Chapter 25 Principals of Inheritance and Variation Part 2
Whenever the blood of a person receives the foreign proteins (antigen) his blood plasma starts forming the antibodies in order to neutralize the foreign antigens.
having blood group A does not possess antigen B but antibody ‘b’ is found in his blood plasma. Persons with blood group O possess none of the antigens and that is why their blood possesses both the antibodies ‘a’ and ‘b’.
Antigen |
Antibody |
Type of blood group |
% in society |
(1) A |
Anti-B or ‘b’ |
A |
23.5 |
(2) B |
Anti-A or ‘a’ |
B |
34.5 |
(3) A, B |
Absent |
AB |
7.5 |
(4) None |
‘a’ and ‘b’ |
O |
34.5 |
In this way when blood with M group is injected in rabbit it will produce antibodies in the blood serum which will bring about agglutination with blood group M and MN but not with blood of N group. In the same way on injecting blood of N group into the rabbit it will bring about agglutination with blood group N and MN and not with blood having blood group M.
Persons with blood group AB are called universal recipients because both antigens A and B are found in their blood and the two antibodies ‘a’ and ‘b’ are absent. Therefore, such persons can receive blood of all the blood groups. In the same way persons who have blood group O– are universal donors as they lack both the antigens and Rh– person can donate to Rh+ person as well as Rh– person but Rh+ person cannot donate blood to Rh– person. But at the same time such persons can not be given the blood of any other blood group except blood group O because their blood possesses both the antibodies ‘a’ and ‘b’. Persons belonging to blood group A and B contain only one antigen and one antibody against it, in their blood. Such persons can therefore receive blood either of the blood group of their own or the blood group O.
Blood group |
Can accept from |
Can donate to |
Agglutination |
Specific mention |
|||
|
|
|
A |
B |
AB |
O |
|
(1) A |
A, O |
A, AB |
No |
Yes |
No |
Yes |
|
(2) B |
B, O |
B, AB |
Yes |
No |
No |
Yes |
|
(3) AB |
A, B, AB, O |
AB only |
Yes |
Yes |
No |
Yes |
Universal recipient |
(4) O |
O only |
A, B, AB, O |
No |
No |
No |
No |
Universal donor |
Artificial anticoagulants are used to prevent blood clotting in the blood banks. These anticoagulants are added to the blood preserved in bottle. Such anticoagulants include sodium citrate, double oxalates (sodium and ammonium), dicumarol and EDTA (ethylene diamine tetra acetic acid). The whole blood in this way can be stored for a maximum period of 21 days.
parents. Genes controlling blood group in man are three instead of two and are called multiple alleles. All these three genes or alleles are located on the same locus on homologous chromosomes. A person can have only two of these three genes at a time which may be either similar or dissimilar in nature. These genes control the production of blood group/antigens in the offspring. The gene which produces antigen A is denoted by Ia, gene for antigen B by Ib and the gene for the absence of both antigens by Io. it is customary to use the letter I (Isohaemagglutinogen) as a basic symbol for the gene at a locus. Based on this, six genotypes are possible for four blood groups in human population.
|
Genotype |
Nature of gene |
Type of blood group |
(1) |
Ia Ia |
Homozygous Dominant |
A |
(2) |
Ia Io |
Heterozygous |
A |
(3) |
Ib Ib |
Homozygous Dominant |
B |
(4) |
Ib Io |
Heterozygous |
B |
(5) |
Ia Ib |
Codominant |
AB |
(6) |
Io Io |
Homozygous Recessive |
O |
The alleles Ia and Ib of human blood group are said to be codominant because both are expressed in the phenotype AB. Each produces its antigen and neither checks the expression of the other. There is codominance as well as dominant recessive inheritance in the case of the alleles for the blood groups in human beings. The alleles Ia and Ib are codominant and are dominant over the allele Io (Ia = Ib > Io). The human blood groups illustrate both multiple allelism and codominance. This blood group are inherited in the simple Mendelian fashion. Thus offsprings with all four kinds of blood groups are possible. If the parents are heterozygous for blood groups A and B which is shown below.
Male (Heterozygous for blood group A)
Gametes Ia Io
Female (Heterozygous for blood group B) Ib Io |
|
Ia |
Io |
Ib |
Ia Ib |
Ib Io |
|
|
Group AB |
Group B |
|
Io |
Ia Io |
Io Io |
|
|
Group A |
Group O |
If we know the blood groups of a couple the blood groups of their children can easily be predicted as shown below.
