3. The Lethal Genes.

In some livings, some genes that control some characteristics when found in the homozygous state, lead to the retardation of growth and cause death to almost one fourth of the offsprings at different stages of life. These genes are referred to as lethal genes. Here is an example for these cases:

Sickle-cell Anaemia in Man:

Is a genetic disease that arises from genetic disorder in the gene responsible for Haemoglobin synthesis in the red blood cells. As a result, a change in the Haemoglobin molecule occurs. Consequently, the red blood cells become folded, and sickle-shaped. These abnormal red blood cells are unable to pick up Oxygen used in respiration. This leads in many cases to malfunctioning of the body, and finally to the death of the sick person.

If (S) represents the gene for normal Haemoglobin.

And (s) represents the gene for the deformed Haemoglobin.

There are three possible genotypes for Human individuals:

• SS : Normal persons. They are homozygous, and have normal Haemoglobin.

• Ss : Carrier persons. They are heterozygous, and the symptoms of the disease

appear on them after physical exertion, or if present in places where

Oxygen is scarce.

• ss : Sick persons that die before maturity. They are homozygous recessive.

If a carrier man marries a carrier woman, the following crossing represents f1 resulted:

A carrier man X A carrier woman

P1 Ss Ss

G1

SS Ss Ss ss

F 1

Normal Carrier Sick and die

1 : 2 : 1

N .B.: This disease represents a case of incomplete dominance. Since, the heterozygous individuals (the carriers) have a special phenotype when Oxygen is scarce. This is different from Lack of dominance.

3. The Multiple Alleles.

S ome of the genetic traits are controlled by more than one pair of allelomorphic genes, and so called multiple alleles. However, each individual carries only two of these allelic genes in a specific locus on a homologous pair of chromosomes. Here is an example for these cases:

1. Blood groups in Man:

After many bad incidents due to blood transfusion, scientists discovered that Human blood differs in some respects from one person to another. As a results of their studies, Human blood was classified into four main groups: (A), (B), (AB), and (O). It was also discovered that these blood groups are inherited as multiple alleles.

There are three allelic genes (A), (B), and (O) from which six genotypes are formed:

(AA), (BB), (OO), (AB), (AO), (BO).

But since the allele (O) is recessive to both (A), and (B) alleles, and there is a case of lack of dominance between (A), and (B) alleles, there are only four phenotypes, and these are the blood groups (A), (B), (AB), and (O). The genotypes can be determined by following up the phenotypes of the parents, and their children. This helps in judging the paternity of disputed children or referring mixed children to their real parents.

The following example illustrates the case of two newly born children that were mixed up in the hospital. Each of the two children belongs to one of the following families:

The 1^st.family: The blood groups of the parents are (A) and (AB), and the blood group of the baby is (O).

The 2^nd.family: The blood groups of the parents are (B) and (O), and the blood group of the baby is (B).

By knowing how the blood groups are inherited, the first family cannot produce a child with blood group (O), while the second family can have babies with blood groups (B) and (O). Thus, the first child (O) cannot belong to the first family. Though this cannot be proven that this child or the other (B) belongs to any of the two families. Despite that, the blood group analysis proved to which family the two children belong. Blood group analysis is helpful only in denying the paternity of a man to a certain child. Blood group analysis helps also in Human race taxonomy, and is used in blood transfusion.

Blood transfusion:

Blood can be transfused between persons of the same blood groups, or between persons of other blood groups according to a definite system. This is due to the presence of:

• Antegins on the surfaces of red blood cells.

• Antibodies in the blood plasma.

. In blood group (A): there is antigen (A) on surfaces of red blood cells, and antibody (B) in the plasma.

. In blood group (B): there is antigen (B) on surfaces of red blood cells, and antibody (A) in the plasma.

. In blood group (AB): there are antigen (A), and antigen (B) on surfaces of red blood cells, and there are no antibodies in the blood plasma.

. In blood group (O): there are no antigens on surfaces of red blood cells, and there are antibodies (A) and (B) in the blood plasma.

The following table summarizes the information about blood groups, and their relation to blood transfusion:

Blood groups	The genotype	Antigen	Antibody	Donor to	Recipient from
A	AA or AO	A	B	A and AB	A and O
B	BB or BO	B	A	B and AB	B and O
AB	AB	A and B	--	AB	All groups
O	OO	--	A and B	All groups	O

N.B.:

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

2. 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: (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) (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 gametes parental 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: