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Blood from group (O) can be safely transfused to any other group. Persons with blood group (O) are called universal donors.




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Persons with blood group (AB) are called universal recipients, because blood can be transfused to them from any other group safely.

Blood group determination:

In order to determine an individual’s blood group, we should have anti. (A) Serum, anti. (B) Serum, and two drops of the individual’s blood put at the two sides of a microscopic slide. By adding a drop of anti. (A) Serum on one of the two blood drops, and a drop of anti. (B) Serum on the other drop, we observe what happens after stirring separately:

- If agglutination occurs with anti. (A) serum only: The blood group is (A)

- If agglutination occurs with anti. (B) serum only: The blood group is (B)

- If agglutination occurs with both sera: The blood group is (AB)

- If agglutination does not occur with any of the two sera: The blood group is (O)

The Rhesus factor (Rh):

The blood cells have other antigens other than antigens (A) and (B), one of them is known as Rhesus factor, as it was discovered in the blood of a species of monkey known as Rhesus before it was discovered in the Human blood. These antigens are found in about 85% of the Human individuals, who are known as Rh+ persons, whereas the other 15% are free from this antigen, and they are called Rh- persons.

Production of Rhesus antigens is controlled by three pairs of genes. These genes are very close in their effect, and they are close to each other on the pair of chromosomes that carries them.

- The presence of any pair of these pairs of genes in the dominant state leads to an Rh+ person. Thus, Rh+ persons may be homozygous, that is the three pairs of genes are found in the dominant state, or heterozygous, where some of the three pairs of genes are dominant, while the rest are recessive.

- In the Rh- persons, all his genes are recessive.

If a Rh- woman is married to Rh+ man, their children will be:

. 100% Rh+ children, if the father was homozygous.

. 50% Rh+ children, and 50% Rh- children, if the father was heterozygous.

If the fetus is Rh+ and the mother is Rh-, some of the blood that leaks from the fetus to the mother’s blood stimulates it to produce antibodies against the Rhesus factor, these antibodies move to the fetus through the placenta and causes disintegration of the fetal red blood cells, infecting the fetus with sever anaemia.



The first baby is not usually affected.

The second baby receives large amount of the antibodies that may lead to his death, unless his blood is changed, or the mother is injected with a protective serum after the birth of the first baby.

 

Linkage and Crossing Over

Introduction –EXTRA INFORMATION On studying the inheritance of two characteristics together, there are two different probabilities: 1. (INDEPENDENT ASSORTMENT) The two characteristics are controlled by two pairs of genes that are carried on two different pairs of homologous chromosomes: In this case, the genes are assorted independently on the gametes, so that the dihybrid individual (the individual which is heterozygous in two characteristic) forms 4 different types of gametes in equal ratio. The 2nd. Mendelian law (Law of independent assortment) is applied here. EX.: The pea plant which is tall-stemmed, and red-flowered, with the genotype TtRr (heterozygous in the two characteristics), during meiosis and formation of gametes forms 4 different types of gametes in equal ratio: TtRr 2. (LINKAGE) The two characteristics are controlled by two pairs of genes that are carried on the same pair of homologous chromosomes: In this case, the genes are assorted mostly as one unit, so that the 2nd. Mendelian law (law of independent assortment) in not applied here. There are two case of linkage: a) (COMPLETE LINKAGE): If the distance between the linked genes (that are carried on the same chromosomes) is too small, i.e. no chance for crossing over to take place between them: The dihybrid individual (the individual which is heterozygous in two characteristic) forms 2 different types of gametes in equal ratio: EX.: The Drosophila insect which is grey-coloured, and long-winged, with the genotype (GLgl) – (supposing that the distance between the two genes g and l is too small) (heterozygous in the two characteristics), during meiosis and formation of gametes forms 2 different types of gametes in equal ratio: (GLgl) b) (INCOMPLETE LINKAGE - LINKAGE AND CROSSING OVER): If the distance between the linked genes (that are carried on the same chromosomes) is large enough for crossing over to take place between them: The dihybrid individual (the individual which is heterozygous in two characteristic) forms 4 different types of gametes: 2 different types of parental gametes in equal ratio, in addition to 2 different types of recombinant gametes in small ratios proportional to the distance between the linked genes. EX.: The Drosophila insect which is grey-coloured, and long-winged, with the genotype (GLgl) - (supposing that the distance between the two genes g and l is large enough, allowing crossing over to take place) (heterozygous in the two characteristics), during meiosis and formation of gametes forms: (GLgl) Parental gametes recombinant gametesparental gametes 1 : in small ratios proportional to the : 1 distance between the linked genes

Each chromosome carries a great number of genes. The number of genes is far greater than the number of chromosomes. These chromosomes are transferred from generation to generation through the gametes. The genes that are carried on the chromosomes may be inherited independently from each other, or inherited together on one chromosome that is inherited as one unit, as that happening in the following example:





Inheritance of the colour of body, and the length of wings in Drosophila insect:

When crossing occurred between a male homozygous fly with grey-coloured, and long-winged (GLGL) and a female fly homozygous with black colour, and vestigial wings (glgl), where:

(G): Represents the gene for the grey colour of the body that dominates (g) the gene for the black colour of the body.

(L): Represents the gene for the long wings that dominates (l) the gene for the vestigial wings.


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