CHAPTER 5 INHERITANCE ( part 2)

DIHYBRID INHERITANCE
MENDEL'S SECOND LAW / LAW OF INDEPENDENT ASSORTMENT:
allele pairs separate independently during the formation of gametes. This means that traits are transmitted to offspring independently of one another.


-Dihybrid inheritance: The genetic cross between two parents involving two different characteristics.

e.g. a pea plant with round yellow seed  (RRYY) is crossed with a pea plant with wrinkled green seed (rryy) 

INHERITANCE OF ABO BLOOD GROUP
-is controlled by three alleles iA, iB and ii


-this is known as multiple allele where one gene is controlled 


 by more than 2 alleles.All alleles control the same


 characteristic.

-

- both alleles iA and iB are codominant whereas allele ii is

 recessive.







compatibility of blood group

-compatibility of blood group of a donor's blood and the recipient must be ensured in blood transfusion.
-Otherwise,coagulation of the red blood cells may occur.





INHERITANCE OF RHESUS FACTOR IN HUMAN
-Rheses factor (Rh factor) is an antigen on the membrane surface of 
 red blood cell.
-Rh+ ==> people with Rh factor
-Rh-  ==> people without Rh factor
-allele for Rh+ is dominant whereas allele for Rh- is recessive



























-The presence or absence of Rh factor in the blood will not threaten a person's health.
-But,complications will arise during pregnancy if the mother's blood is Rh- and the foetal's blood is Rh+

• Some of the foetal blood cells may enter the mother's blood via placenta.
• This will trigger the mother's lymphocytes to produce anti-Rh antibodies to react with the Rhesus antigen in the foetal blood.
• However,the first pregnancy does not usually cause any harmful effect to the foetus because the antibodies are not produced fast enough to enter the foetal's bloodstream before birth.
• The anti-Rh antibodies will remain in the mother's bloodstream after delivering.
• In the second pregnancy,the existing anti-Rh antibodies may enter the foetal bloodstream through the placenta. The antibodies may cause agglutination and haemolysis of the foetal red blood cells.
• This condition is known as erythroblastosis fetalis
• The foetus may suffer from severe anaemia,damage to the heart,liver and brain or may die.
• Solution: A mother is given an intravenous injection of anti-Rh globulin after the first child is born to destroy and prevent the formation of anti-Rh antibodies in the next pregnancy.

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