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Multiple alleles and Pseudoalleles
Contents : Definition and characteristics of multiple alleles; Examples of multiple alleles; ABO blood groups in man; Other blood antigens in man; Pseudoalleles; Rh antigen in man; Differences between multiple alleles and pseudoalleles; Medico-legal applications of blood groups in man; Suggested reading.
Definition of multiple alleles A series of three or more genes that occupy the same locus in a given pair of homologous chromosomes are referred to as multiple alleles. Note: Multiple alleles were unknown to Mendel; but the alleles follow Mendel’s laws of inheritance
Characteristics of multiple alleles
- The multiple alleles always occupy the same locus on the homologous chromosomes;
- No crossing-over takes place between the members of the multiple alleles;
- Each member of the multiple alleles has its own phenotype, but together they control the same character;
- The wild-type (normal) is dominant or codominant to mutant alleles, and two such alleles may express an intermediate trait ( e.g. AB blood group);
- Crosses between any two mutant alleles result in a mutant-like phenotype, but not like the wild-type phenotype.
Examples of multiple alleles
- ABO blood groups in man: Three alleles (IA, IB^ and IO) control four blood groups: A, B, AB and O.
- Coat colour in rabbits: Four alleles (C+, Cch, Ch^ and Ca) control four coat colours: agouti, chinchilla, Himalayan and albino.
- Genes for haemoglobin variants in man: Numerous alleles control Hb variants, important ones are HbA, HbS, HbC, HbG^ etc.
ABO blood groups in man Blood components of vertebrates: Blood corpuscles, plasma and blood serum
1. Blood corpuscles: Cellular part; composed of erythrocytes (RBC), leucocytes (WBC) and thrombocytes (blood platelets); 2. Plasma: Non-cellular, colourless and liquid part; 3. Blood serum: Transparent, pale yellow part; blood serum= (plasma-fibrinogen) *Fibrinogen is the blood coagulating component.
ABO blood groups: Antigen-antibody reaction Human blood contains two antigens: Antigen A and antigen B; Blood antigens (=agglutinogens) are mucopolysaccharides (protein + sugar) that are present on the surface of the RBC. The protein is identical in both blood antigens A and B, but sugar in the basis for antigen-antibody reaction specificity;
Antibodies (= immunoglobulins) are also proteins, produced in blood serum by the immune system of the body in response to a particular antigen; So, human RBC has two blood antigens: A and B, and blood serum produces two antibodies: anti A (σ) and anti B (β).
Discovery of ABO blood groups Dr. Karl Landsteiner (1900) discovered two types of blood antigens, A and B, and he divided human blood into three groups: A, B and O; He was awarded the Nobel Prize in 1930. Two years later (1902), two students of Dr. Landsteiner: Alfred Von DeCastello and Adriano Sturli discovered the 4th^ blood group AB; So, human blood is divided into four blood groups: A, B, AB and O.
Classification of ABO blood groups in man Depending on the antigen-antibody reactions, humans belong to the following four blood groups:
Note: AB+^ is a universal recipient and O-^ is a universal donor. But, another blood antigen (Rh
- or -) must be taken into consideration during blood transfusion.
Inheritance of ABO blood groups in man ABO blood groups are controlled by three multiple alleles: IA, IB^ and IO^ (=i); where I= isohaemagglutinogen or isoagglutinogen; The gene responsible for blood groups is located on the long arm of chromosome 9 in man; The dominance relationship between the alleles: (IA^ = IB) > IO; So, there are 4 phenotypes and 6 genotypes of the human blood groups as follows: Phenotypes Genotypes A (^) IA IA (homozygote), IA IO (heterozygote) B (^) IB IB (homozygote), IB IO (heterozygote) AB (^) IA IB (always heterozygote) O (^) IO IO (always homozygote)
Frequency of ABO blood groups in man O group ~ 48% A group ~ 37% B group ~ 11% AB group ~ 4%
Fig 7.1 Pie diagram showing the frequency (%) of ABO blood groups in man Subtypes of A and B blood groups Blood groups Subtypes A A 1 , A 2 and A 3 AB A 1 B, A 2 B and A 3 B
Other blood antigens in man: Apart from A and B blood antigens, there is a large number of other blood antigens in man. These include: Rh, MN, Bombay, Cellano, Diago, Duffy, Kidd, Kell, Lervis, Lewis, Lutheran and Xg etc.
