Multiple alleles: Blood group genetics Prepared by | Lecture notes Genetics

Course: Fundamentals of Genetics

Class: - Ist Year, IInd Semester

Lecture No. XV

Title of topic: - Multiple alleles: Blood group genetics

Prepared by- Vinod Kumar, Assistant Professor, (PB & G)

College of Agriculture, Powarkheda

Alleles

Alternate forms of a gene is known as allele. Alleles are of two types viz., either

dominant and recessive or wild type and mutant type.

Characteristic features of alleles:

1. They occupy the same locus on a particular chromosome.

2. They govern the same character of an individual. (T and t – control plant height)

3. A haploid cell has a single copy of an allele for a character. A diploid cell has two

copies of an allele for a character, while a polyploidy cell has more than two copies of

an allele for a character.

4. An individual may have identical alleles at the corresponding locus of homologous

chromosomes in the homozygote or two different alleles in the heterozygote.

5. The alleles may be dominant and recessive or wild and mutant types.

Multiple alleles

Generally a gene has two alternative forms called alleles. Usually one of them is

dominant over the other. The two alleles of a gene determine the two contrasting forms

of a single character. Ex. Tall (T) and dwarf (t) plant height in garden pea. But in many

cases, several alleles of a single gene are known to exist and each one of them governs

a distinct form of the concerned character or trait. Such a situation is known as multiple

allelism and all the alleles of a single gene are called multiple alleles. Many genes in

both animals and plants exhibit multiple alleles. Ex: Blood group in human beings, fur

colour / coat colour in rabbits and self-incompatibility alleles in plants.

1. Blood groups in human beings: On the basis of presence / absence of certain

antigens, four blood groups in human beings have been established by Karl Landsteiner

in 1900. The blood group system in human beings is believed to be controlled by a

single gene generally designated as “ I.” The gene “ I” has three alleles. – IA, IB and i.

Allele IA controls the production of antigen A, IB controls the production of antigen B and i

does not produce any antigen. Individuals with the genotype IAIA or IAi produces antigen

‘A’ and are classified in blood group A. individuals with genotype IBIB or IBi are classified

in blood group B. Individuals with genotype ii are grouped in ‘O’ blood group and such

individuals produce neither antigen A nor antigen B. individuals with genotype IAIB

produce both antigens A and B and hence classified as ‘AB’ blood group.

Human blood groups, their antigen, antibody and compatible blood groups for

transfusion:

Blood group

Genotypes

Antigen found

Antibody present

Compatible blood

group

IA IA or IA i

A and O

IB IB or IB i

B and O

IA IB

None

A, B, AB and O

Universal recipient

None

Universal donor

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Course: Fundamentals of Genetics Class: - Ist^ Year, IInd^ Semester Lecture No. XV Title of topic: - Multiple alleles: Blood group genetics Prepared by- Vinod Kumar, Assistant Professor, (PB & G) College of Agriculture, Powarkheda Alleles Alternate forms of a gene is known as allele. Alleles are of two types viz ., either dominant and recessive or wild type and mutant type. Characteristic features of alleles:

They occupy the same locus on a particular chromosome.
They govern the same character of an individual. (T and t – control plant height)
A haploid cell has a single copy of an allele for a character. A diploid cell has two copies of an allele for a character, while a polyploidy cell has more than two copies of an allele for a character.
An individual may have identical alleles at the corresponding locus of homologous chromosomes in the homozygote or two different alleles in the heterozygote.
The alleles may be dominant and recessive or wild and mutant types. Multiple alleles Generally a gene has two alternative forms called alleles. Usually one of them is dominant over the other. The two alleles of a gene determine the two contrasting forms of a single character. Ex. Tall (T) and dwarf (t) plant height in garden pea. But in many cases, several alleles of a single gene are known to exist and each one of them governs a distinct form of the concerned character or trait. Such a situation is known as multiple allelism and all the alleles of a single gene are called multiple alleles. Many genes in both animals and plants exhibit multiple alleles. Ex: Blood group in human beings, fur colour / coat colour in rabbits and self-incompatibility alleles in plants. 1. Blood groups in human beings: On the basis of presence / absence of certain antigens, four blood groups in human beings have been established by Karl Landsteiner in 1900. The blood group system in human beings is believed to be controlled by a single gene generally designated as “ I.” The gene “ I” has three alleles. – IA, IB and i. Allele IA controls the production of antigen A, IB controls the production of antigen B and i does not produce any antigen. Individuals with the genotype IAIA or IAi produces antigen ‘A’ and are classified in blood group A. individuals with genotype IBIB or IBi are classified in blood group B. Individuals with genotype ii are grouped in ‘O’ blood group and such individuals produce neither antigen A nor antigen B. individuals with genotype IAIB produce both antigens A and B and hence classified as ‘AB’ blood group. Human blood groups, their antigen, antibody and compatible blood groups for transfusion: Blood group Genotypes Antigen found Antibody present Compatible blood group A IA IA or IA i A B A and O B IB IB or IB i B A B and O AB IA IB AB None A, B, AB and O Universal recipient O ii None AB O Universal donor

2. Fur or coat colour in rabbits: The fur colour in rabbits is a well known example of multiple alleles. In rabbits, the fur colour is of four types viz ., agouti, chinchilla, himalayan and albino. It is due to multiple alleles of a single gene ‘C’. Phenotype Gene symbol Genotype

Agouti C CC, Ccch, Cch, Cc
Chinchilla (^) Cch^ cch^ cch, cch^ ch, cchc
Himalayan Ch^ ch^ ch, ch^ c
Albino c cc The order of dominance for fur colour in rabbits can be represented as follows : C > cch > ch > c Agouti > Chinchilla > Himalayan > Albino Full colour Mixture of Main body is No or wild type coloured and white, while the pigment white hairs tips of ears, feet, and with over the body tail and snout pure are black white fur colour 3. Self incompatibility alleles in plants: A series of self incompatibility alleles insures cross pollination in many plants. Such alleles were described first in toabacco and later they were found in several other plant species like Brassica , radish, tomato, potato etc. In these species, self incompatibility is governed by a single gene ‘S’ which has multiple alleles viz ., S 1 , S 2 , S 3 , S 4 and so on. Characteristic features of multiple alleles:
Multiple alleles always belong to the same locus in a chromosome.
One allele is present at a locus at a time in a chromosome.
Multiple alleles always control the same character of an individual. However, the phenotypic expression of the character will differ depending on the alleles present.
There is no crossing over in a multiple allelic series.
In a multiple allelic series, wild type is almost always dominant over the mutant type.
A cross between two strains homozygous for mutant alleles will always produce a mutant phenotype and never a wild phenotype. In other words, multiple alleles do not show complementation.
Further, F 2 generations from such crosses show typical monohybrid ratio for the concerned trait.

Multiple alleles: Blood group genetics Prepared by, Lecture notes of Genetics

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