5 Genetics and Evolution
5.5 Multiple allelism, Recombination, Sex linkage
Mutliple alleles, Recombination, Linkage
Dr V Malathi
Multiple allelism
According to Mendel, only two alleles, one dominant and one recessive, could exist for a given gene.
Most diploid organisms including humans can only have two alleles for a given gene
However multiple alleles may exist at the population level .
In such cases many combinations of two alleles are observed.
When many alleles exist for the same gene, it is convention to denote the most common phenotype or genotype as the wild type (often abbreviated “+”);
All other phenotypes or genotypes are considered variants of this standard, meaning that they deviate from the wild type. The variant may be recessive or dominant to the wild-type allele.
Example : Coat color in rabbits .
Here, four alleles exist for the c gene.
- The wild-type version, has Genotype : C+C+, is expressed as brown fur.
- The chinchilla , genotype : has cchcch, is expressed as black-tipped white fur.
- The Himalayan phenotype, has genotype : chch, has black fur on the extremities and white fur elsewhere.
- The albino, or “colorless” phenotype, genotype cc, is expressed as white fur.
In cases of multiple alleles, dominance hierarchies can exist.
In above case, the wild-type allele is dominant over all the others, chinchilla is incompletely dominant over Himalayan and albino, and Himalayan is dominant over albino.
“Four different alleles of rabbit” by Lumen learning is licensed under CC BY 4.0
Another example for multiple allelism is ABO blood grouping in humans
The ABO blood groups is determined according to the type of the antigen protein expressed on the surface of the Red Blood Cells .The three alleles: IA, IB, and i determine the blood group
- The A blood group blood RBCs express the Antigen A on their surface ; Expressed by the alleles IA IA and IA i
- The B blood group blood RBCs express the Antigen B on their surface ; Expressed by the alleles IB IB and IB i
- The A Bblood group blood RBCs express both the Antigen A and B on their surface ; Expressed by the alleles IA IB
- The O blood group blood RBCs do not express any Antigen A on their surface ; These possess the alleles ii
Alleles IA and IB are codominant with respect to one another, and both are dominant to i. These three alleles in the population result in six genotypes, and four phenotypes.
” ABO blood system in humans “ by Melissa Hardy is licensed under CC BY-NC 4.0
Watch the video from Amoeba Sisters on Multiple alleles
Genetic Recombination
Genetic recombination involves the exhange of genetic material between organisms. This leads to the production of offspring with combinations of traits that differ from either parent.
In eukaryotes ,genetic recombination occurs during meiosis .
Recombination can be classified in to two types
- Interchromosomal recombination : This occurs through independent assortment of alleles, which are on different but on homologous chromosomes.
- Int