood cells, causing erythroblastosis fetalis.

Step-by-Step Solution

1
Determine parental genotypes for the Rhesus factor.
Mother is Rhesus-negative (rrrr). Father is Rhesus-positive, but since his mother was Rhesus-negative (rrrr), he must be heterozygous (RrRr).
An individual receiving a recessive allele from one parent must carry that allele.
2
Calculate the genetic probability of the fetus inheriting the Rhesus-positive phenotype.
Cross between mother (rrrr) and father (RrRr) yields genotypes 1/2 Rr1/2\ Rr (Rhesus-positive) and 1/2 rr1/2\ rr (Rhesus-negative). Probability of Rhesus-positive fetus = 50%.
Monohybrid testcross of a heterozygote with a homozygous recessive individual.
3
Evaluate the immunological cause of erythroblastosis fetalis in sensitized mothers.
Sensitization occurred during the delivery of the first Rhesus-positive child when fetal RBCs entered maternal circulation, producing anti-Rh IgG antibodies. If the second child is Rhesus-positive (RrRr), maternal anti-Rh IgG antibodies cross the placenta, causing hemolysis.
IgG antibodies cross the placental barrier, whereas IgM antibodies (such as ABO isoagglutinins) generally do not.

Key Concept

Rhesus Factor Incompatibility and Erythroblastosis Fetalis
Estimated Time:2m 0s
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Topics

Applications of Genetics in Medicine and Agriculture

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