As we discussed in “lecture”, Rh is the most complex of the blood group types, i

Question

As we discussed in “lecture”, Rh is the most complex of the blood group types, involving at least 45 different antigens. The most clinically important antigen, D or RhO, is encoded by the gene RhD which is found on chromosome 1. Individuals that are Rh-positive have either one or two RhD genes, whereas the Rh-negative phenotype is caused by the absence of the RhD gene. (The antithetical allele d does not exist, however the letter "d" is used to indicate the D-negative phenotype). For the purpose of this homework, we will simplify things. Assume that the Rh blood group has only two alleles: the Rh-positive allele (D) and the Rh-negative allele (d).

            Erythroblastosis fetalis (EF) is a condition that causes the mother's red blood cells to attack those of the baby as if they were any foreign invaders. It is referred to as hemolytic anemia of the newborn. It is caused by anti-Rh antibodies from the mother which pass through the placenta and attack fetal blood cells that happen to be Rh-positive. Babies that are at risk for this condition are those with Rh-positive blood, whose mothers are Rh-negative (dd).

Side note: Here’s a headline that addresses this condition. It’s well worth a read!

http://www.cnn.com/2015/06/09/health/james-harrison-golden-arm-blood-rhesus/index.html

            Consider a population under Hardy-Weinberg equilibrium, where the frequency of the Rh-negative allele, d, is 0.3. What is the frequency of crosses that could potentially produce children with erythroblastosis fetalis?

Explanation / Answer

Hemolytic disease of new born is known as hemolytic anemia or erythroblastosis fetalis occurring in the new born infant due to sensitization of mother to foreign blood group antigen present in the red cells of fetus. These antigens are not found in the mother and hence immune response is triggered in the mother against fetal red blood cell antigens. Different blood group antigens may cause hemolytic disease and best known antigen is RH antigen.

Hemolytic disease does not occur when first time the blood group incompatible pregnancy occurs. This is because the fetal red blood cells are not produce in significant numbers to sensitize mother immune cells in circulation till the placenta separates from the uterus after delivery. At the time of delivery mother’s circulation is sensitized with foreign blood group antigen and antibodies begin to form. The mothers are affected by immune sensitization against fetal red blood antigens which may or may not exhibit any symptoms

In a subsequent pregnancy with fetus having same blood group antigen, the mother immune system is induced. The antibodies cause destruction of red blood cells leading to hemolytic anemia and jaundice in the affected infant.

When the father is Rh positive (DD) and mother is Rh negative (dd), then there is 100% chance for offspring to be Rh positive and affected with erythroblastosis fetalis.

if the father is heterozygous for Rh factor (Dd) and mother is also heterozygous for Rh factor then there is 75% of chances for offspring getting affected.

If the father is heterozygous (Dd) and mother is homozygous recessive (dd) for Rh factor, then there is 50% chances for the offspring getting affected.

If the father is homozygous dominant (DD) and mother is heterozygous (Dd), then there is 50% chances for the offspring getting affected.

If the father is homozygous recessive (dd) and mother is homozygous dominant (DD), then there is 50% chances for the offspring getting affected.

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