|
Blood groups of parents (known) |
Genotype of parents (known) |
Blood groups of children |
|
Possible |
Not possible |
|||
(1) |
O and O |
Io Io ´ Io Io |
O |
A, B, AB |
(2) |
O and A |
Io Io ´ Ia Io |
O, A |
B, AB |
(3) |
A and A |
Ia Io ´ Ia Io |
O, A |
B, AB |
(4) |
O and B |
Io Io ´ Ib Io |
O, B |
A, AB |
(5) |
B and B |
Ib Io ´ Ib Io |
O, B |
A, AB |
(6) |
A and B |
Ia Ia ´ Ib Ib Ia Ia ´ Ib Io Ia Io ´ Ib Io |
O, A, B, AB |
None |
(7) |
O and AB |
Io Io ´ Ia Ib |
A, B |
O, AB |
(8) |
A and AB |
Ia Io ´ Ia Ib |
A, B, AB |
O |
(9) |
B and AB |
Ib Io ´ Ia Ib |
A, B, AB |
O |
(10) |
AB and AB |
Ia Ib ´ Ia Ib |
A, B, AB |
O |
S.No. |
Blood group of child (known) |
Genotype of child (known) |
Blood group of father or mother (known) |
Blood group of other parent |
|
Possible |
Not possible |
||||
(1) |
O |
Io Io |
O A B |
A, B O, B O, A |
AB |
(2) |
A |
Ia Io, Ia Ia |
O, B |
A, AB |
O, B |
(3) |
B |
Ib Io , Ib Ib |
O, A A |
B, AB B, AB |
O, A O, A |
(4) |
AB |
Ia Ib |
B AB |
A, AB A, B, AB |
O, B O |
Rh antibodies. As mother’s blood is Rh– i.e. devoid of Rh antigen, it causes no harm to her. These Rh antibodies alongwith the mother’s blood on reaching the foetal circulation cause clamping of foetal RBCs or agglutination reaction. The first child is some how born normal because by that time the number of antibodies in mother’s blood remain lesser but they increase with successive pregnancies. Thus the foetus following the first child dies either within the womb or just after its birth. This condition is known as erythroblastosis foetalis. So a marriage between Rh+ boy and Rh– girl is considered biologically incompatible.
Boy |
Girl |
Type of biological marriage |
Rh+ |
Rh+ |
Compatible marriage |
Rh– |
Rh– |
Compatible marriage |
Rh– |
Rh+ |
Compatible marriage |
Rh+ |
Rh– |
Incompatible marriage |
However, there is no danger if both parents are Rh– or mother is Rh+ and father is Rh–. Rh factor serum has been developed which when given to the Rh– mother after each child birth saves the next child. This serum contains Rh antibodies which destroy the Rh antigens of foetus before they can initiate formation of Rh antibodies in the mother.
Rh-positive
FATHER
homozygous
RR X rr Rr
Rh-negative
MOTHER
homozygous
First pregnancy
Rh antibodies formed
Rh-negative mother
Rh+foetus
heterozygous
First pregnancy child survives
Rh antibodies
enter foetus
Rh-positive second pregnancy foetus dies
(B)
Father Rh+
×
(A)
Rh positive erythrocytes (antigens) of foetus enter maternal circulation and stimulate production of Rh antibodies
Rh+
Rh– Mother
Foetus
Antibodies of mother enter foetal circulation (subsequent pregnancies) and destroy foetal erythrocytes causing erthroblastosis or death
(C)
Fig : Foetal death in the womb due to erythroblastosis foetalis
condition while those denoted by small letter to Rh– condition. Rh+ condition is dominant over Rh– condition. Thus Rh+ person may be homozygous (RR) or heterozygous (Rr) while Rh– persons are always homozygous(rr). Hereditary trait for Rh– factor is inherited according to Mendelian principle.
PARENTS
Homozygous Homozygous Heterozygous Heterozygous
Rh-positive FATHER
Rh-negative MOTHER
Rh-positive FATHER
Rh-positive MOTHER
RR rr Rr
R R ×
Gametes
r r × R r ×
Rr Rr rr
100%
Rh+
50%
Rh+
50%
Rh–
25%
Rh+
50%
Rh+
25%
Rh–
Offsprings
Fig : Inheritance of Rh antigen
The idea of mutation first originated from the observations of a Dutch botanist Hugo de Vries (1880) on variations in plants of Oenothera lamarckiana. The mutation can be defined as sudden, stable discontinuous and inheritable variations which appear in organism due to permanent change in their genotype. Mutation is mainly of two types :
Tautomeres are the alternate forms of bases and are produced by rearrangements of electrons and proton in
the molecules.
Common Form
NH2
Uncommon Tautomer
H N
Cytosine 5
|
3
Pyrimidines
N 2 N
O O
Thymine
CH3
5
4
O
1 NH
CH3 OH
N
|
O O
N NH
Adenine 5
N 4 1 N
3 2
H N
|
Purines
Guanine
N
N
N
5
4 1 NH
3 2
N
O
N OH
N N
N N
NH2 NH
Fig : Few tautomers
Tautomerism is caused by certain chemical mutagens. In the next replication purines pair with pyrimidines and the base pair is altered at a particular locus. The uncommon forms are unstable and at the next replication, cycle revert back to their normal forms.
A T G C T G G T
II II III III II III III II Original DNA
T