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Q: Why is O the most common blood type when it is recessive?

Krishna: Blood type is inherited. O type is the most common despite being a recessive gene because it is more highly expressed in the gene pool, while type A and type B are dominant (and type AB is codominant) but are less common because they are less expressed in the gene pool.

Q: Can two parents with O blood type have a child with A?

Krishna: Two O parents will get an O child nearly all of the time. But it is technically possible for two O-type parents to have a child with A or B blood, and maybe even AB (although this is really unlikely). In fact, a child can get almost any kind of blood type if you consider the effect of mutations.
Q: Which blood group parents pass on harmful blood types and consequences?
Krishna: Harm comes in when both parents have a different blood type. There are two blood types, Rhesus negative and Rhesus positive. The Rhesus factor is that sign + or – that appears after your blood group. Remember parents pass their blood groups allele to their children
Q: What blood types should not have babies together?
Q: I have heard that certain combinations of blood types in parents cannot possibly give rise to particular blood types in their offspring - indeed, several plays and stories have used this as a plot device to reveal at some point that a person's social father could not have been his/her biological father. Is this true, or a myth? And if true, what are the "impossible" combinations?
Krishna: If a person of O blood group breeds with a person of B group all the children must be either B or O. If the child is A or AB one of the individuals cannot be the parent. An O and B crossing can not produce an A or AB child. An AB with an O can produce A children or B children but not O.
Q: Is it possible, through mutation, for two blood type O (OO) parents to have a non type O child? Along the same lines, it is possible for mutation to account for any blood type in a child or are there limitations on which mutations are possible?
Krishna: Yes, changes in the DNA -- also known as mutations -- can cause these kinds of uncommon scenarios. In fact, there are documented cases where things like this have happened!
Q: What harm can ABO incompatibility cause?
Krishna: The new born can have jaundice because of ABO incompatibility.
ABO incompatibility is the most common maternal-fetal blood group incompatibility and the most common cause of hemolytic disease of the newborn (HDN).
ABO incompatibility is one of the diseases which can cause jaundiceABO incompatibility happens when a mother's blood type is O, and her baby's blood type is A or B. The mother's immune system may react and make antibodies against her baby's red blood cells.
The expression of ABO incompatibility in most of the cases is mild due to the lower expression of A and B Antigens on fetal red cells. ABO incompatibility has affected the first pregnancy and is milder in the subsequent pregnancies.

ABO incompatibility in the newborn generally presents as neonatal jaundice due to a Coombs positive hemolytic anemia and occurs in 0.5-1% of newborns. In contrast to the severe intrauterine or neonatal hemolytic anemia associated with Rh sensitization, clinically important neonatal anemia due to ABO incompatibility occurs infrequently. The major clinical issue with HDN due to ABO incompatibility is jaundice.

Several reasons have been proposed to account for lack of intrauterine hemolysis due to ABO incompatibility. These include less well developed A and B antigens on fetal red blood cells to stimulate maternal antibody production, and the ubiquitous distribution of A and B antigens in other tissues resulting in fewer antibodies that cross the placenta to bind to antigens on fetal red cells. The most important reason that ABO incompatibility does not cause hydrops fetalis is that naturally occurring anti-A and anti-B antibodies are IgM and do not cross the placenta.

It is estimated that <1% of type-O mothers have clinically significant anti-A or anti-B antibody which is IgG. ABO incompatibility with transplacental transfer of IgG anti-A antibody, or more commonly, anti-B antibody has rarely been reported in association with intrauterine hemolysis leading to hydrops fetalis. It should be noted that other causes of nonimmune hydrops fetalis have not been systematically excluded in these case reports. The most likely explanation for these rare cases is that exposure to antigens similar to the A or B antigen may occur from sources other than the fetus and stimulates IgG production in the mother. For example, an antigen similar to the B antigen is found in E.coli, and exposure to this organism may stimulate production of IgG anti-B antibody in type O individuals.

In contrast to Rh incompatibility, which tends to become more severe with each subsequent Rh positive pregnancy, ABO incompatibility does not demonstrate any consistent pattern. Thus, the patient’s first offspring may have clinically important hemolytic disease of the newborn due to ABO incompatibility, while subsequent newborns may be unaffected or very mildly affected. Because of the rarity of severe intrauterine hemolysis due to ABO incompatibility, assessment for intrauterine fetal anemia is not recommended based on the mother having type O blood. Measurement of IgG anti-A and anti-B antibody may be considered part of the evaluation of unexplained signs of fetal anemia such as ascites or hydrops when the mother is type O. Collecting a cord blood sample at birth for blood type and direct antibody testing should be considered when the mother is type O and a previous child had hemolytic disease of the newborn due to ABO incompatibility.

Q: How does ABO incompatibility happen?
Krishna: ABO incompatibility happens when a mother's blood type is O, and her baby's blood type is A or B. The mother's immune system may react and make antibodies against her baby's red blood cells. The consequences and treatment are similar to Rhesus disease.
Q: What causes blood type incompatibility?
Krishna: Blood type incompatibility only becomes a problem after a mother develops antibodies against her baby's blood cells. These antibodies don't develop until a mother is “sensitized,” which occurs when the mother's and baby's blood mix during pregnancy.
Q: How is it treated?
Krishna: ABO incompatibility is treated in newborns by light therapy (phototherapy). On rare occasions an exchange transfusion may be necessary. Full recovery usually occurs with no lasting repercussions.
Q: How long does ABO incompatibility last in a baby?
Krishna: Can persist up to 12 weeks after birth.
Q: What is the golden blood type?
Krishna: Rhnull, the Rarest Blood Type on Earth, Has Been Called the "Golden Blood" The extremely rare blood type is shared by only a handful of people on Earth. There are eight common blood types.
Q: What causes Coombs disease?
Krishna: A blood disease called autoimmune hemolytic anemia happens when antibodies destroy your own red blood cells faster than your body can make them. You can get it because of: Diseases like lupus and leukemia. Infections such as mononucleosis.
Q: Which is the most powerful blood group?
Krishna: Of the eight main blood types, people with type O have the lowest risk for heart disease. People with types AB and B are at the greatest risk, which could be a result of higher rates of inflammation for these blood types. A heart-healthy lifestyle is particularly important for people with types AB and B blood.

Q: Which blood type is the best?
Krishna: Types O negative and O positive are best suited to donate red blood cells. O negative is the universal blood type, meaning that anyone can receive your blood. And O- and O+ blood are both extra special when it comes to traumas where there is no time for blood typing.
Q: What blood type is most common?
Krishna: O-positive. In general, the rarest blood type is AB-negative and the most common is O-positive.

Q: Can your blood type change?
Krishna: Almost always, an individual has the same blood group for life, but very rarely an individual's blood type changes through addition or suppression of an antigen in infection, malignancy, or autoimmune disease. Another more common cause of blood type change is a bone marrow transplant.

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