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Multiple allele

Multiple alleles
BIO 202
Multiple alleles-
When more than two different form of given gene exist in a
species they are called as multiple allele
Normally individual has two alleles
During his study on Genetic Mendel also assumed only two alleles of one trait
There can be large number of possible allelic forms in that same population.
Features of Multiple Alleles
Multiple alleles always occupy the same locus on the chromosome.
Multiple alleles always influence the same character.
No crossing over among the member alleles of the same multiple
allelic series.
Examples of Multiple Alleles
a.) Blood group (ABO) in human.
b.) Coat color in mice.
c.) Coat color in rabbit.
d.) Eye color in Drosophila.
Blood group (ABO) in human
Blood group (ABO) in human
• Classical and most common example of multiple allele is ABO
blood group of human being
• Gene coding blood group has three alleles rather than two.
• For the ABO gene, the three alleles are the IA, IB and i alleles. We
typically call these alleles "A," "B," and "O,"
• “letter "I,“ stands for "immunoglobin."
• The IA and IB show co-dominance - an individual who is
heterozygous for these two alleles, the phenotypes of both alleles are
completely expressed, thus producing blood type AB.
• The i allele (the "O" allele) is recessive to both the IA and IB alleles (the
"A" and "B" alleles).
are alleles that cause the death of the organism that carries them. They
are usually a result of mutations in genes that are essential for growth or
Lethal alleles can cause death of an organism prenatally or any time after
Lethal alleles may be recessive or dominant.
Lethal alleles fall into four categories.
1. Early onset- lethal alleles which result in death of an organism at early
stage of life, for example, during embryogenesis.
2. Late onset- lethal allele which kills organism at their final stage of life
3. Conditional- lethal allele which kill an organism under certain
environmental conditions only. e.g., some temperature sensitive alleles kill
organisms only at high temperature.
4. Semi lethal- Lethal allele which kill only some individuals of the
population but not all are know as semi lethal.
Recessive Lethal Gene
A pair of identical alleles that are both present in an
organism that ultimately results in death
Dominant lethal allele kills both in homozygous and heterozygous states.
Individuals with a dominant lethal allele die before they can leave progeny.
Therefore, the mutant dominant lethal is removed from the population in
the same generation in which it arose.
Dominant lethal genes are rarely detected due to their rapid elimination
from populations. One example of a disease caused by a dominant lethal
allele is Huntington’s disease.
Huntington disease is a progressive brain disorder that causes
uncontrolled movements, emotional problems and loss of thinking
Symptoms usually begin between 30 and 50 years of age, but can start at
any age. The disease may develop earlier in life in each successive
generation. About eight percent of cases start before the age of 20 years
and typically present with symptoms more similar to Parkinson's diseas e
A gene that work with other genes to control the expression of a particular
For example, one gene controls whether eye colour is blue or brown but
other (modifier) genes can also influence the colour by affecting the
amount or distribution of pigment in the iris.