the father is Rh-positive and a homozygous, and the mother is Rh-negative. their first child is normal, but the second has hemolytic disease. Why is the first child normal?
2006-08-27 08:08:23 · 4 answers · asked by Anonymous in Science & Mathematics ➔ Biology
You are talking about erythroblastosis fetalis, when an Rh- woman has an Rh+ baby inside her. Usualy not a problem with the first pregnancy, but after multiple Rh+ pregnancies in the Rh- woman, her body has LEARNED over time how to make anti-Rh+ antibodies and her antibodies can cross the placenta and attack her Rh+ babies blood, causing LYSIS of the baby's rbcs. First child is normal because most mixing of mom-baby's blood occurs AFTER birth of the first child after the placenta tears off. Blood from placenta (which is a mix of mother and baby blood) then goes into the wound where the placenta came off, thus the woman's blood is exposed and LEARNS how to make those antibodies for the NEXT time the Rh antibody-antigen rx is triggered. Each pregnancy can be worse danger for that baby.
2006-08-27 08:14:07 · answer #1 · answered by gopigirl 4 · 0⤊ 0⤋
The mother may be heterozygous for Rh factor with one Rh positive and one Rh negative gene, in which case you would expect 50% children who are homozygous Rh positive and 50% who are phenotypically normal (carriers) but are heterozygous for Rh factor.
To have any children with Rh - related hemolysis something else must be involved.
First off, if the mother was homozygous Rh negative the fact that the firstborn was normal could be related to the fact that the mother would have to have an immune response against the Rh positive protein. During gestation, small amounts of blood leak across the placenta. If the mother's immune system recognizes the Rh positive proteins of the baby's cells late in pregnancy, like close to delivery or at delivery, the immune response may not have had a enough time to develop antibodies against the baby's Rh positive blood cells and the first baby would be born normal but all others would be affected by these antibodies.
Penetrability of the trait could also be related to what happened.
(1) The father may be Rh positive homozygous but the fact that there was a child that suffered from hemolytic disease raises an important question. Was one of the father's Rh aleles defective even though it was Rh positive. This could allow penetrance of the Rh negative trait the mother was carrying (we don't know if she has two or one Rh negative genes).
(2) There might also be an ABO blood compatibility that caused the hemolysis in the second child and we haven't been told ABO blood types of the parents so we can only speculate that this may be a cause.
2006-08-27 15:21:58 · answer #2 · answered by Art 3 · 0⤊ 0⤋
Hemolytic Disease of the Newborn (Kernicterus)
occurs when mother is Rh- and fetus is Rh+
Rh factor contains several antigens. It is found in red blood cells only.
Rh+ means that red blood cells have antigenic agglutinogen D
Rh- has no antigenic agglutinogen and, when in contact with Rh+ blood, forms anti-agglutinogen antibodies which destroy red blood cells
massive destruction of red blood cells in the fetus and newborn induces sever hyperbilirubinemia (kernicterus); bilirubin accumulates in the brain causing impaired development
first child less affected than following siblings
Treatment: passive immunization of the mother by administration of anti-Rh antibodies
2006-08-28 02:31:08 · answer #3 · answered by hamdi_batriyshah 3 · 0⤊ 0⤋
the mother doesn't form antibodies until the birth of the first child; at birth a small amount of the blood from the child enters the mother
2006-08-27 15:11:36 · answer #4 · answered by Ellen N 4 · 0⤊ 0⤋