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9.6.1. Multiple allelism

Multiple allelism

Mendel, in his experiments, established that for each locus  of a certain character there were only two different alleles: A and a, B and b, etc. But if it is the case that there are several different possible alleles, it will be a multiple allelism phenomenon, and the set of alleles belonging to said locus forms an allelic series.

An example of multiple allelism that is very familiar to us is the one that determines the human blood groups of the AB0 system.

In principle, people were classified according to the four possible phenotypes (A, B, AB, and 0), depending on whether or not agglutination occurred when mixing erythrocytes with another blood group.

Agglutination occurs because antigenic substances on the surface of erythrocytes react with amino acids present in blood serum.

People in group A have antigenic specificity A in their erythrocytes, people in group B, with antigenic specificity B, and those in group AB with antigenic specificities A and B, are called anti A and Anti B.

The blood sera of type A people carry anti-B, those of type B carry anti-A, those of type AB none, and those of type 0 carry both.

This case consists of an allelic series made up of three alleles A, B and 0. Alleles A and B determine the production of antigens A and B, respectively, while 0 does not produce antigen. Alleles A and B are codominant among themselves and both are dominant in relation to 0. The AB0 system can be summarized as follows:

Genotype

Phenotype

Antigenic specificity

Serum antibodies

AA, A0

TO

TO

Anti-B

BB, B0

B

B

Anti-A

AB

AB

A and B

None

00

0

None

Anti-A and Anti-B

There can be problems with blood transfusions when the recipient's amino acids react with the donor's red blood cells. The inverse case is not important, since the amino acids of the serum are diluted in the receptor before producing agglutination. Therefore, people with blood group AB are universal recipients and those with blood group 0, universal donors.

Nomenclature for genetic problems

In multiple allelism problems, the trait is determined by more than two alleles for a gene. The most typical example is the blood group of the AB0 system. Each individual presents in his genotype only two of the possible alleles. To make this idea clear, a nomenclature will be used in which all the alleles for that gene are represented using the same letter (I), and differing in the superscripts (IA , IB , I0) or subscripts IA, IB, I0). The allele I0 can also be represented by i to express its recessivity with respect to the other two.

Remember that the number 0 is not the letter O.

Blood group inheritance

Blood type 0 A B AB
Genotype ii (00) IAi (AO) IAIA (AA) IBi (BO) IBIB (BB) IAIB (AB)
0 ii (00) 0
OO OO OO OO
0 o A
A0 00 A0 00
A
AO AO AO AO
0 o B
B0 00 B0 00
B
BO BO BO BO
A o B
AO BO AO BO
A IAi (AO) O o A
AO AO OO OO
O o A
AA AO AO OO
A
AA AA AO AO
O, A, B o AB
AB AO BO OO
B o AB
AB AB BO BO
A, B o AB
AA AB AO BO
IAIA (AA) A
AO AO AO AO
A
AA AO AA AO
A
AA AA AA AA
A o AB
AB AO AB AO
AB
AB AB AB AB
A o AB
AA AB AA AB
B IBi (BO) O o B
BO BO OO OO
O, A, B o AB
AB BO AO OO
A o AB
AB AB AO AO
O o B
BB BO BO OO
B
BB BB BO BO
A, B o AB
AB BB AO BO
IBIB (BB) B
BO BO BO BO
B or AB
AB BO AB BO
AB
AB AB AB AB
B
BB BO BB BO
B
BB BB BB BB
B or AB
AB BB AB BB
AB IAIB (AB) A or B
AO AO BO BO
A, B or AB
AA AO AB BO
A or AB
AA AA AB AB
A, B or AB
AB AO BB BO
B or AB
AB AB BB BB
A, B, or AB
AA AB AB BB

Other blood groups

Although it is not another case of multiple allelism, as we have talked about blood groups, we take the opportunity to comment on another system such as the Rh factor (its name comes from the rhesus monkey).

The Rh factor is an integral membrane protein of red blood cells. Rh positive is dominant over Rh negative. It is said to be Rh positive if the red blood cells have a certain protein on their surface, and Rh negative if it lacks it. 85% of the population has a dominant structure in this protein, which corresponds to a certain sequence of amino acids that in common language are usually called Rh+.

The transfusion of blood from an Rh+ to an Rh– that does not have this agglutinogen causes the formation of antibodies, which in successive donations can agglutinate the blood (form clots). Hence, this factor is taken into account in blood and organ donations.

If an Rh- woman has a child with an Rh+ man, the child is likely to have problems. If the fetus is Rh+, within its mother Rh-, and it is the mother's first pregnancy, there is usually no problem because the fetus' blood does not enter the mother's circulatory system during pregnancy.

If the two bloods mix during delivery when the placenta is detached, the mother's body recognizes the Rh protein as foreign and produces antibodies against them.

If the Rh- woman becomes pregnant again with another Rh+ child, her Rh antibodies will identify the Rh proteins on the surface of the red blood cells as foreign bodies and pass into the baby's blood to destroy them. This problem is known as hemolytic disease or Rh disease of the newborn. There is a treatment with Rh globulin.

Genetic problems solved: Rh blood groups (4th ESO).

Another system to classify blood groups is through the MN system, which can be MM, NN or MN. Alleles that determine the system the MN blood group are codominant and are represented by the symbols LM   and LN. The base letter (L) is used in honor of the discoverers of the blood group (Landsteiner and Levine).

Questions that have come out in University entrance exams (Selectividad, EBAU, EvAU)

Genetic problems solved:  Rh blood groups  (4th ESO). It contains questions that have come out on EvAU exams from various universities.