A couple of problems and their solutions Problem 1: A woman of blood group A married a man of blood group O. They have three children bearing O, A and AB blood groups. Which child does not belong to the couple? (Or, which child is adopted?)
Solution: The genotype of the man is IOIO. (a) If the woman is homozygous A: IAIA^ × IOIO^ = Children will be A (IAIO) only; (b) If the woman is heterozygous A: IAIO^ × IOIO^ = Children will be A (IAIO) and O (IOIO).
So, the possible blood groups of the children are A and O. The child with blood group AB does not belong to the couple, which means he/she is adopted.
Problem 2: A female belongs to blood group A and she married a person having blood group B. What would be the possible blood groups of their children?
Solution: The female is either IAIA^ or IAIO^ and the male is either IBIB^ or IBIO. So, the possible blood groups of the children from this couple would be: A, B, AB and O which is shown below:
♂♂/♀♀ IAIA^ IAIO IBIB^ AB B, AB IBIO^ A, AB A, B, AB, O
Pseudoalleles Definition Pseudoalleles occupy different loci that are very close to each other, but not exactly the same loci. As a result, crossing-over between the pseudoalleles takes place at extremely low frequencies.
Examples of traits controlled by pseudoalleles
- Rh antigen in man (3-10 alleles): Rh++, Rh+-. Rh-^ -^ are major ones.
- Eye colour in Drosophila (up to 16 alleles): white eye (w), apricot (wa), coral (wc), chery (wch), ivory (wi) etc are common ones.
Rh antigen (Rh factor) in man Discovery: Landsteiner and Wiener (1940) discovered Rh antigen in man. Rhesus monkeys Macaca rhesus have the same antigen, hence the name. Subsequently, it was found that about 85% people have similar antigen (Rh+) and about 15% people lack this antigen (Rh-) in the blood. The responsible gene for the Rh antigen is located on the short arm of chromosome 1 in man.
Number of alleles: Rh antigen forms a complex system involving over 45 antigens. According to Fisher and Wiener, however, 3-10 pseudoalleles regulate the Rh antigen in man. For simplicity, we may assume that there are 3 genotypes: Rh++, Rh+-^ and Rh- -^ in the human population.
Importance of Rh antigen in man: Rh antigen is very important for blood groupings in man, because mixing of Rh+ and Rh- blood (incompatible match) may cause blood clotting as well as death of the blood recipients and foetus/ newborn babies.
Rh incompatibility in man An example: P: Husband Rh positive (Rh++/Rh+-) × Wife Rh negative (Rh- -) G: Rh+^ and Rh-^ Rh- F 1 : Rh+-^ Rh- -^ Rh+-^ Rh- - Mother sensitized Normal Die of Erythroblastosis Normal baby baby foetalis baby
Features of Erythroblastosis foetalis in man: o The foetus suffers from haemolytic anaemia; o It has severe jaundice; and o It results in stillbirth or neonatal death.
Legal applications:
- Criminal investigations;
- Disputed parentage settlements;
- Identification of disaster victims;
- Description of human populations and races etc.
Suggested reading: Ayala FJ & Kiger Jr. JA, 1980. Modern Genetics. Burns, GW. 1980. The Science of Genetics. Gardner et al. 1991. Principles of Genetics (8th^ edn) Islam, MS. 2018. Selected Lectures on Genetics. LAP Lambert Academic Publishing, Germany. Sinnott et al. 1973. Principles of Genetics (5th^ edn) Stansfield, WD. 1991. Theory and Problems of Genetics (3rd^ edn) Strickberger, MW. 1976. Introduction to Genetics. Winchester, AM. 1966. Genetics (3rd^ edn) Wikipedia: www.wikipedia.com Bmjvg, g.mv., Lvb, nv.mv. I ivbv, g.nv.Zv. 2017| †R‡bwU·: wgj I Awg‡ji weÁvb | Ab¨cÖKvk, XvKv|
