Principle of Multiple AlleleThe 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.

the human blood relationship

• 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 colorblindnessTable 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)