Multiple allele - Genetics

Principle of Multiple Allele
The genes of a particular trait are
located in specific loci in the
chromosomes. Through the help of the
modern technology, geneticists
discovered that there are some traits
that are not controlled only by two
alleles but by multiple alleles.

Human blood groups
• one example of the traits controlled by multiple
alleles is the human blood groups: A, B, AB and
O. These 3 letters refer to 2 types of
carbohydrates designated as A and B that are
incorporated in the membranes of red blood
cells.
• Although an individual can only have 2 alleles per
gene, 3 alleles control this characteristic, which in
various combination, produce the 4 human blood
groups: A, AB, B and O.

• It shows that the alleles for A (I^A) and B (I^B)
are dominant over the O (i) allele.
• Persons with type O carry the homozygous
alleles for O (ii). This means that they lack the
A and B alleles in their blood. A person
heterozygous for blood type AB carries the
Alleles for A and B and since both alleles are
expressed these alleles codominant with each
other.

• a person's blood type can be done through a
simple test. This is the reason why it is used as
evidence in paternity suits. Blood tests can be
used as evidence whether the man could be
or could not be the father of a certain child,
although blood type alone does not prove that
he is.

Coat Color in Rabbits
• Another example of a trait controlled by multiple
genes is coat color in rabbits. There are 4 types of
coat color in rabbits and each type denots
specific alleles.
• Agouti coat is pure black or yellow and
sometimes with patches.
• Chinchilla coat - silvery gray
• Himalayan - white coats with black color in the
extremities.
• Albino coat- pure white due to absence of
pigmentation.

This shows the genotypes of the different coat colors of in
rabbit. Take not that Agouti is the most dominant among
the coat colors, followed by Chinchilla

Cross between Agouti and Chinchilla coat color

sex-linked traits
• Traits that are controlled by genes found in the
sex chromosomes (found in both X and Y)
• sex-linked traits are recessive. In most cases, the
recessive genes or allele was inherited from one
or both of the parents. sex-linked traits mostly
affect the male offspring. this is because they
have only one X-chromosome, which they inherit
from their mother. If the X chromosome carries a
genetic disorder, such disorder will be expressed
in them.

• In contrast, the two X chromosomes of the
female offspring should both carry the disorder
before it can be expressed. examples of sex-
linked trait: hemophilia and color blindness.
• Hemophilia- recessive genetic disorder wherein
the blood does not clot. this is caused by the lack
of genes that synthesize the protein that is
needed to initiate the blood clotting process.

• hemophiliacs bleed excessively, if not
medically controlled can cause death.

• Cross male hemophiliac (XHY) and normal
female (XhXh)
• Cross between a male hemophiliac (XHY) and
a carrier (XHXh)

Color blindness
• It is another example of sex-linked trait. It is a
condition wherein the individual is unable to
distinguish among some or all colors. For
them, some colors appear as shades of gray. It
results from several common recessive
disorders associated with the X chromosomes.
This happens since some mutant forms of the
genes change the light-absorbing capacity of
the sensory receptors inside the cells.

• There are two alleles fro eye vision, the dominant
normal eye vision (N) and the recessive colorblind
vision(n). Both alleles are found in the X
chromosome. Females have 2 X chromosomes. If
both X chromosomes carrry the dominant gene
for normal vision, the female individual definitely
has a normal eye vision. If both X chromosomes
carry the recessive gene (homozygous alleles);
the female individual will be color blind. If the
female carries heterozygous alleles, that female
individual is a carrier, but not color blind.

Genetic table for colorblindness
Table A: Female Eye vision Table B: Male Eye vision
Phenotype Genotype Phenotype Genotype
Normal Vision XNXN Normal vision XN Y
Normal vision
(carrier)
XNXn Color blind Xn Y
Color blind XnXn

• Sex-linked trait is also carried by the genes in
the males’ Y chromosome. Sex-linked traits
associated with the Y chromosomes are called
holandric traits. One typical example of a
holandric trait is trichocysts, which is the
growth of hair in the ears of aging males.

Sex – influenced trait
• It is carried by the autosomes and not the sex
chromosomes. So this trait is not resctricted to
male humans alone. 2 alleles control this trait –
the bald (b), the recessive gene and the non-bald
(B), the dominant gene. But the manner by which
the trait is expressed (phenotype) is unusual. The
phenotypical expression of the trait is controlled
by the hormone testosterone. Both male and
female humans have the hormone, however,
males have the higher levels of testosterone than
females.

• As a result, the recessive allele for baldness (b)
behaves like a dominant allel resulting in what is
classically described as “male pattern baldness”.
Among females, the gene for baldness behaves
like a recessive allele. So that in all cases, a male
heterozygous for baldness will experience hair
loss but a heterozygous female will not. Also, a
homozygous female may only experience
receding hairlines, bald spots in the head or
simply thinning of the hair.

• Thus baldness trait can be inherited from
either parent. If the father is bald and the son
is bald, it can be inferred that the baldness
trait was inherited from the father. However, if
the father is not bald and the son is bald, it
can also be inferred that the son inherited the
trait from his mother. This is the reason why
baldness is sometimes mistakenly identified as
X linked trait.

Genetic table for baldness in Male and
Female humans
Male Female
Phenotype Genotype Phenotype Genotype
Bald XBYb Bald XbXb
Bald XbYb Not bald XBXb
Not bald XBYb Not bald XBXB

• Cross between a bald male (XbYb) and a not
bald female (carrier) (XBXb)

Multiple allele - Genetics

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